How do you convert a byte array to a hexadecimal string and vice versa

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How can you convert a byte array to a hexadecimal string  and vice versa

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There is also XmlWriter WriteBinHex  see the MSDN page   This is very useful if you need to put the hexadecimal string into an XML stream   Here is a standalone method to see how it works       public static string ToBinHex byte   bytes                XmlWriterSettings xmlWriterSettings   new XmlWriterSettings            xmlWriterSettings ConformanceLevel   ConformanceLevel Fragment          xmlWriterSettings CheckCharacters   false          xmlWriterSettings Encoding   ASCIIEncoding ASCII          MemoryStream memoryStream   new MemoryStream            using  XmlWriter xmlWriter   XmlWriter Create memoryStream  xmlWriterSettings                         xmlWriter WriteBinHex bytes  0  bytes Length                     return Encoding ASCII GetString memoryStream ToArray

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Inverse function for Waleed Eissa code  Hex String To Byte Array        public static byte   HexToBytes this string hexString                        byte   b   new byte hexString Length   2                       char c          for  int i   0  i  lt  hexString Length   2  i                          c   hexString i   2               b i     byte   c  lt  0x40   c - 0x30    c  lt  0x47   c - 0x37   c - 0x57    lt  lt  4               c   hexString i   2   1               b i      byte  c  lt  0x40   c - 0x30    c  lt  0x47   c - 0x37   c - 0x57                       return b          Waleed Eissa function with lower case support       public static string BytesToHex this byte   barray  bool toLowerCase   true                byte addByte   0x37          if  toLowerCase  addByte   0x57          char   c   new char barray Length   2           byte b          for  int i   0  i  lt  barray Length    i                        b     byte  barray i   gt  gt  4                c i   2     char  b  gt  9   b   addByte   b   0x30               b     byte  barray i   amp  0xF                c i   2   1     char  b  gt  9   b   addByte   b   0x30                      return new string c

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Safe versions   public static class HexHelper        System Diagnostics Contracts Pure      public static string ToHex this byte   value                if  value    null              throw new ArgumentNullException  value             const string hexAlphabet     0123456789ABCDEF            var chars   new char checked value Length   2            unchecked                       for  int i   0  i  lt  value Length  i                                  chars i   2    hexAlphabet value i   gt  gt  4                   chars i   2   1    hexAlphabet value i   amp  0xF                                   return new string chars               System Diagnostics Contracts Pure      public static byte   FromHex this string value                if  value    null              throw new ArgumentNullException  value            if  value Length   2    0              throw new ArgumentException  Hexadecimal value length must be even     value             unchecked                       byte   result   new byte value Length   2               for  int i   0  i  lt  result Length  i                                     0 48  - 9 57  - gt  0 - 9                    A 65  - F 70  - gt  10 - 15                 int b   value i   2      High 4 bits                  int val     b -  0         9  - b   gt  gt  31   amp  -7    lt  lt  4                  b   value i   2   1      Low 4 bits                  val     b -  0         9  - b   gt  gt  31   amp  -7                   result i    checked  byte val                             return result                      Unsafe versions For those who prefer performance and do not afraid of unsafeness  About 35  faster ToHex and 10  faster FromHex   public static class HexUnsafeHelper        System Diagnostics Contracts Pure      public static unsafe string ToHex this byte   value                if  value    null              throw new ArgumentNullException  value             const string alphabet     0123456789ABCDEF            string result   new string      checked value Length   2            fixed  char  alphabetPtr   alphabet          fixed  char  resultPtr   result                        char  ptr   resultPtr              unchecked                               for  int i   0  i  lt  value Length  i                                           ptr       alphabetPtr    value i   gt  gt  4                         ptr       alphabetPtr    value i   amp  0xF                                                      return result              System Diagnostics Contracts Pure      public static unsafe byte   FromHex this string value                if  value    null              throw new ArgumentNullException  value            if  value Length   2    0              throw new ArgumentException  Hexadecimal value length must be even     value             unchecked                       byte   result   new byte value Length   2               fixed  char  valuePtr   value                                char  valPtr   valuePtr                  for  int i   0  i  lt  result Length  i                                             0 48  - 9 57  - gt  0 - 9                        A 65  - F 70  - gt  10 - 15                     int b    valPtr       High 4 bits                      int val     b -  0         9  - b   gt  gt  31   amp  -7    lt  lt  4                      b    valPtr       Low 4 bits                      val     b -  0         9  - b   gt  gt  31   amp  -7                       result i    checked  byte val                                               return result                      BTW For benchmark testing initializing alphabet every time convert function called is wrong  alphabet must be const  for string  or static readonly  for char     Then alphabet-based conversion of byte   to string becomes as fast as byte manipulation versions   And of course test must be compiled in Release  with optimization  and with debug option  Suppress JIT optimization  turned off  same for  Enable Just My Code  if code must be debuggable

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In terms of speed  this seems to be better than anything here     public static string ToHexString byte   data        byte b      int i  j  k      int l   data Length      char   r   new char l   2       for  i   0  j   0  i  lt  l    i          b   data i         k   b  gt  gt  4        r j       char  k  gt  9   k   0x37   k   0x30         k   b  amp  15        r j       char  k  gt  9   k   0x37   k   0x30             return new string r

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As of  NET 5 RC2 you can use   Convert ToHexString byte   inArray  which returns a string and Convert FromHexString string s  which returns a byte     Overloads are available that take span parameters

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And for inserting into an SQL string  if you re not using command parameters    public static String ByteArrayToSQLHexString byte   Source        return    0x    BitConverter ToString Source  Replace  -

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Fastest method for old school people    miss you pointers     static public byte   HexStrToByteArray string str                byte   res   new byte  str Length   2    0   0   str Length   2      check and allocate memory         for  int i   0  j   0  j  lt  res Length  i    2  j      convert loop             res j     byte   str i    32   9    25   16    str i   1    32   9    25           return res

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Another way is by using stackalloc to reduce GC memory pressure   static string ByteToHexBitFiddle byte   bytes            var c   stackalloc char bytes Length   2   1           int b           for  int i   0  i  lt  bytes Length    i                        b   bytes i   gt  gt  4              c i   2     char  55   b      b - 10   gt  gt  31   amp  -7                b   bytes i   amp  0xF              c i   2   1     char  55   b      b - 10   gt  gt  31   amp  -7                      c bytes Length   2       0           return new string c

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I did not get the code you suggested to work  Olipro  hex i    hex i 1  apparently returned an int   I did  however have some success by taking some hints from Waleeds code and hammering this together  It s ugly as hell but it seems to work and performs at 1 3 of the time compared to the others according to my tests  using patridges testing mechanism   Depending on input size  Switching around the   s to separate out 0-9 first would probably yield a slightly faster result since there are more numbers than letters   public static byte   StringToByteArray2 string hex        byte   bytes   new byte hex Length 2       int bl   bytes Length      for  int i   0  i  lt  bl    i                bytes i     byte   hex 2   i   gt   F    hex 2   i  - 0x57   hex 2   i   gt   9    hex 2   i  - 0x37   hex 2   i  - 0x30   lt  lt  4           bytes i      byte  hex 2   i   1   gt   F    hex 2   i   1  - 0x57   hex 2   i   1   gt   9    hex 2   i   1  - 0x37   hex 2   i   1  - 0x30             return bytes

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Either   public static string ByteArrayToString byte   ba      StringBuilder hex   new StringBuilder ba Length   2     foreach  byte b in ba      hex AppendFormat   0 x2    b     return hex ToString        or   public static string ByteArrayToString byte   ba      return BitConverter ToString ba  Replace  -           There are even more variants of doing it  for example here   The reverse conversion would go like this   public static byte   StringToByteArray String hex      int NumberChars   hex Length    byte   bytes   new byte NumberChars   2     for  int i   0  i  lt  NumberChars  i    2      bytes i   2    Convert ToByte hex Substring i  2   16     return bytes        Using Substring is the best option in combination with Convert ToByte  See this answer for more information  If you need better performance  you must avoid Convert ToByte before you can drop SubString

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You can use the BitConverter ToString method   byte   bytes    0  1  2  4  8  16  32  64  128  256  Console WriteLine  BitConverter ToString bytes      Output      00-01-02-04-08-10-20-40-80-FF   More information  BitConverter ToString Method  Byte

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Inverse function for Waleed Eissa code  Hex String To Byte Array        public static byte   HexToBytes this string hexString                        byte   b   new byte hexString Length   2                       char c          for  int i   0  i  lt  hexString Length   2  i                          c   hexString i   2               b i     byte   c  lt  0x40   c - 0x30    c  lt  0x47   c - 0x37   c - 0x57    lt  lt  4               c   hexString i   2   1               b i      byte  c  lt  0x40   c - 0x30    c  lt  0x47   c - 0x37   c - 0x57                       return b          Waleed Eissa function with lower case support       public static string BytesToHex this byte   barray  bool toLowerCase   true                byte addByte   0x37          if  toLowerCase  addByte   0x57          char   c   new char barray Length   2           byte b          for  int i   0  i  lt  barray Length    i                        b     byte  barray i   gt  gt  4                c i   2     char  b  gt  9   b   addByte   b   0x30               b     byte  barray i   amp  0xF                c i   2   1     char  b  gt  9   b   addByte   b   0x30                      return new string c

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a safe version of the lookup solution              public static string ByteArrayToHexViaLookup32Safe byte   bytes  bool withZeroX                if  bytes Length    0                        return withZeroX    0x                           int length   bytes Length   2    withZeroX   2   0           StateSmall stateToPass   new StateSmall bytes  withZeroX           return string Create length  stateToPass   chars  state    gt                        int offset0x   0              if  state WithZeroX                                chars 0     0                   chars 1     x                   offset0x    2                             Span lt uint gt  charsAsInts   MemoryMarshal Cast lt char  uint gt  chars Slice offset0x                int targetLength   state Bytes Length              for  int i   0  i  lt  targetLength  i    1                                uint val   Lookup32 state Bytes i                    charsAsInts i    val                                       private struct StateSmall               public StateSmall byte   bytes  bool withZeroX                        Bytes   bytes              WithZeroX   withZeroX                     public byte   Bytes          public bool WithZeroX

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Here s my shot at it  I ve created a pair of extension classes to extend string and byte  On the large file test  the performance is comparable to Byte Manipulation 2   The code below for ToHexString is an optimized implementation of the lookup and shift algorithm  It is almost identical to the one by Behrooz  but it turns out using a foreach to iterate and a counter is faster than an explicitly indexing for   It comes in 2nd place behind Byte Manipulation 2 on my machine and is very readable code  The following test results are also of interest   ToHexStringCharArrayWithCharArrayLookup  41 589 69 average ticks  over 1000 runs   1 5X ToHexStringCharArrayWithStringLookup  50 764 06 average ticks  over 1000 runs   1 2X ToHexStringStringBuilderWithCharArrayLookup  62 812 87 average ticks  over 1000 runs   1 0X  Based on the above results it seems safe to conclude that    The penalties for indexing into a string to perform the lookup vs  a char array are significant in the large file test  The penalties for using a StringBuilder of known capacity vs  a char array of known size to create the string are even more significant    Here s the code   using System   namespace ConversionExtensions       public static class ByteArrayExtensions               private readonly static char   digits   new char      0    1    2    3    4    5    6    7    8    9    A    B    C    D    E    F              public static string ToHexString this byte   bytes                        char   hex   new char bytes Length   2               int index   0               foreach  byte b in bytes                                hex index      digits b  gt  gt  4                   hex index      digits b  amp  0x0F                              return new string hex                       using System  using System IO   namespace ConversionExtensions       public static class StringExtensions               public static byte   ToBytes this string hexString                        if   string IsNullOrEmpty hexString   amp  amp  hexString Length   2    0                                throw new FormatException  Hexadecimal string must not be empty and must contain an even number of digits to be valid                                hexString   hexString ToUpperInvariant                byte   data   new byte hexString Length   2                for  int index   0  index  lt  hexString Length  index    2                                int highDigitValue   hexString index   lt    9    hexString index  -  0    hexString index  -  A    10                  int lowDigitValue   hexString index   1   lt    9    hexString index   1  -  0    hexString index   1  -  A    10                   if  highDigitValue  lt  0    lowDigitValue  lt  0    highDigitValue  gt  15    lowDigitValue  gt  15                                        throw new FormatException  An invalid digit was encountered  Valid hexadecimal digits are 0-9 and A-F                                       else                                       byte value    byte   highDigitValue  lt  lt  4     lowDigitValue  amp  0x0F                        data index   2    value                                               return data                      Below are the test results that I got when I put my code in  patridge s testing project on my machine  I also added a test for converting to a byte array from hexadecimal  The test runs that exercised my code are ByteArrayToHexViaOptimizedLookupAndShift and HexToByteArrayViaByteManipulation  The HexToByteArrayViaConvertToByte was taken from XXXX  The HexToByteArrayViaSoapHexBinary is the one from  Mykroft s answer      Intel Pentium III Xeon processor      Cores  4  lt br  gt      Current Clock Speed  1576  lt br  gt      Max Clock Speed  3092  lt br  gt              Converting array of bytes into hexadecimal string representation            ByteArrayToHexViaByteManipulation2  39 366 64 average ticks  over 1000 runs   22 4X      ByteArrayToHexViaOptimizedLookupAndShift  41 588 64 average ticks    over 1000 runs   21 2X      ByteArrayToHexViaLookup  55 509 56 average ticks  over 1000 runs   15 9X      ByteArrayToHexViaByteManipulation  65 349 12 average ticks  over 1000 runs   13 5X      ByteArrayToHexViaLookupAndShift  86 926 87 average ticks  over 1000   runs   10 2X      ByteArrayToHexStringViaBitConverter  139 353 73 average   ticks  over 1000 runs  6 3X      ByteArrayToHexViaSoapHexBinary  314 598 77 average ticks  over 1000 runs   2 8X      ByteArrayToHexStringViaStringBuilderForEachByteToString  344 264 63   average ticks  over 1000 runs   2 6X      ByteArrayToHexStringViaStringBuilderAggregateByteToString  382 623 44   average ticks  over 1000 runs   2 3X      ByteArrayToHexStringViaStringBuilderForEachAppendFormat  818 111 95   average ticks  over 1000 runs   1 1X      ByteArrayToHexStringViaStringConcatArrayConvertAll  839 244 84 average   ticks  over 1000 runs   1 1X      ByteArrayToHexStringViaStringBuilderAggregateAppendFormat  867 303 98   average ticks  over 1000 runs   1 0X      ByteArrayToHexStringViaStringJoinArrayConvertAll  882 710 28 average   ticks  over 1000 runs   1 0X

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This version of ByteArrayToHexViaByteManipulation could be faster   From my reports    ByteArrayToHexViaByteManipulation3  1 68 average ticks  over 1000 runs   17 5X ByteArrayToHexViaByteManipulation2  1 73 average ticks  over 1000 runs   16 9X ByteArrayToHexViaByteManipulation  2 90 average ticks  over 1000 runs   10 1X ByteArrayToHexViaLookupAndShift  3 22 average ticks  over 1000 runs   9 1X      static private readonly char   hexAlphabet   new char         0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F    static string ByteArrayToHexViaByteManipulation3 byte   bytes        char   c   new char bytes Length   2       byte b      for  int i   0  i  lt  bytes Length  i                  b     byte  bytes i   gt  gt  4            c i   2    hexAlphabet b           b     byte  bytes i   amp  0xF            c i   2   1    hexAlphabet b             return new string c        And I think this one is an optimization       static private readonly char   hexAlphabet   new char             0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        static string ByteArrayToHexViaByteManipulation4 byte   bytes                char   c   new char bytes Length   2           for  int i   0  ptr   0  i  lt  bytes Length  i    ptr    2                        byte b   bytes i               c ptr    hexAlphabet b  gt  gt  4               c ptr   1    hexAlphabet b  amp  0xF                     return new string c

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Safe versions   public static class HexHelper        System Diagnostics Contracts Pure      public static string ToHex this byte   value                if  value    null              throw new ArgumentNullException  value             const string hexAlphabet     0123456789ABCDEF            var chars   new char checked value Length   2            unchecked                       for  int i   0  i  lt  value Length  i                                  chars i   2    hexAlphabet value i   gt  gt  4                   chars i   2   1    hexAlphabet value i   amp  0xF                                   return new string chars               System Diagnostics Contracts Pure      public static byte   FromHex this string value                if  value    null              throw new ArgumentNullException  value            if  value Length   2    0              throw new ArgumentException  Hexadecimal value length must be even     value             unchecked                       byte   result   new byte value Length   2               for  int i   0  i  lt  result Length  i                                     0 48  - 9 57  - gt  0 - 9                    A 65  - F 70  - gt  10 - 15                 int b   value i   2      High 4 bits                  int val     b -  0         9  - b   gt  gt  31   amp  -7    lt  lt  4                  b   value i   2   1      Low 4 bits                  val     b -  0         9  - b   gt  gt  31   amp  -7                   result i    checked  byte val                             return result                      Unsafe versions For those who prefer performance and do not afraid of unsafeness  About 35  faster ToHex and 10  faster FromHex   public static class HexUnsafeHelper        System Diagnostics Contracts Pure      public static unsafe string ToHex this byte   value                if  value    null              throw new ArgumentNullException  value             const string alphabet     0123456789ABCDEF            string result   new string      checked value Length   2            fixed  char  alphabetPtr   alphabet          fixed  char  resultPtr   result                        char  ptr   resultPtr              unchecked                               for  int i   0  i  lt  value Length  i                                           ptr       alphabetPtr    value i   gt  gt  4                         ptr       alphabetPtr    value i   amp  0xF                                                      return result              System Diagnostics Contracts Pure      public static unsafe byte   FromHex this string value                if  value    null              throw new ArgumentNullException  value            if  value Length   2    0              throw new ArgumentException  Hexadecimal value length must be even     value             unchecked                       byte   result   new byte value Length   2               fixed  char  valuePtr   value                                char  valPtr   valuePtr                  for  int i   0  i  lt  result Length  i                                             0 48  - 9 57  - gt  0 - 9                        A 65  - F 70  - gt  10 - 15                     int b    valPtr       High 4 bits                      int val     b -  0         9  - b   gt  gt  31   amp  -7    lt  lt  4                      b    valPtr       Low 4 bits                      val     b -  0         9  - b   gt  gt  31   amp  -7                       result i    checked  byte val                                               return result                      BTW For benchmark testing initializing alphabet every time convert function called is wrong  alphabet must be const  for string  or static readonly  for char     Then alphabet-based conversion of byte   to string becomes as fast as byte manipulation versions   And of course test must be compiled in Release  with optimization  and with debug option  Suppress JIT optimization  turned off  same for  Enable Just My Code  if code must be debuggable

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Yet another variation for diversity   public static byte   FromHexString string src        if  String IsNullOrEmpty src           return null       int index   src Length      int sz   index   2      if  sz  lt   0          return null       byte   rc   new byte sz        while  --sz  gt   0                char lo   src --index           char hi   src --index            rc sz     byte                                  hi  gt    0   amp  amp  hi  lt    9     hi -  0                     hi  gt    a   amp  amp  hi  lt    f     hi -  a    10                    hi  gt    A   amp  amp  hi  lt    F     hi -  A    10                   0                            lt  lt  4                                   lo  gt    0   amp  amp  lo  lt    9     lo -  0                     lo  gt    a   amp  amp  lo  lt    f     lo -  a    10                    lo  gt    A   amp  amp  lo  lt    F     lo -  A    10                   0                                     return rc

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The following expands the excellent answer here by allowing native lower case option as well  and also handles null or empty input and makes this an extension method            lt summary gt          Converts the byte array to a hex string very fast  Excellent job         with code lightly adapted from  community wiki  here  https   stackoverflow com a 14333437 264031          the function was originally named  ByteToHexBitFiddle   Now allows a native lowerCase option         to be input and allows null or empty inputs  null returns null  empty returns empty            lt  summary gt      public static string ToHexString this byte   bytes  bool lowerCase   false                if  bytes    null              return null          else if  bytes Length    0              return              char   c   new char bytes Length   2            int b          int xAddToAlpha   lowerCase   87   55          int xAddToDigit   lowerCase   -39   -7           for  int i   0  i  lt  bytes Length  i                   b   bytes i   gt  gt  4              c i   2     char  xAddToAlpha   b      b - 10   gt  gt  31   amp  xAddToDigit                 b   bytes i   amp  0xF              c i   2   1     char  xAddToAlpha   b      b - 10   gt  gt  31   amp  xAddToDigit                       string val   new string c           return val             public static string ToHexString this IEnumerable lt byte gt  bytes  bool lowerCase   false                if  bytes    null              return null          byte   arr   bytes ToArray            return arr ToHexString lowerCase

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You can use the BitConverter ToString method   byte   bytes    0  1  2  4  8  16  32  64  128  256  Console WriteLine  BitConverter ToString bytes      Output      00-01-02-04-08-10-20-40-80-FF   More information  BitConverter ToString Method  Byte

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Basic Solution With Extension Support  public static class Utils       public static byte   ToBin this string hex                int NumberChars   hex Length          byte   bytes   new byte NumberChars   2           for  int i   0  i  lt  NumberChars  i    2              bytes i   2    Convert ToByte hex Substring i  2   16           return bytes            public static string ToHex this byte   ba                return  BitConverter ToString ba  Replace  -                  And use this class like below      byte   arr1   new byte     1  2  3        string hex1   arr1 ToHex        byte   arr2   hex1 ToBin

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I ll enter this bit fiddling competition as I have an answer that also uses bit-fiddling to decode hexadecimals  Note that using character arrays may be even faster as calling StringBuilder methods will take time as well   public static String ToHex  byte   data        int dataLength   data Length         pre-create the stringbuilder using the length of the data   2  precisely enough     StringBuilder sb   new StringBuilder  dataLength   2       for  int i   0  i  lt  dataLength  i              int b   data  i               check using calculation over bits to see if first tuple is a letter            isLetter is zero if it is a digit  1 if it is a letter         int isLetter    b  gt  gt  7   amp    b  gt  gt  6     b  gt  gt  5    amp  1              calculate the code using a multiplication to make up the difference between            a digit character and an alphanumerical character         int code    0      b  gt  gt  4   amp  0xF    isLetter     A  -  9  - 1              now append the result  after casting the code point to a character         sb Append   Char code               do the same with the lower  less significant  tuple         isLetter    b  gt  gt  3   amp    b  gt  gt  2     b  gt  gt  1    amp  1          code    0     b  amp  0xF    isLetter     A  -  9  - 1           sb Append   Char code             return sb ToString        public static byte   FromHex  String hex            pre-create the array     int resultLength   hex Length   2      byte   result   new byte resultLength          set validity   0  0   valid  anything else is not valid      int validity   0      int c  isLetter  value  validDigitStruct  validDigit  validLetterStruct  validLetter      for  int i   0  hexOffset   0  i  lt  resultLength  i    hexOffset    2            c   hex  hexOffset               check using calculation over bits to see if first char is a letter            isLetter is zero if it is a digit  1 if it is a letter  upper  amp  lowercase          isLetter    c  gt  gt  6   amp  1              calculate the tuple value using a multiplication to make up the difference between            a digit character and an alphanumerical character            minus 1 for the fact that the letters are not zero based         value     c  amp  0xF    isLetter    -1   10    lt  lt  4              check validity of all the other bits         validity    c  gt  gt  7     changed to  gt  gt   maybe not OK  use UInt           validDigitStruct    c  amp  0x30    0x30          validDigit     c  amp  0x8   gt  gt  3     c  amp  0x6           validity     isLetter   1     validDigitStruct   validDigit            validLetterStruct   c  amp  0x18          validLetter      c - 1   amp  0x4   gt  gt  2      c - 1   amp  0x2           validity    isLetter    validLetterStruct   validLetter               do the same with the lower  less significant  tuple         c   hex  hexOffset   1           isLetter    c  gt  gt  6   amp  1          value     c  amp  0xF    isLetter    -1   10           result  i     byte value              check validity of all the other bits         validity    c  gt  gt  7     changed to  gt  gt   maybe not OK  use UInt           validDigitStruct    c  amp  0x30    0x30          validDigit     c  amp  0x8   gt  gt  3     c  amp  0x6           validity     isLetter   1     validDigitStruct   validDigit            validLetterStruct   c  amp  0x18          validLetter      c - 1   amp  0x4   gt  gt  2      c - 1   amp  0x2           validity    isLetter    validLetterStruct   validLetter              if  validity    0            throw new ArgumentException   Hexadecimal encoding incorrect for input     hex              return result      Converted from Java code

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If you want more flexibility than BitConverter  but don t want those clunky 1990s-style explicit loops  then you can do   String Join String Empty  Array ConvertAll bytes  x   gt  x ToString  X2        Or  if you re using  NET 4 0   String Concat Array ConvertAll bytes  x   gt  x ToString  X2         The latter from a comment on the original post

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I came up with a different code that is tolerant to extra characters  whitespace  dash      It is mostly inspired from some acceptably-fast answers here  It allows parsing of the following  file   00-aa-84-fb 12 32 FF CD 12 00 12 32 FF CD 1200d5e68a        lt summary gt Reads a hex string into bytes lt  summary gt  public static IEnumerable lt byte gt  HexadecimalStringToBytes string hex        if  hex    null          throw new ArgumentNullException nameof hex         char c  c1   default char       bool hasc1   false      unchecked             for  int i   0  i  lt  hex Length  i                  c   hex i               bool isValid    A   lt   c  amp  amp  c  lt    f      a   lt   c  amp  amp  c  lt    f      0   lt   c  amp  amp  c  lt    9               if   hasc1                    if  isValid                        hasc1   true                                  else                   hasc1   false                  if  isValid                        yield return  byte   GetHexVal c1   lt  lt  4    GetHexVal c                                                 c1   c                           lt summary gt Reads a hex string into a byte array lt  summary gt  public static byte   HexadecimalStringToByteArray string hex        if  hex    null          throw new ArgumentNullException nameof hex         var bytes   new List lt byte gt  hex Length   2       foreach  var item in HexadecimalStringToBytes hex             bytes Add item              return bytes ToArray       private static byte GetHexVal char val        return  byte  val -  val  lt  0x3A   0x30   val  lt  0x5B   0x37   0x57                                                                                                       digits 0-9       upper char A-Z     a-z     Please refer to full code when copying  Unit tests included   Some might say it is too much tolerant to extra chars  So don t rely on this code to perform validation  or change it

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Extension methods  disclaimer  completely untested code  BTW       public static class ByteExtensions       public static string ToHexString this byte   ba                StringBuilder hex   new StringBuilder ba Length   2            foreach  byte b in ba                        hex AppendFormat   0 x2    b                     return hex ToString              etc   Use either of Tomalak s three solutions  with the last one being an extension method on a string

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When writing crypto code it s common to avoid data dependent branches and table lookups to ensure the runtime doesn t depend on the data  since data dependent timing can lead to side-channel attacks   It s also pretty fast   static string ByteToHexBitFiddle byte   bytes        char   c   new char bytes Length   2       int b      for  int i   0  i  lt  bytes Length  i              b   bytes i   gt  gt  4          c i   2     char  55   b      b-10  gt  gt 31  amp -7            b   bytes i   amp  0xF          c i   2   1     char  55   b      b-10  gt  gt 31  amp -7              return new string c       Ph nglui mglw nafh Cthulhu R lyeh wgah nagl fhtagn       Abandon all hope  ye who enter here   An explanation of the weird bit fiddling    bytes i   gt  gt  4 extracts the high nibble of a byte bytes i   amp  0xF extracts the low nibble of a byte b - 10 is  lt  0 for values b  lt  10  which will become a decimal digit is  gt   0 for values b  gt  10  which will become a letter from A to F  Using i  gt  gt  31 on a signed 32 bit integer extracts the sign  thanks to sign extension  It will be -1 for i  lt  0 and 0 for i  gt   0  Combining 2  and 3   shows that  b-10  gt  gt 31 will be 0 for letters and -1 for digits  Looking at the case for letters  the last summand becomes 0  and b is in the range 10 to 15  We want to map it to A 65  to F 70   which implies adding 55   A -10   Looking at the case for digits  we want to adapt the last summand so it maps b from the range 0 to 9 to the range 0 48  to 9 57   This means it needs to become -7   0  - 55   Now we could just multiply with 7  But since -1 is represented by all bits being 1  we can instead use  amp  -7 since  0  amp  -7     0 and  -1  amp  -7     -7    Some further considerations    I didn t use a second loop variable to index into c  since measurement shows that calculating it from i is cheaper   Using exactly i  lt  bytes Length as upper bound of the loop allows the JITter to eliminate bounds checks on bytes i   so I chose that variant  Making b an int allows unnecessary conversions from and to byte

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This problem could also be solved using a look-up table  This would require a small amount of static memory for both the encoder and decoder  This method will however be fast    Encoder table 512 nbsp bytes or 1024 nbsp bytes  twice the size if both upper and lower case is needed  Decoder table 256 nbsp bytes or 64 nbsp KiB  either a single char look-up or dual char look-up    My solution uses 1024 nbsp bytes for the encoding table  and 256 nbsp bytes for decoding   Decoding  private static readonly byte   LookupTable   new byte       0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07  0x08  0x09  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     private static byte Lookup char c      var b   LookupTable c     if  b    255      throw new IOException  Expected a hex character  got     c     return b     public static byte ToByte char   chars  int offset      return  byte  Lookup chars offset    lt  lt  4   Lookup chars offset   1         Encoding  private static readonly char     LookupTableUpper  private static readonly char     LookupTableLower   static Hex       LookupTableLower   new char 256       LookupTableUpper   new char 256       for  var i   0  i  lt  256  i            LookupTableLower i    i ToString  x2   ToCharArray        LookupTableUpper i    i ToString  X2   ToCharArray           public static char   ToCharLower byte   b  int bOffset      return LookupTableLower b bOffset       public static char   ToCharUpper byte   b  int bOffset      return LookupTableUpper b bOffset        Comparison  StringBuilderToStringFromBytes    106148 BitConverterToStringFromBytes      15783 ArrayConvertAllToStringFromBytes   54290 ByteManipulationToCharArray         8444 TableBasedToCharArray               5651       this solution  Note  During decoding IOException and IndexOutOfRangeException could occur  if a character has a too high value   256   Methods for de encoding streams or arrays should be implemented  this is just a proof of concept

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This works to go from string to byte array     public static byte   StrToByteArray string str                Dictionary lt string  byte gt  hexindex   new Dictionary lt string  byte gt             for  byte i   0  i  lt  255  i                hexindex Add i ToString  X2    i            List lt byte gt  hexres   new List lt byte gt             for  int i   0  i  lt  str Length  i    2              hexres Add hexindex str Substring i  2              return hexres ToArray

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Extension methods  disclaimer  completely untested code  BTW       public static class ByteExtensions       public static string ToHexString this byte   ba                StringBuilder hex   new StringBuilder ba Length   2            foreach  byte b in ba                        hex AppendFormat   0 x2    b                     return hex ToString              etc   Use either of Tomalak s three solutions  with the last one being an extension method on a string

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Extension methods  disclaimer  completely untested code  BTW       public static class ByteExtensions       public static string ToHexString this byte   ba                StringBuilder hex   new StringBuilder ba Length   2            foreach  byte b in ba                        hex AppendFormat   0 x2    b                     return hex ToString              etc   Use either of Tomalak s three solutions  with the last one being an extension method on a string

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Not to pile on to the many answers here  but I found a fairly optimal   4 5x better than accepted   straightforward implementation of the hex string parser  First  output from my tests  the first batch is my implementation    Give me that string  04c63f7842740c77e545bb0b2ade90b384f119f6ab57b680b7aa575a2f40939f  Time to parse 100 000 times  50 4192 ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F  Accepted answer   StringToByteArray  Time to parse 100000 times  233 1264ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F  With Mono s implementation  Time to parse 100000 times  777 2544ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F  With SoapHexBinary  Time to parse 100000 times  845 1456ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F   The base64 and  BitConverter d  lines are there to test for correctness  Note that they are equal   The implementation   public static byte   ToByteArrayFromHex string hexString      if  hexString Length   2    0  throw new ArgumentException  String must have an even length      var array   new byte hexString Length   2     for  int i   0  i  lt  hexString Length  i    2          array i 2    ByteFromTwoChars hexString i   hexString i   1          return array     private static byte ByteFromTwoChars char p  char p 2      byte ret    if  p  lt    9   amp  amp  p  gt    0           ret    byte    p -  0    lt  lt  4         else if  p  lt    f   amp  amp  p  gt    a           ret    byte    p -  a    10   lt  lt  4         else if  p  lt    F   amp  amp  p  gt    A           ret    byte    p -  A    10   lt  lt  4       else throw new ArgumentException  Char is not a hex digit      p  p       if  p 2  lt    9   amp  amp  p 2  gt    0           ret     byte    p 2 -  0           else if  p 2  lt    f   amp  amp  p 2  gt    a           ret     byte    p 2 -  a    10          else if  p 2  lt    F   amp  amp  p 2  gt    A           ret     byte    p 2 -  A    10        else throw new ArgumentException  Char is not a hex digit      p 2   p 2       return ret      I tried some stuff with unsafe and moving the  clearly redundant  character-to-nibble if sequence to another method  but this was the fastest it got    I concede that this answers half the question  I felt that the string- byte   conversion was underrepresented  while the byte  - string angle seems to be well covered  Thus  this answer

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Either   public static string ByteArrayToString byte   ba      StringBuilder hex   new StringBuilder ba Length   2     foreach  byte b in ba      hex AppendFormat   0 x2    b     return hex ToString        or   public static string ByteArrayToString byte   ba      return BitConverter ToString ba  Replace  -           There are even more variants of doing it  for example here   The reverse conversion would go like this   public static byte   StringToByteArray String hex      int NumberChars   hex Length    byte   bytes   new byte NumberChars   2     for  int i   0  i  lt  NumberChars  i    2      bytes i   2    Convert ToByte hex Substring i  2   16     return bytes        Using Substring is the best option in combination with Convert ToByte  See this answer for more information  If you need better performance  you must avoid Convert ToByte before you can drop SubString

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I guess its speed is worth 16 extra bytes       static char   hexes   new char    0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        public static string ToHexadecimal  this byte   Bytes                char   Result   new char Bytes Length  lt  lt  1           int Offset   0          for  int i   0  i    Bytes Length  i                  Result Offset      hexes Bytes i   gt  gt  4               Result Offset      hexes Bytes i   amp  0x0F                     return new string Result

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Yet another variation for diversity   public static byte   FromHexString string src        if  String IsNullOrEmpty src           return null       int index   src Length      int sz   index   2      if  sz  lt   0          return null       byte   rc   new byte sz        while  --sz  gt   0                char lo   src --index           char hi   src --index            rc sz     byte                                  hi  gt    0   amp  amp  hi  lt    9     hi -  0                     hi  gt    a   amp  amp  hi  lt    f     hi -  a    10                    hi  gt    A   amp  amp  hi  lt    F     hi -  A    10                   0                            lt  lt  4                                   lo  gt    0   amp  amp  lo  lt    9     lo -  0                     lo  gt    a   amp  amp  lo  lt    f     lo -  a    10                    lo  gt    A   amp  amp  lo  lt    F     lo -  A    10                   0                                     return rc

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Basic Solution With Extension Support  public static class Utils       public static byte   ToBin this string hex                int NumberChars   hex Length          byte   bytes   new byte NumberChars   2           for  int i   0  i  lt  NumberChars  i    2              bytes i   2    Convert ToByte hex Substring i  2   16           return bytes            public static string ToHex this byte   ba                return  BitConverter ToString ba  Replace  -                  And use this class like below      byte   arr1   new byte     1  2  3        string hex1   arr1 ToHex        byte   arr2   hex1 ToBin

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If performance matters  here s an optimized solution       static readonly char    hexDigits    0123456789abcdef  ToCharArray        public static string ToHexString this byte   bytes                char   digits   new char bytes Length   2           for  int i   0  i  lt  bytes Length  i                          int d1  d2              d1   Math DivRem bytes i   16  out d2               digits 2   i     hexDigits d1               digits 2   i   1     hexDigits d2                     return new string digits           It s about 2 5 times faster that BitConverter ToString  and about 7 times faster that BitConverter ToString   removal of the  -  chars

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Two mashups which folds the two nibble operations into one   Probably pretty efficient version   public static string ByteArrayToString2 byte   ba        char   c   new char ba Length   2       for  int i   0  i  lt  ba Length   2    i                byte b    byte   ba i gt  gt 1   gt  gt  4   i amp 1  1    amp  0xF           c i     char  55   b      b-10  gt  gt 31  amp -7              return new string  c        Decadent linq-with-bit-hacking version   public static string ByteArrayToString byte   ba        return string Concat  ba SelectMany  b   gt  new int     b  gt  gt  4  b  amp  0xF    Select  b   gt   char  55   b      b-10  gt  gt 31  amp -7           And reverse   public static byte   HexStringToByteArray  string s         byte   ab   new byte s Length gt  gt 1       for  int i   0  i  lt  s Length  i                   int b   s i           b    b -  0         9  - b  gt  gt 31  amp -7           ab i gt  gt 1      byte  b  lt  lt  4   i amp 1  1              return ab

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If you want more flexibility than BitConverter  but don t want those clunky 1990s-style explicit loops  then you can do   String Join String Empty  Array ConvertAll bytes  x   gt  x ToString  X2        Or  if you re using  NET 4 0   String Concat Array ConvertAll bytes  x   gt  x ToString  X2         The latter from a comment on the original post

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You can use the BitConverter ToString method   byte   bytes    0  1  2  4  8  16  32  64  128  256  Console WriteLine  BitConverter ToString bytes      Output      00-01-02-04-08-10-20-40-80-FF   More information  BitConverter ToString Method  Byte

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As of  NET 5 RC2 you can use   Convert ToHexString byte   inArray  which returns a string and Convert FromHexString string s  which returns a byte     Overloads are available that take span parameters

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This version of ByteArrayToHexViaByteManipulation could be faster   From my reports    ByteArrayToHexViaByteManipulation3  1 68 average ticks  over 1000 runs   17 5X ByteArrayToHexViaByteManipulation2  1 73 average ticks  over 1000 runs   16 9X ByteArrayToHexViaByteManipulation  2 90 average ticks  over 1000 runs   10 1X ByteArrayToHexViaLookupAndShift  3 22 average ticks  over 1000 runs   9 1X      static private readonly char   hexAlphabet   new char         0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F    static string ByteArrayToHexViaByteManipulation3 byte   bytes        char   c   new char bytes Length   2       byte b      for  int i   0  i  lt  bytes Length  i                  b     byte  bytes i   gt  gt  4            c i   2    hexAlphabet b           b     byte  bytes i   amp  0xF            c i   2   1    hexAlphabet b             return new string c        And I think this one is an optimization       static private readonly char   hexAlphabet   new char             0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        static string ByteArrayToHexViaByteManipulation4 byte   bytes                char   c   new char bytes Length   2           for  int i   0  ptr   0  i  lt  bytes Length  i    ptr    2                        byte b   bytes i               c ptr    hexAlphabet b  gt  gt  4               c ptr   1    hexAlphabet b  amp  0xF                     return new string c

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If you want to get the  4x speed increase  reported by wcoenen  then if it s not obvious  replace hex Substring i  2  with hex i  hex i 1   You could also take it a step further and get rid of the i  2 by using i   in both places

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Not optimized for speed  but more LINQy than most answers   NET 4 0     lt Extension   gt  Public Function FromHexToByteArray hex As String  As Byte       hex   If hex  String Empty      If hex Length Mod 2   1 Then hex    0   amp  hex     Return Enumerable Range 0  hex Length   2  Select Function i  Convert ToByte hex Substring i   2  2   16   ToArray End Function   lt Extension   gt  Public Function ToHexString bytes As IEnumerable Of Byte   As String     Return String Concat bytes Select Function b  b ToString  X2     End Function

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This works to go from string to byte array     public static byte   StrToByteArray string str                Dictionary lt string  byte gt  hexindex   new Dictionary lt string  byte gt             for  byte i   0  i  lt  255  i                hexindex Add i ToString  X2    i            List lt byte gt  hexres   new List lt byte gt             for  int i   0  i  lt  str Length  i    2              hexres Add hexindex str Substring i  2              return hexres ToArray

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This problem could also be solved using a look-up table  This would require a small amount of static memory for both the encoder and decoder  This method will however be fast    Encoder table 512 nbsp bytes or 1024 nbsp bytes  twice the size if both upper and lower case is needed  Decoder table 256 nbsp bytes or 64 nbsp KiB  either a single char look-up or dual char look-up    My solution uses 1024 nbsp bytes for the encoding table  and 256 nbsp bytes for decoding   Decoding  private static readonly byte   LookupTable   new byte       0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07  0x08  0x09  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     private static byte Lookup char c      var b   LookupTable c     if  b    255      throw new IOException  Expected a hex character  got     c     return b     public static byte ToByte char   chars  int offset      return  byte  Lookup chars offset    lt  lt  4   Lookup chars offset   1         Encoding  private static readonly char     LookupTableUpper  private static readonly char     LookupTableLower   static Hex       LookupTableLower   new char 256       LookupTableUpper   new char 256       for  var i   0  i  lt  256  i            LookupTableLower i    i ToString  x2   ToCharArray        LookupTableUpper i    i ToString  X2   ToCharArray           public static char   ToCharLower byte   b  int bOffset      return LookupTableLower b bOffset       public static char   ToCharUpper byte   b  int bOffset      return LookupTableUpper b bOffset        Comparison  StringBuilderToStringFromBytes    106148 BitConverterToStringFromBytes      15783 ArrayConvertAllToStringFromBytes   54290 ByteManipulationToCharArray         8444 TableBasedToCharArray               5651       this solution  Note  During decoding IOException and IndexOutOfRangeException could occur  if a character has a too high value   256   Methods for de encoding streams or arrays should be implemented  this is just a proof of concept

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Either   public static string ByteArrayToString byte   ba      StringBuilder hex   new StringBuilder ba Length   2     foreach  byte b in ba      hex AppendFormat   0 x2    b     return hex ToString        or   public static string ByteArrayToString byte   ba      return BitConverter ToString ba  Replace  -           There are even more variants of doing it  for example here   The reverse conversion would go like this   public static byte   StringToByteArray String hex      int NumberChars   hex Length    byte   bytes   new byte NumberChars   2     for  int i   0  i  lt  NumberChars  i    2      bytes i   2    Convert ToByte hex Substring i  2   16     return bytes        Using Substring is the best option in combination with Convert ToByte  See this answer for more information  If you need better performance  you must avoid Convert ToByte before you can drop SubString

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I did not get the code you suggested to work  Olipro  hex i    hex i 1  apparently returned an int   I did  however have some success by taking some hints from Waleeds code and hammering this together  It s ugly as hell but it seems to work and performs at 1 3 of the time compared to the others according to my tests  using patridges testing mechanism   Depending on input size  Switching around the   s to separate out 0-9 first would probably yield a slightly faster result since there are more numbers than letters   public static byte   StringToByteArray2 string hex        byte   bytes   new byte hex Length 2       int bl   bytes Length      for  int i   0  i  lt  bl    i                bytes i     byte   hex 2   i   gt   F    hex 2   i  - 0x57   hex 2   i   gt   9    hex 2   i  - 0x37   hex 2   i  - 0x30   lt  lt  4           bytes i      byte  hex 2   i   1   gt   F    hex 2   i   1  - 0x57   hex 2   i   1   gt   9    hex 2   i   1  - 0x37   hex 2   i   1  - 0x30             return bytes

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Not to pile on to the many answers here  but I found a fairly optimal   4 5x better than accepted   straightforward implementation of the hex string parser  First  output from my tests  the first batch is my implementation    Give me that string  04c63f7842740c77e545bb0b2ade90b384f119f6ab57b680b7aa575a2f40939f  Time to parse 100 000 times  50 4192 ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F  Accepted answer   StringToByteArray  Time to parse 100000 times  233 1264ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F  With Mono s implementation  Time to parse 100000 times  777 2544ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F  With SoapHexBinary  Time to parse 100000 times  845 1456ms Result as base64  BMY eEJ0DHflRbsLKt6Qs4TxGfarV7aAt6pXWi9Ak58  BitConverter d  04-C6-3F-78-42-74-0C-77-E5-45-BB-0B-2A-DE-90-B3-84-F1-19-F6-AB-5 7-B6-80-B7-AA-57-5A-2F-40-93-9F   The base64 and  BitConverter d  lines are there to test for correctness  Note that they are equal   The implementation   public static byte   ToByteArrayFromHex string hexString      if  hexString Length   2    0  throw new ArgumentException  String must have an even length      var array   new byte hexString Length   2     for  int i   0  i  lt  hexString Length  i    2          array i 2    ByteFromTwoChars hexString i   hexString i   1          return array     private static byte ByteFromTwoChars char p  char p 2      byte ret    if  p  lt    9   amp  amp  p  gt    0           ret    byte    p -  0    lt  lt  4         else if  p  lt    f   amp  amp  p  gt    a           ret    byte    p -  a    10   lt  lt  4         else if  p  lt    F   amp  amp  p  gt    A           ret    byte    p -  A    10   lt  lt  4       else throw new ArgumentException  Char is not a hex digit      p  p       if  p 2  lt    9   amp  amp  p 2  gt    0           ret     byte    p 2 -  0           else if  p 2  lt    f   amp  amp  p 2  gt    a           ret     byte    p 2 -  a    10          else if  p 2  lt    F   amp  amp  p 2  gt    A           ret     byte    p 2 -  A    10        else throw new ArgumentException  Char is not a hex digit      p 2   p 2       return ret      I tried some stuff with unsafe and moving the  clearly redundant  character-to-nibble if sequence to another method  but this was the fastest it got    I concede that this answers half the question  I felt that the string- byte   conversion was underrepresented  while the byte  - string angle seems to be well covered  Thus  this answer

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Extension methods  disclaimer  completely untested code  BTW       public static class ByteExtensions       public static string ToHexString this byte   ba                StringBuilder hex   new StringBuilder ba Length   2            foreach  byte b in ba                        hex AppendFormat   0 x2    b                     return hex ToString              etc   Use either of Tomalak s three solutions  with the last one being an extension method on a string

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If you want more flexibility than BitConverter  but don t want those clunky 1990s-style explicit loops  then you can do   String Join String Empty  Array ConvertAll bytes  x   gt  x ToString  X2        Or  if you re using  NET 4 0   String Concat Array ConvertAll bytes  x   gt  x ToString  X2         The latter from a comment on the original post

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And for inserting into an SQL string  if you re not using command parameters    public static String ByteArrayToSQLHexString byte   Source        return    0x    BitConverter ToString Source  Replace  -

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In terms of speed  this seems to be better than anything here     public static string ToHexString byte   data        byte b      int i  j  k      int l   data Length      char   r   new char l   2       for  i   0  j   0  i  lt  l    i          b   data i         k   b  gt  gt  4        r j       char  k  gt  9   k   0x37   k   0x30         k   b  amp  15        r j       char  k  gt  9   k   0x37   k   0x30             return new string r

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Shortest way and  net core supported       public static string BytesToString byte   ba    gt          ba Aggregate new StringBuilder 32    sb  b    gt  sb Append b ToString  X2     ToString

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Why make it complex  This is simple in Visual nbsp Studio nbsp 2008   C    string hex   BitConverter ToString YourByteArray  Replace  -          VB   Dim hex As String   BitConverter ToString YourByteArray  Replace  -

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If you want to get the  4x speed increase  reported by wcoenen  then if it s not obvious  replace hex Substring i  2  with hex i  hex i 1   You could also take it a step further and get rid of the i  2 by using i   in both places

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From Microsoft s developers  a nice  simple conversion   public static string ByteArrayToString byte   ba            Concatenate the bytes into one long string     return ba Aggregate new StringBuilder 32                                sb  b    gt  sb Append b ToString  X2                                  ToString        While the above is clean and compact  performance junkies will scream about it using enumerators  You can get peak performance with an improved version of Tomalak s original answer   public static string ByteArrayToString byte   ba             StringBuilder hex   new StringBuilder ba Length   2          for int i 0  i  lt  ba Length  i              lt -- Use for loop is faster than foreach           hex Append ba i  ToString  X2           lt -- ToString is faster than AppendFormat        return hex ToString            This is the fastest of all the routines I ve seen posted here so far  Don t just take my word for it    performance test each routine and inspect its CIL code for yourself

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Two mashups which folds the two nibble operations into one   Probably pretty efficient version   public static string ByteArrayToString2 byte   ba        char   c   new char ba Length   2       for  int i   0  i  lt  ba Length   2    i                byte b    byte   ba i gt  gt 1   gt  gt  4   i amp 1  1    amp  0xF           c i     char  55   b      b-10  gt  gt 31  amp -7              return new string  c        Decadent linq-with-bit-hacking version   public static string ByteArrayToString byte   ba        return string Concat  ba SelectMany  b   gt  new int     b  gt  gt  4  b  amp  0xF    Select  b   gt   char  55   b      b-10  gt  gt 31  amp -7           And reverse   public static byte   HexStringToByteArray  string s         byte   ab   new byte s Length gt  gt 1       for  int i   0  i  lt  s Length  i                   int b   s i           b    b -  0         9  - b  gt  gt 31  amp -7           ab i gt  gt 1      byte  b  lt  lt  4   i amp 1  1              return ab

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There s a class called SoapHexBinary that does exactly what you want   using System Runtime Remoting Metadata W3cXsd2001   public static byte   GetStringToBytes string value        SoapHexBinary shb   SoapHexBinary Parse value       return shb Value     public static string GetBytesToString byte   value        SoapHexBinary shb   new SoapHexBinary value       return shb ToString

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Performance Analysis  Note  new leader as of 2015-08-20   I ran each of the various conversion methods through some crude Stopwatch performance testing  a run with a random sentence  n 61  1000 iterations  and a run with a Project Gutenburg text  n 1 238 957  150 iterations   Here are the results  roughly from fastest to slowest  All measurements are in ticks  10 000 ticks   1 ms  and all relative notes are compared to the  slowest  StringBuilder implementation  For the code used  see below or the test framework repo where I now maintain the code for running this   Disclaimer  WARNING  Do not rely on these stats for anything concrete  they are simply a sample run of sample data  If you really need top-notch performance  please test these methods in an environment representative of your production needs with data representative of what you will use   Results   Lookup by byte unsafe  via CodesInChaos   added to test repo by airbreather    Text  4 727 85  105 2X  Sentence  0 28  99 7X   Lookup by byte  via CodesInChaos    Text  10 853 96  45 8X faster  Sentence  0 65  42 7X faster   Byte Manipulation 2  via CodesInChaos    Text  12 967 69  38 4X faster  Sentence  0 73  37 9X faster   Byte Manipulation  via Waleed Eissa    Text  16 856 64  29 5X faster  Sentence  0 70  39 5X faster   Lookup Shift  via Nathan Moinvaziri    Text  23 201 23  21 4X faster  Sentence  1 24  22 3X faster   Lookup by nibble  via Brian Lambert    Text  23 879 41  20 8X faster  Sentence  1 15  23 9X faster   BitConverter  via Tomalak    Text  113 269 34  4 4X faster  Sentence  9 98  2 8X faster    SoapHexBinary  ToString  via Mykroft    Text  178 601 39  2 8X faster  Sentence  10 68  2 6X faster    byte  ToString  X2    using foreach   derived from Will Dean s answer    Text  308 805 38  2 4X faster  Sentence  16 89  2 4X faster    byte  ToString  X2    using  IEnumerable  Aggregate  requires System Linq   via Mark    Text  352 828 20  2 1X faster  Sentence  16 87  2 4X faster   Array ConvertAll  using string Join   via Will Dean    Text  675 451 57  1 1X faster  Sentence  17 95  2 2X faster   Array ConvertAll  using string Concat  requires  NET 4 0   via Will Dean    Text  752 078 70  1 0X faster  Sentence  18 28  2 2X faster    StringBuilder  AppendFormat  using foreach   via Tomalak    Text  672 115 77  1 1X faster  Sentence  36 82  1 1X faster    StringBuilder  AppendFormat  using  IEnumerable  Aggregate  requires System Linq   derived from Tomalak s answer    Text  718 380 63  1 0X faster  Sentence  39 71  1 0X faster     Lookup tables have taken the lead over byte manipulation  Basically  there is some form of precomputing what any given nibble or byte will be in hex  Then  as you rip through the data  you simply look up the next portion to see what hex string it would be  That value is then added to the resulting string output in some fashion  For a long time byte manipulation  potentially harder to read by some developers  was the top-performing approach   Your best bet is still going to be finding some representative data and trying it out in a production-like environment  If you have different memory constraints  you may prefer a method with fewer allocations to one that would be faster but consume more memory   Testing Code  Feel free to play with the testing code I used  A version is included here but feel free to clone the repo and add your own methods  Please submit a pull request if you find anything interesting or want to help improve the testing framework it uses    Add the new static method  Func lt byte    string gt   to  Tests ConvertByteArrayToHexString Test cs  Add that method s name to the TestCandidates return value in that same class  Make sure you are running the input version you want  sentence or text  by toggling the comments in GenerateTestInput in that same class  Hit F5 and wait for the output  an HTML dump is also generated in the  bin folder       static string ByteArrayToHexStringViaStringJoinArrayConvertAll byte   bytes        return string Join string Empty  Array ConvertAll bytes  b   gt  b ToString  X2        static string ByteArrayToHexStringViaStringConcatArrayConvertAll byte   bytes        return string Concat Array ConvertAll bytes  b   gt  b ToString  X2        static string ByteArrayToHexStringViaBitConverter byte   bytes        string hex   BitConverter ToString bytes       return hex Replace  -          static string ByteArrayToHexStringViaStringBuilderAggregateByteToString byte   bytes        return bytes Aggregate new StringBuilder bytes Length   2    sb  b    gt  sb Append b ToString  X2     ToString      static string ByteArrayToHexStringViaStringBuilderForEachByteToString byte   bytes        StringBuilder hex   new StringBuilder bytes Length   2       foreach  byte b in bytes          hex Append b ToString  X2         return hex ToString      static string ByteArrayToHexStringViaStringBuilderAggregateAppendFormat byte   bytes        return bytes Aggregate new StringBuilder bytes Length   2    sb  b    gt  sb AppendFormat   0 X2    b   ToString      static string ByteArrayToHexStringViaStringBuilderForEachAppendFormat byte   bytes        StringBuilder hex   new StringBuilder bytes Length   2       foreach  byte b in bytes          hex AppendFormat   0 X2    b       return hex ToString      static string ByteArrayToHexViaByteManipulation byte   bytes        char   c   new char bytes Length   2       byte b      for  int i   0  i  lt  bytes Length  i              b     byte  bytes i   gt  gt  4            c i   2     char  b  gt  9   b   0x37   b   0x30           b     byte  bytes i   amp  0xF            c i   2   1     char  b  gt  9   b   0x37   b   0x30             return new string c     static string ByteArrayToHexViaByteManipulation2 byte   bytes        char   c   new char bytes Length   2       int b      for  int i   0  i  lt  bytes Length  i              b   bytes i   gt  gt  4          c i   2     char  55   b      b - 10   gt  gt  31   amp  -7            b   bytes i   amp  0xF          c i   2   1     char  55   b      b - 10   gt  gt  31   amp  -7              return new string c     static string ByteArrayToHexViaSoapHexBinary byte   bytes        SoapHexBinary soapHexBinary   new SoapHexBinary bytes       return soapHexBinary ToString      static string ByteArrayToHexViaLookupAndShift byte   bytes        StringBuilder result   new StringBuilder bytes Length   2       string hexAlphabet    0123456789ABCDEF       foreach  byte b in bytes            result Append hexAlphabet  int  b  gt  gt  4             result Append hexAlphabet  int  b  amp  0xF               return result ToString      static readonly uint   lookup32UnsafeP    uint  GCHandle Alloc  Lookup32  GCHandleType Pinned  AddrOfPinnedObject    static string ByteArrayToHexViaLookup32UnsafeDirect byte   bytes        var lookupP    lookup32UnsafeP      var result   new string  char 0  bytes Length   2       fixed  byte  bytesP   bytes      fixed  char  resultP   result            uint  resultP2    uint  resultP          for  int i   0  i  lt  bytes Length  i                  resultP2 i    lookupP bytesP i                        return result    static uint    Lookup32   Enumerable Range 0  255  Select i   gt        string s   i ToString  X2        return   uint s 0       uint s 1   lt  lt  16      ToArray    static string ByteArrayToHexViaLookupPerByte byte   bytes        var result   new char bytes Length   2       for  int i   0  i  lt  bytes Length  i                  var val    Lookup32 bytes i            result 2 i     char val          result 2 i   1     char   val  gt  gt  16             return new string result     static string ByteArrayToHexViaLookup byte   bytes        string   hexStringTable   new string              00    01    02    03    04    05    06    07    08    09    0A    0B    0C    0D    0E    0F            10    11    12    13    14    15    16    17    18    19    1A    1B    1C    1D    1E    1F            20    21    22    23    24    25    26    27    28    29    2A    2B    2C    2D    2E    2F            30    31    32    33    34    35    36    37    38    39    3A    3B    3C    3D    3E    3F            40    41    42    43    44    45    46    47    48    49    4A    4B    4C    4D    4E    4F            50    51    52    53    54    55    56    57    58    59    5A    5B    5C    5D    5E    5F            60    61    62    63    64    65    66    67    68    69    6A    6B    6C    6D    6E    6F            70    71    72    73    74    75    76    77    78    79    7A    7B    7C    7D    7E    7F            80    81    82    83    84    85    86    87    88    89    8A    8B    8C    8D    8E    8F            90    91    92    93    94    95    96    97    98    99    9A    9B    9C    9D    9E    9F            A0    A1    A2    A3    A4    A5    A6    A7    A8    A9    AA    AB    AC    AD    AE    AF            B0    B1    B2    B3    B4    B5    B6    B7    B8    B9    BA    BB    BC    BD    BE    BF            C0    C1    C2    C3    C4    C5    C6    C7    C8    C9    CA    CB    CC    CD    CE    CF            D0    D1    D2    D3    D4    D5    D6    D7    D8    D9    DA    DB    DC    DD    DE    DF            E0    E1    E2    E3    E4    E5    E6    E7    E8    E9    EA    EB    EC    ED    EE    EF            F0    F1    F2    F3    F4    F5    F6    F7    F8    F9    FA    FB    FC    FD    FE    FF              StringBuilder result   new StringBuilder bytes Length   2       foreach  byte b in bytes            result Append hexStringTable b              return result ToString        Update  2010-01-13   Added Waleed s answer to analysis  Quite fast   Update  2011-10-05   Added string Concat Array ConvertAll variant for completeness  requires  NET 4 0   On par with string Join version   Update  2012-02-05   Test repo includes more variants such as StringBuilder Append b ToString  X2     None upset the results any  foreach is faster than  IEnumerable  Aggregate  for instance  but BitConverter still wins   Update  2012-04-03   Added Mykroft s SoapHexBinary answer to analysis  which took over third place   Update  2013-01-15   Added CodesInChaos s byte manipulation answer  which took over first place  by a large margin on large blocks of text    Update  2013-05-23   Added Nathan Moinvaziri s lookup answer and the variant from Brian Lambert s blog  Both rather fast  but not taking the lead on the test machine I used  AMD Phenom 9750    Update  2014-07-31   Added  CodesInChaos s new byte-based lookup answer  It appears to have taken the lead on both the sentence tests and the full-text tests   Update  2015-08-20   Added airbreather s optimizations and unsafe variant to this answer s repo  If you want to play in the unsafe game  you can get some huge performance gains over any of the prior top winners on both short strings and large texts

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There is a simple one-liner solution not yet mentioned that will convert hex strings into byte arrays  we don t care about negative interpretation here as it does not matter    BigInteger Parse str  System Globalization NumberStyles HexNumber  ToByteArray   Reverse   ToArray

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For performance I would go with drphrozens solution  A tiny optimization for the decoder could be to use a table for either char to get rid of the   lt  lt  4    Clearly the two method calls are costly  If some kind of check is made either on input or output data  could be CRC  checksum or whatever  the if  b    255     could be skipped and thereby also the method calls altogether   Using offset   and offset instead of offset and offset   1 might give some theoretical benefit but I suspect the compiler handles this better than me   private static readonly byte   LookupTableLow   new byte       0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07  0x08  0x09  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     private static readonly byte   LookupTableHigh   new byte       0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0x00  0x10  0x20  0x30  0x40  0x50  0x60  0x70  0x80  0x90  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xA0  0xB0  0xC0  0xD0  0xE0  0xF0  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xA0  0xB0  0xC0  0xD0  0xE0  0xF0  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     private static byte LookupLow char c      var b   LookupTableLow c     if  b    255      throw new IOException  Expected a hex character  got     c     return b     private static byte LookupHigh char c      var b   LookupTableHigh c     if  b    255      throw new IOException  Expected a hex character  got     c     return b     public static byte ToByte char   chars  int offset      return  byte  LookupHigh chars offset       LookupLow chars offset         This is just off the top of my head and has not been tested or benchmarked

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Why make it complex  This is simple in Visual nbsp Studio nbsp 2008   C    string hex   BitConverter ToString YourByteArray  Replace  -          VB   Dim hex As String   BitConverter ToString YourByteArray  Replace  -

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Another lookup table based approach  This one uses only one lookup table for each byte  instead of a lookup table per nibble   private static readonly uint    lookup32   CreateLookup32     private static uint   CreateLookup32         var result   new uint 256       for  int i   0  i  lt  256  i                  string s i ToString  X2            result i      uint s 0       uint s 1   lt  lt  16             return result     private static string ByteArrayToHexViaLookup32 byte   bytes        var lookup32    lookup32      var result   new char bytes Length   2       for  int i   0  i  lt  bytes Length  i                  var val   lookup32 bytes i            result 2 i     char val          result 2 i   1     char   val  gt  gt  16             return new string result       I also tested variants of this using ushort  struct char X1  X2   struct byte X1  X2  in the lookup table   Depending on the compilation target  x86  X64  those either had the approximately same performance or were slightly slower than this variant     And for even higher performance  its unsafe sibling   private static readonly uint    lookup32Unsafe   CreateLookup32Unsafe    private static readonly uint   lookup32UnsafeP    uint  GCHandle Alloc  lookup32Unsafe GCHandleType Pinned  AddrOfPinnedObject     private static uint   CreateLookup32Unsafe         var result   new uint 256       for  int i   0  i  lt  256  i                  string s i ToString  X2            if BitConverter IsLittleEndian              result i      uint s 0       uint s 1   lt  lt  16           else             result i      uint s 1       uint s 0   lt  lt  16             return result     public static string ByteArrayToHexViaLookup32Unsafe byte   bytes        var lookupP    lookup32UnsafeP      var result   new char bytes Length   2       fixed byte  bytesP   bytes      fixed  char  resultP   result                uint  resultP2    uint  resultP          for  int i   0  i  lt  bytes Length  i                          resultP2 i    lookupP bytesP i                        return new string result       Or if you consider it acceptable to write into the string directly   public static string ByteArrayToHexViaLookup32UnsafeDirect byte   bytes        var lookupP    lookup32UnsafeP      var result   new string  char 0  bytes Length   2       fixed  byte  bytesP   bytes      fixed  char  resultP   result                uint  resultP2    uint  resultP          for  int i   0  i  lt  bytes Length  i                          resultP2 i    lookupP bytesP i                        return result

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static string ByteArrayToHexViaLookupPerByte2 byte   bytes                            var result3   new uint bytes Length           for  int i   0  i  lt  bytes Length  i                    result3 i     Lookup32 bytes i            var handle   GCHandle Alloc result3  GCHandleType Pinned           try                           var result   Marshal PtrToStringUni handle AddrOfPinnedObject    bytes Length   2                   return result                    finally                           handle Free                  This functions in my tests is always the second entry after the unsafe implementation   Unfortunately  the test bench is not so reliable    if you run it multiple times the list got shuffled so much that who knows after the unsafe which is really the fastest  It doesn t take into a account pre-warming  jit compilation time  and GC performance hits  I would like to have rewritten it to have more information  but I didn t had really the time for it

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Combined a few answers into a class for my later copy and paste convenience       lt summary gt      Extension methods to quickly convert byte array to string and back       lt  summary gt  public static class HexConverter            lt summary gt          Map values to hex digits          lt  summary gt      private static readonly char   HexDigits                          0    1    2    3    4    5    6    7    8    9    A    B    C    D    E    F                       lt summary gt          Map 56 characters between   0    F   to their hex equivalents  and set invalid characters         such that they will overflow byte to fail conversion           lt  summary gt      private static readonly ushort   HexValues                         0x0000  0x0001  0x0002  0x0003  0x0004  0x0005  0x0006  0x0007  0x0008  0x0009  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100              0x000A  0x000B  0x000C  0x000D  0x000E  0x000F  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100              0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x000A  0x000B              0x000C  0x000D  0x000E  0x000F                      lt summary gt          Empty byte array           lt  summary gt      private static readonly byte   Empty   new byte 0             lt summary gt          Convert a byte array to a hexadecimal string           lt  summary gt           lt param name  quot bytes quot  gt          The input byte array           lt  param gt           lt returns gt          A string of hexadecimal digits           lt  returns gt      public static string ToHexString this byte   bytes                var c   new char bytes Length   2           for  int i   0  j   0  i  lt  bytes Length  i                          c j      HexDigits bytes i   gt  gt  4               c j      HexDigits bytes i   amp  0x0F                      return new string c                   lt summary gt          Parse a string of hexadecimal digits into a byte array           lt  summary gt           lt param name  quot hexadecimalString quot  gt          The hexadecimal string           lt  param gt           lt returns gt          The parsed  lt see cref  quot byte   quot   gt  array           lt  returns gt           lt exception cref  quot ArgumentException quot  gt          The input string either contained invalid characters  or was of an odd length           lt  exception gt      public static byte   ToByteArray string hexadecimalString                if   TryParse hexadecimalString  out var value                         throw new ArgumentException  quot Invalid hexadecimal string quot   nameof hexadecimalString                       return value                  lt summary gt          Parse a hexadecimal string to bytes          lt  summary gt           lt param name  quot hexadecimalString quot  gt          The hexadecimal string  which must be an even number of characters           lt  param gt           lt param name  quot value quot  gt          The parsed value if successful           lt  param gt           lt returns gt          True if successful           lt  returns gt      public static bool TryParse string hexadecimalString  out byte   value                if  hexadecimalString Length    0                        value   Empty              return true                     if  hexadecimalString Length   2    0                        value   Empty              return false                     try                        value   new byte hexadecimalString Length   2               for  int i   0  j   0  j  lt  hexadecimalString Length  i                                  value i     byte   HexValues hexadecimalString j    -  0    lt  lt  4                                      HexValues hexadecimalString j    -  0                                return true                    catch  OverflowException                        value   Empty              return false

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With Java 8   we ca use Byte toUnsignedInt  public static String convertBytesToHex byte   bytes        StringBuilder result   new StringBuilder        for  byte byt   bytes            int decimal   Byte toUnsignedInt byt           String hex   Integer toHexString decimal           result append hex             return result toString

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If you want more flexibility than BitConverter  but don t want those clunky 1990s-style explicit loops  then you can do   String Join String Empty  Array ConvertAll bytes  x   gt  x ToString  X2        Or  if you re using  NET 4 0   String Concat Array ConvertAll bytes  x   gt  x ToString  X2         The latter from a comment on the original post

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This is an answer to revision 4 of Tomalak s highly popular answer  and subsequent edits    I ll make the case that this edit is wrong  and explain why it could be reverted  Along the way  you might learn a thing or two about some internals  and see yet another example of what premature optimization really is and how it can bite you   tl dr  Just use Convert ToByte and String Substring if you re in a hurry   Original code  below   it s the best combination if you don t want to re-implement Convert ToByte  Use something more advanced  see other answers  that doesn t use Convert ToByte if you need performance  Do not use anything else other than String Substring in combination with Convert ToByte  unless someone has something interesting to say about this in the comments of this answer   warning  This answer may become obsolete if a Convert ToByte char    Int32  overload is implemented in the framework  This is unlikely to happen soon   As a general rule  I don t much like to say  don t optimize prematurely   because nobody knows when  premature  is  The only thing you must consider when deciding whether to optimize or not is   Do I have the time and resources to investigate optimization approaches properly    If you don t  then it s too soon  wait until your project is more mature or until you need the performance  if there is a real need  then you will make the time   In the meantime  do the simplest thing that could possibly work instead   Original code       public static byte   HexadecimalStringToByteArray Original string input                var outputLength   input Length   2          var output   new byte outputLength           for  var i   0  i  lt  outputLength  i                output i    Convert ToByte input Substring i   2  2   16           return output          Revision 4       public static byte   HexadecimalStringToByteArray Rev4 string input                var outputLength   input Length   2          var output   new byte outputLength           using  var sr   new StringReader input                         for  var i   0  i  lt  outputLength  i                    output i    Convert ToByte new string new char 2     char sr Read     char sr Read       16                     return output          The revision avoids String Substring and uses a StringReader instead  The given reason is      Edit  you can improve performance for long strings by using a single   pass parser  like so    Well  looking at the reference code for String Substring  it s clearly  single-pass  already  and why shouldn t it be  It operates at byte-level  not on surrogate pairs   It does allocate a new string however  but then you need to allocate one to pass to Convert ToByte anyway  Furthermore  the solution provided in the revision allocates yet another object on every iteration  the two-char array   you can safely put that allocation outside the loop and reuse the array to avoid that       public static byte   HexadecimalStringToByteArray string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           using  var sr   new StringReader input                         for  var i   0  i  lt  outputLength  i                                  numeral 0     char sr Read                    numeral 1     char sr Read                    output i    Convert ToByte new string numeral   16                                   return output          Each hexadecimal numeral represents a single octet using two digits  symbols    But then  why call StringReader Read twice  Just call its second overload and ask it to read two characters in the two-char array at once  and reduce the amount of calls by two       public static byte   HexadecimalStringToByteArray string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           using  var sr   new StringReader input                         for  var i   0  i  lt  outputLength  i                                  var read   sr Read numeral  0  2                   Debug Assert read    2                   output i    Convert ToByte new string numeral   16                                   return output          What you re left with is a string reader whose only added  value  is a parallel index  internal  pos  which you could have declared yourself  as j for example   a redundant length variable  internal  length   and a redundant reference to the input string  internal  s   In other words  it s useless   If you wonder how Read  reads   just look at the code  all it does is call String CopyTo on the input string  The rest is just book-keeping overhead to maintain values we don t need   So  remove the string reader already  and call CopyTo yourself  it s simpler  clearer  and more efficient       public static byte   HexadecimalStringToByteArray string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           for  int i   0  j   0  i  lt  outputLength  i    j    2                        input CopyTo j  numeral  0  2               output i    Convert ToByte new string numeral   16                     return output          Do you really need a j index that increments in steps of two parallel to i  Of course not  just multiply i by two  which the compiler should be able to optimize to an addition        public static byte   HexadecimalStringToByteArray BestEffort string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           for  int i   0  i  lt  outputLength  i                          input CopyTo i   2  numeral  0  2               output i    Convert ToByte new string numeral   16                     return output          What does the solution look like now  Exactly like it was at the beginning  only instead of using String Substring to allocate the string and copy the data to it  you re using an intermediary array to which you copy the hexadecimal numerals to  then allocate the string yourself and copy the data again from the array and into the string  when you pass it in the string constructor   The second copy might be optimized-out if the string is already in the intern pool  but then String Substring will also be able to avoid it in these cases   In fact  if you look at String Substring again  you see that it uses some low-level internal knowledge of how strings are constructed to allocate the string faster than you could normally do it  and it inlines the same code used by CopyTo directly in there to avoid the call overhead   String Substring   Worst-case  One fast allocation  one fast copy  Best-case  No allocation  no copy    Manual method   Worst-case  Two normal allocations  one normal copy  one fast copy  Best-case  One normal allocation  one normal copy    Conclusion  If you want to use Convert ToByte String  Int32   because you don t want to re-implement that functionality yourself   there doesn t seem to be a way to beat String Substring  all you do is run in circles  re-inventing the wheel  only with sub-optimal materials    Note that using Convert ToByte and String Substring is a perfectly valid choice if you don t need extreme performance  Remember  only opt for an alternative if you have the time and resources to investigate how it works properly   If there was a Convert ToByte char    Int32   things would be different of course  it would be possible to do what I described above and completely avoid String    I suspect that people who report better performance by  avoiding String Substring  also avoid Convert ToByte String  Int32   which you should really be doing if you need the performance anyway  Look at the countless other answers to discover all the different approaches to do that   Disclaimer  I haven t decompiled the latest version of the framework to verify that the reference source is up-to-date  I assume it is   Now  it all sounds good and logical  hopefully even obvious if you ve managed to get so far   But is it true   Intel R  Core TM  i7-3720QM CPU   2 60GHz     Cores  8     Current Clock Speed  2600     Max Clock Speed  2600 -------------------- Parsing hexadecimal string into an array of bytes -------------------- HexadecimalStringToByteArray Original  7 777 09 average ticks  over 10000 runs   1 2X HexadecimalStringToByteArray BestEffort  8 550 82 average ticks  over 10000 runs   1 1X HexadecimalStringToByteArray Rev4  9 218 03 average ticks  over 10000 runs   1 0X   Yes   Props to Partridge for the bench framework  it s easy to hack  The input used is the following SHA-1 hash repeated 5000 times to make a 100 000 bytes long string   209113288F93A9AB8E474EA78D899AFDBB874355   Have fun   But optimize with moderation

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Complement to answer by  CodesInChaos  reversed method   public static byte   HexToByteUsingByteManipulation string s        byte   bytes   new byte s Length   2       for  int i   0  i  lt  bytes Length  i                  int hi   s i 2  - 65          hi   hi   10     hi  gt  gt  31   amp  7            int lo   s i 2   1  - 65          lo   lo   10     lo  gt  gt  31   amp  7   amp  0x0f           bytes i     byte   lo   hi  lt  lt  4             return bytes      Explanation    amp  0x0f is to support  also lower case letters  hi   hi   10     hi  gt  gt  31   amp  7   is the same as   hi   ch-65   10      ch-65   gt  gt  31   amp  7    For  0    9  it is the same as hi   ch - 65   10   7  which is hi   ch - 48  this is because of 0xffffffff  amp  7    For  A    F  it is hi   ch - 65   10   this is because of 0x00000000  amp  7    For  a    f  we have to big numbers so we must subtract 32 from default version by making some bits 0 by using  amp  0x0f   65 is code for  A   48 is code for  0   7 is the number of letters between  9  and  A  in the ASCII table     456789   lt   gt   ABCD

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Another fast function     private static readonly byte   HexNibble   new byte         0x0  0x1  0x2  0x3  0x4  0x5  0x6  0x7      0x8  0x9  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0xA  0xB  0xC  0xD  0xE  0xF  0x0      0x0  0x0  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0x0  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0x0  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0xA  0xB  0xC  0xD  0xE  0xF     public static byte   HexStringToByteArray  string str         int byteCount   str Length  gt  gt  1      byte   result   new byte byteCount    str Length  amp  1        for  int i   0  i  lt  byteCount  i             result i     byte   HexNibble str i  lt  lt  1  - 48   lt  lt  4   HexNibble str  i  lt  lt  1    1  - 48        if   str Length  amp  1     0           result byteCount     byte  HexNibble str str Length - 1  - 48       return result

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There is a simple one-liner solution not yet mentioned that will convert hex strings into byte arrays  we don t care about negative interpretation here as it does not matter    BigInteger Parse str  System Globalization NumberStyles HexNumber  ToByteArray   Reverse   ToArray

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Combined a few answers into a class for my later copy and paste convenience       lt summary gt      Extension methods to quickly convert byte array to string and back       lt  summary gt  public static class HexConverter            lt summary gt          Map values to hex digits          lt  summary gt      private static readonly char   HexDigits                          0    1    2    3    4    5    6    7    8    9    A    B    C    D    E    F                       lt summary gt          Map 56 characters between   0    F   to their hex equivalents  and set invalid characters         such that they will overflow byte to fail conversion           lt  summary gt      private static readonly ushort   HexValues                         0x0000  0x0001  0x0002  0x0003  0x0004  0x0005  0x0006  0x0007  0x0008  0x0009  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100              0x000A  0x000B  0x000C  0x000D  0x000E  0x000F  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100              0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x0100  0x000A  0x000B              0x000C  0x000D  0x000E  0x000F                      lt summary gt          Empty byte array           lt  summary gt      private static readonly byte   Empty   new byte 0             lt summary gt          Convert a byte array to a hexadecimal string           lt  summary gt           lt param name  quot bytes quot  gt          The input byte array           lt  param gt           lt returns gt          A string of hexadecimal digits           lt  returns gt      public static string ToHexString this byte   bytes                var c   new char bytes Length   2           for  int i   0  j   0  i  lt  bytes Length  i                          c j      HexDigits bytes i   gt  gt  4               c j      HexDigits bytes i   amp  0x0F                      return new string c                   lt summary gt          Parse a string of hexadecimal digits into a byte array           lt  summary gt           lt param name  quot hexadecimalString quot  gt          The hexadecimal string           lt  param gt           lt returns gt          The parsed  lt see cref  quot byte   quot   gt  array           lt  returns gt           lt exception cref  quot ArgumentException quot  gt          The input string either contained invalid characters  or was of an odd length           lt  exception gt      public static byte   ToByteArray string hexadecimalString                if   TryParse hexadecimalString  out var value                         throw new ArgumentException  quot Invalid hexadecimal string quot   nameof hexadecimalString                       return value                  lt summary gt          Parse a hexadecimal string to bytes          lt  summary gt           lt param name  quot hexadecimalString quot  gt          The hexadecimal string  which must be an even number of characters           lt  param gt           lt param name  quot value quot  gt          The parsed value if successful           lt  param gt           lt returns gt          True if successful           lt  returns gt      public static bool TryParse string hexadecimalString  out byte   value                if  hexadecimalString Length    0                        value   Empty              return true                     if  hexadecimalString Length   2    0                        value   Empty              return false                     try                        value   new byte hexadecimalString Length   2               for  int i   0  j   0  j  lt  hexadecimalString Length  i                                  value i     byte   HexValues hexadecimalString j    -  0    lt  lt  4                                      HexValues hexadecimalString j    -  0                                return true                    catch  OverflowException                        value   Empty              return false

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Shortest way and  net core supported       public static string BytesToString byte   ba    gt          ba Aggregate new StringBuilder 32    sb  b    gt  sb Append b ToString  X2     ToString

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static string ByteArrayToHexViaLookupPerByte2 byte   bytes                            var result3   new uint bytes Length           for  int i   0  i  lt  bytes Length  i                    result3 i     Lookup32 bytes i            var handle   GCHandle Alloc result3  GCHandleType Pinned           try                           var result   Marshal PtrToStringUni handle AddrOfPinnedObject    bytes Length   2                   return result                    finally                           handle Free                  This functions in my tests is always the second entry after the unsafe implementation   Unfortunately  the test bench is not so reliable    if you run it multiple times the list got shuffled so much that who knows after the unsafe which is really the fastest  It doesn t take into a account pre-warming  jit compilation time  and GC performance hits  I would like to have rewritten it to have more information  but I didn t had really the time for it

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Another lookup table based approach  This one uses only one lookup table for each byte  instead of a lookup table per nibble   private static readonly uint    lookup32   CreateLookup32     private static uint   CreateLookup32         var result   new uint 256       for  int i   0  i  lt  256  i                  string s i ToString  X2            result i      uint s 0       uint s 1   lt  lt  16             return result     private static string ByteArrayToHexViaLookup32 byte   bytes        var lookup32    lookup32      var result   new char bytes Length   2       for  int i   0  i  lt  bytes Length  i                  var val   lookup32 bytes i            result 2 i     char val          result 2 i   1     char   val  gt  gt  16             return new string result       I also tested variants of this using ushort  struct char X1  X2   struct byte X1  X2  in the lookup table   Depending on the compilation target  x86  X64  those either had the approximately same performance or were slightly slower than this variant     And for even higher performance  its unsafe sibling   private static readonly uint    lookup32Unsafe   CreateLookup32Unsafe    private static readonly uint   lookup32UnsafeP    uint  GCHandle Alloc  lookup32Unsafe GCHandleType Pinned  AddrOfPinnedObject     private static uint   CreateLookup32Unsafe         var result   new uint 256       for  int i   0  i  lt  256  i                  string s i ToString  X2            if BitConverter IsLittleEndian              result i      uint s 0       uint s 1   lt  lt  16           else             result i      uint s 1       uint s 0   lt  lt  16             return result     public static string ByteArrayToHexViaLookup32Unsafe byte   bytes        var lookupP    lookup32UnsafeP      var result   new char bytes Length   2       fixed byte  bytesP   bytes      fixed  char  resultP   result                uint  resultP2    uint  resultP          for  int i   0  i  lt  bytes Length  i                          resultP2 i    lookupP bytesP i                        return new string result       Or if you consider it acceptable to write into the string directly   public static string ByteArrayToHexViaLookup32UnsafeDirect byte   bytes        var lookupP    lookup32UnsafeP      var result   new string  char 0  bytes Length   2       fixed  byte  bytesP   bytes      fixed  char  resultP   result                uint  resultP2    uint  resultP          for  int i   0  i  lt  bytes Length  i                          resultP2 i    lookupP bytesP i                        return result

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For performance I would go with drphrozens solution  A tiny optimization for the decoder could be to use a table for either char to get rid of the   lt  lt  4    Clearly the two method calls are costly  If some kind of check is made either on input or output data  could be CRC  checksum or whatever  the if  b    255     could be skipped and thereby also the method calls altogether   Using offset   and offset instead of offset and offset   1 might give some theoretical benefit but I suspect the compiler handles this better than me   private static readonly byte   LookupTableLow   new byte       0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07  0x08  0x09  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0x0A  0x0B  0x0C  0x0D  0x0E  0x0F  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     private static readonly byte   LookupTableHigh   new byte       0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0x00  0x10  0x20  0x30  0x40  0x50  0x60  0x70  0x80  0x90  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xA0  0xB0  0xC0  0xD0  0xE0  0xF0  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xA0  0xB0  0xC0  0xD0  0xE0  0xF0  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF    0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF  0xFF     private static byte LookupLow char c      var b   LookupTableLow c     if  b    255      throw new IOException  Expected a hex character  got     c     return b     private static byte LookupHigh char c      var b   LookupTableHigh c     if  b    255      throw new IOException  Expected a hex character  got     c     return b     public static byte ToByte char   chars  int offset      return  byte  LookupHigh chars offset       LookupLow chars offset         This is just off the top of my head and has not been tested or benchmarked

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a safe version of the lookup solution              public static string ByteArrayToHexViaLookup32Safe byte   bytes  bool withZeroX                if  bytes Length    0                        return withZeroX    0x                           int length   bytes Length   2    withZeroX   2   0           StateSmall stateToPass   new StateSmall bytes  withZeroX           return string Create length  stateToPass   chars  state    gt                        int offset0x   0              if  state WithZeroX                                chars 0     0                   chars 1     x                   offset0x    2                             Span lt uint gt  charsAsInts   MemoryMarshal Cast lt char  uint gt  chars Slice offset0x                int targetLength   state Bytes Length              for  int i   0  i  lt  targetLength  i    1                                uint val   Lookup32 state Bytes i                    charsAsInts i    val                                       private struct StateSmall               public StateSmall byte   bytes  bool withZeroX                        Bytes   bytes              WithZeroX   withZeroX                     public byte   Bytes          public bool WithZeroX

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I just encountered the very same problem today  and I came across this code   private static string ByteArrayToHex byte   barray        char   c   new char barray Length   2       byte b      for  int i   0  i  lt  barray Length    i                b     byte  barray i   gt  gt  4            c i   2     char  b  gt  9   b   0x37   b   0x30           b     byte  barray i   amp  0xF            c i   2   1     char  b  gt  9   b   0x37   b   0x30             return new string c       Source  Forum post byte   Array to Hex String  see the post by PZahra   I modified the code a little to remove the 0x prefix   I did some performance testing to the code and it was almost eight times faster than using BitConverter ToString    the fastest according to patridge s post

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Not optimized for speed  but more LINQy than most answers   NET 4 0     lt Extension   gt  Public Function FromHexToByteArray hex As String  As Byte       hex   If hex  String Empty      If hex Length Mod 2   1 Then hex    0   amp  hex     Return Enumerable Range 0  hex Length   2  Select Function i  Convert ToByte hex Substring i   2  2   16   ToArray End Function   lt Extension   gt  Public Function ToHexString bytes As IEnumerable Of Byte   As String     Return String Concat bytes Select Function b  b ToString  X2     End Function

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When writing crypto code it s common to avoid data dependent branches and table lookups to ensure the runtime doesn t depend on the data  since data dependent timing can lead to side-channel attacks   It s also pretty fast   static string ByteToHexBitFiddle byte   bytes        char   c   new char bytes Length   2       int b      for  int i   0  i  lt  bytes Length  i              b   bytes i   gt  gt  4          c i   2     char  55   b      b-10  gt  gt 31  amp -7            b   bytes i   amp  0xF          c i   2   1     char  55   b      b-10  gt  gt 31  amp -7              return new string c       Ph nglui mglw nafh Cthulhu R lyeh wgah nagl fhtagn       Abandon all hope  ye who enter here   An explanation of the weird bit fiddling    bytes i   gt  gt  4 extracts the high nibble of a byte bytes i   amp  0xF extracts the low nibble of a byte b - 10 is  lt  0 for values b  lt  10  which will become a decimal digit is  gt   0 for values b  gt  10  which will become a letter from A to F  Using i  gt  gt  31 on a signed 32 bit integer extracts the sign  thanks to sign extension  It will be -1 for i  lt  0 and 0 for i  gt   0  Combining 2  and 3   shows that  b-10  gt  gt 31 will be 0 for letters and -1 for digits  Looking at the case for letters  the last summand becomes 0  and b is in the range 10 to 15  We want to map it to A 65  to F 70   which implies adding 55   A -10   Looking at the case for digits  we want to adapt the last summand so it maps b from the range 0 to 9 to the range 0 48  to 9 57   This means it needs to become -7   0  - 55   Now we could just multiply with 7  But since -1 is represented by all bits being 1  we can instead use  amp  -7 since  0  amp  -7     0 and  -1  amp  -7     -7    Some further considerations    I didn t use a second loop variable to index into c  since measurement shows that calculating it from i is cheaper   Using exactly i  lt  bytes Length as upper bound of the loop allows the JITter to eliminate bounds checks on bytes i   so I chose that variant  Making b an int allows unnecessary conversions from and to byte

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Another way is by using stackalloc to reduce GC memory pressure   static string ByteToHexBitFiddle byte   bytes            var c   stackalloc char bytes Length   2   1           int b           for  int i   0  i  lt  bytes Length    i                        b   bytes i   gt  gt  4              c i   2     char  55   b      b - 10   gt  gt  31   amp  -7                b   bytes i   amp  0xF              c i   2   1     char  55   b      b - 10   gt  gt  31   amp  -7                      c bytes Length   2       0           return new string c

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I guess its speed is worth 16 extra bytes       static char   hexes   new char    0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F        public static string ToHexadecimal  this byte   Bytes                char   Result   new char Bytes Length  lt  lt  1           int Offset   0          for  int i   0  i    Bytes Length  i                  Result Offset      hexes Bytes i   gt  gt  4               Result Offset      hexes Bytes i   amp  0x0F                     return new string Result

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Either   public static string ByteArrayToString byte   ba      StringBuilder hex   new StringBuilder ba Length   2     foreach  byte b in ba      hex AppendFormat   0 x2    b     return hex ToString        or   public static string ByteArrayToString byte   ba      return BitConverter ToString ba  Replace  -           There are even more variants of doing it  for example here   The reverse conversion would go like this   public static byte   StringToByteArray String hex      int NumberChars   hex Length    byte   bytes   new byte NumberChars   2     for  int i   0  i  lt  NumberChars  i    2      bytes i   2    Convert ToByte hex Substring i  2   16     return bytes        Using Substring is the best option in combination with Convert ToByte  See this answer for more information  If you need better performance  you must avoid Convert ToByte before you can drop SubString

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The following expands the excellent answer here by allowing native lower case option as well  and also handles null or empty input and makes this an extension method            lt summary gt          Converts the byte array to a hex string very fast  Excellent job         with code lightly adapted from  community wiki  here  https   stackoverflow com a 14333437 264031          the function was originally named  ByteToHexBitFiddle   Now allows a native lowerCase option         to be input and allows null or empty inputs  null returns null  empty returns empty            lt  summary gt      public static string ToHexString this byte   bytes  bool lowerCase   false                if  bytes    null              return null          else if  bytes Length    0              return              char   c   new char bytes Length   2            int b          int xAddToAlpha   lowerCase   87   55          int xAddToDigit   lowerCase   -39   -7           for  int i   0  i  lt  bytes Length  i                   b   bytes i   gt  gt  4              c i   2     char  xAddToAlpha   b      b - 10   gt  gt  31   amp  xAddToDigit                 b   bytes i   amp  0xF              c i   2   1     char  xAddToAlpha   b      b - 10   gt  gt  31   amp  xAddToDigit                       string val   new string c           return val             public static string ToHexString this IEnumerable lt byte gt  bytes  bool lowerCase   false                if  bytes    null              return null          byte   arr   bytes ToArray            return arr ToHexString lowerCase

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I suspect the speed of this will knock the socks off most of the other tests     Public Function BufToHex ByVal buf   As Byte  As String     Dim sB As New System Text StringBuilder     For i As Integer   0 To buf Length - 1         sB Append buf i  ToString  x2        Next i     Return sB ToString End Function

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This is a great post  I like Waleed s solution  I haven t run it through patridge s test but it seems to be quite fast  I also needed the reverse process  converting a hex string to a byte array  so I wrote it as a reversal of Waleed s solution  Not sure if it s any faster than Tomalak s original solution  Again  I did not run the reverse process through patridge s test either   private byte   HexStringToByteArray string hexString        int hexStringLength   hexString Length      byte   b   new byte hexStringLength   2       for  int i   0  i  lt  hexStringLength  i    2                int topChar    hexString i   gt  0x40   hexString i  - 0x37   hexString i  - 0x30   lt  lt  4          int bottomChar   hexString i   1   gt  0x40   hexString i   1  - 0x37   hexString i   1  - 0x30          b i   2    Convert ToByte topChar   bottomChar             return b

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From Microsoft s developers  a nice  simple conversion   public static string ByteArrayToString byte   ba            Concatenate the bytes into one long string     return ba Aggregate new StringBuilder 32                                sb  b    gt  sb Append b ToString  X2                                  ToString        While the above is clean and compact  performance junkies will scream about it using enumerators  You can get peak performance with an improved version of Tomalak s original answer   public static string ByteArrayToString byte   ba             StringBuilder hex   new StringBuilder ba Length   2          for int i 0  i  lt  ba Length  i              lt -- Use for loop is faster than foreach           hex Append ba i  ToString  X2           lt -- ToString is faster than AppendFormat        return hex ToString            This is the fastest of all the routines I ve seen posted here so far  Don t just take my word for it    performance test each routine and inspect its CIL code for yourself

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I ll enter this bit fiddling competition as I have an answer that also uses bit-fiddling to decode hexadecimals  Note that using character arrays may be even faster as calling StringBuilder methods will take time as well   public static String ToHex  byte   data        int dataLength   data Length         pre-create the stringbuilder using the length of the data   2  precisely enough     StringBuilder sb   new StringBuilder  dataLength   2       for  int i   0  i  lt  dataLength  i              int b   data  i               check using calculation over bits to see if first tuple is a letter            isLetter is zero if it is a digit  1 if it is a letter         int isLetter    b  gt  gt  7   amp    b  gt  gt  6     b  gt  gt  5    amp  1              calculate the code using a multiplication to make up the difference between            a digit character and an alphanumerical character         int code    0      b  gt  gt  4   amp  0xF    isLetter     A  -  9  - 1              now append the result  after casting the code point to a character         sb Append   Char code               do the same with the lower  less significant  tuple         isLetter    b  gt  gt  3   amp    b  gt  gt  2     b  gt  gt  1    amp  1          code    0     b  amp  0xF    isLetter     A  -  9  - 1           sb Append   Char code             return sb ToString        public static byte   FromHex  String hex            pre-create the array     int resultLength   hex Length   2      byte   result   new byte resultLength          set validity   0  0   valid  anything else is not valid      int validity   0      int c  isLetter  value  validDigitStruct  validDigit  validLetterStruct  validLetter      for  int i   0  hexOffset   0  i  lt  resultLength  i    hexOffset    2            c   hex  hexOffset               check using calculation over bits to see if first char is a letter            isLetter is zero if it is a digit  1 if it is a letter  upper  amp  lowercase          isLetter    c  gt  gt  6   amp  1              calculate the tuple value using a multiplication to make up the difference between            a digit character and an alphanumerical character            minus 1 for the fact that the letters are not zero based         value     c  amp  0xF    isLetter    -1   10    lt  lt  4              check validity of all the other bits         validity    c  gt  gt  7     changed to  gt  gt   maybe not OK  use UInt           validDigitStruct    c  amp  0x30    0x30          validDigit     c  amp  0x8   gt  gt  3     c  amp  0x6           validity     isLetter   1     validDigitStruct   validDigit            validLetterStruct   c  amp  0x18          validLetter      c - 1   amp  0x4   gt  gt  2      c - 1   amp  0x2           validity    isLetter    validLetterStruct   validLetter               do the same with the lower  less significant  tuple         c   hex  hexOffset   1           isLetter    c  gt  gt  6   amp  1          value     c  amp  0xF    isLetter    -1   10           result  i     byte value              check validity of all the other bits         validity    c  gt  gt  7     changed to  gt  gt   maybe not OK  use UInt           validDigitStruct    c  amp  0x30    0x30          validDigit     c  amp  0x8   gt  gt  3     c  amp  0x6           validity     isLetter   1     validDigitStruct   validDigit            validLetterStruct   c  amp  0x18          validLetter      c - 1   amp  0x4   gt  gt  2      c - 1   amp  0x2           validity    isLetter    validLetterStruct   validLetter              if  validity    0            throw new ArgumentException   Hexadecimal encoding incorrect for input     hex              return result      Converted from Java code

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Fastest method for old school people    miss you pointers     static public byte   HexStrToByteArray string str                byte   res   new byte  str Length   2    0   0   str Length   2      check and allocate memory         for  int i   0  j   0  j  lt  res Length  i    2  j      convert loop             res j     byte   str i    32   9    25   16    str i   1    32   9    25           return res

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Here s my shot at it  I ve created a pair of extension classes to extend string and byte  On the large file test  the performance is comparable to Byte Manipulation 2   The code below for ToHexString is an optimized implementation of the lookup and shift algorithm  It is almost identical to the one by Behrooz  but it turns out using a foreach to iterate and a counter is faster than an explicitly indexing for   It comes in 2nd place behind Byte Manipulation 2 on my machine and is very readable code  The following test results are also of interest   ToHexStringCharArrayWithCharArrayLookup  41 589 69 average ticks  over 1000 runs   1 5X ToHexStringCharArrayWithStringLookup  50 764 06 average ticks  over 1000 runs   1 2X ToHexStringStringBuilderWithCharArrayLookup  62 812 87 average ticks  over 1000 runs   1 0X  Based on the above results it seems safe to conclude that    The penalties for indexing into a string to perform the lookup vs  a char array are significant in the large file test  The penalties for using a StringBuilder of known capacity vs  a char array of known size to create the string are even more significant    Here s the code   using System   namespace ConversionExtensions       public static class ByteArrayExtensions               private readonly static char   digits   new char      0    1    2    3    4    5    6    7    8    9    A    B    C    D    E    F              public static string ToHexString this byte   bytes                        char   hex   new char bytes Length   2               int index   0               foreach  byte b in bytes                                hex index      digits b  gt  gt  4                   hex index      digits b  amp  0x0F                              return new string hex                       using System  using System IO   namespace ConversionExtensions       public static class StringExtensions               public static byte   ToBytes this string hexString                        if   string IsNullOrEmpty hexString   amp  amp  hexString Length   2    0                                throw new FormatException  Hexadecimal string must not be empty and must contain an even number of digits to be valid                                hexString   hexString ToUpperInvariant                byte   data   new byte hexString Length   2                for  int index   0  index  lt  hexString Length  index    2                                int highDigitValue   hexString index   lt    9    hexString index  -  0    hexString index  -  A    10                  int lowDigitValue   hexString index   1   lt    9    hexString index   1  -  0    hexString index   1  -  A    10                   if  highDigitValue  lt  0    lowDigitValue  lt  0    highDigitValue  gt  15    lowDigitValue  gt  15                                        throw new FormatException  An invalid digit was encountered  Valid hexadecimal digits are 0-9 and A-F                                       else                                       byte value    byte   highDigitValue  lt  lt  4     lowDigitValue  amp  0x0F                        data index   2    value                                               return data                      Below are the test results that I got when I put my code in  patridge s testing project on my machine  I also added a test for converting to a byte array from hexadecimal  The test runs that exercised my code are ByteArrayToHexViaOptimizedLookupAndShift and HexToByteArrayViaByteManipulation  The HexToByteArrayViaConvertToByte was taken from XXXX  The HexToByteArrayViaSoapHexBinary is the one from  Mykroft s answer      Intel Pentium III Xeon processor      Cores  4  lt br  gt      Current Clock Speed  1576  lt br  gt      Max Clock Speed  3092  lt br  gt              Converting array of bytes into hexadecimal string representation            ByteArrayToHexViaByteManipulation2  39 366 64 average ticks  over 1000 runs   22 4X      ByteArrayToHexViaOptimizedLookupAndShift  41 588 64 average ticks    over 1000 runs   21 2X      ByteArrayToHexViaLookup  55 509 56 average ticks  over 1000 runs   15 9X      ByteArrayToHexViaByteManipulation  65 349 12 average ticks  over 1000 runs   13 5X      ByteArrayToHexViaLookupAndShift  86 926 87 average ticks  over 1000   runs   10 2X      ByteArrayToHexStringViaBitConverter  139 353 73 average   ticks  over 1000 runs  6 3X      ByteArrayToHexViaSoapHexBinary  314 598 77 average ticks  over 1000 runs   2 8X      ByteArrayToHexStringViaStringBuilderForEachByteToString  344 264 63   average ticks  over 1000 runs   2 6X      ByteArrayToHexStringViaStringBuilderAggregateByteToString  382 623 44   average ticks  over 1000 runs   2 3X      ByteArrayToHexStringViaStringBuilderForEachAppendFormat  818 111 95   average ticks  over 1000 runs   1 1X      ByteArrayToHexStringViaStringConcatArrayConvertAll  839 244 84 average   ticks  over 1000 runs   1 1X      ByteArrayToHexStringViaStringBuilderAggregateAppendFormat  867 303 98   average ticks  over 1000 runs   1 0X      ByteArrayToHexStringViaStringJoinArrayConvertAll  882 710 28 average   ticks  over 1000 runs   1 0X

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I suspect the speed of this will knock the socks off most of the other tests     Public Function BufToHex ByVal buf   As Byte  As String     Dim sB As New System Text StringBuilder     For i As Integer   0 To buf Length - 1         sB Append buf i  ToString  x2        Next i     Return sB ToString End Function

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I came up with a different code that is tolerant to extra characters  whitespace  dash      It is mostly inspired from some acceptably-fast answers here  It allows parsing of the following  file   00-aa-84-fb 12 32 FF CD 12 00 12 32 FF CD 1200d5e68a        lt summary gt Reads a hex string into bytes lt  summary gt  public static IEnumerable lt byte gt  HexadecimalStringToBytes string hex        if  hex    null          throw new ArgumentNullException nameof hex         char c  c1   default char       bool hasc1   false      unchecked             for  int i   0  i  lt  hex Length  i                  c   hex i               bool isValid    A   lt   c  amp  amp  c  lt    f      a   lt   c  amp  amp  c  lt    f      0   lt   c  amp  amp  c  lt    9               if   hasc1                    if  isValid                        hasc1   true                                  else                   hasc1   false                  if  isValid                        yield return  byte   GetHexVal c1   lt  lt  4    GetHexVal c                                                 c1   c                           lt summary gt Reads a hex string into a byte array lt  summary gt  public static byte   HexadecimalStringToByteArray string hex        if  hex    null          throw new ArgumentNullException nameof hex         var bytes   new List lt byte gt  hex Length   2       foreach  var item in HexadecimalStringToBytes hex             bytes Add item              return bytes ToArray       private static byte GetHexVal char val        return  byte  val -  val  lt  0x3A   0x30   val  lt  0x5B   0x37   0x57                                                                                                       digits 0-9       upper char A-Z     a-z     Please refer to full code when copying  Unit tests included   Some might say it is too much tolerant to extra chars  So don t rely on this code to perform validation  or change it

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This is an answer to revision 4 of Tomalak s highly popular answer  and subsequent edits    I ll make the case that this edit is wrong  and explain why it could be reverted  Along the way  you might learn a thing or two about some internals  and see yet another example of what premature optimization really is and how it can bite you   tl dr  Just use Convert ToByte and String Substring if you re in a hurry   Original code  below   it s the best combination if you don t want to re-implement Convert ToByte  Use something more advanced  see other answers  that doesn t use Convert ToByte if you need performance  Do not use anything else other than String Substring in combination with Convert ToByte  unless someone has something interesting to say about this in the comments of this answer   warning  This answer may become obsolete if a Convert ToByte char    Int32  overload is implemented in the framework  This is unlikely to happen soon   As a general rule  I don t much like to say  don t optimize prematurely   because nobody knows when  premature  is  The only thing you must consider when deciding whether to optimize or not is   Do I have the time and resources to investigate optimization approaches properly    If you don t  then it s too soon  wait until your project is more mature or until you need the performance  if there is a real need  then you will make the time   In the meantime  do the simplest thing that could possibly work instead   Original code       public static byte   HexadecimalStringToByteArray Original string input                var outputLength   input Length   2          var output   new byte outputLength           for  var i   0  i  lt  outputLength  i                output i    Convert ToByte input Substring i   2  2   16           return output          Revision 4       public static byte   HexadecimalStringToByteArray Rev4 string input                var outputLength   input Length   2          var output   new byte outputLength           using  var sr   new StringReader input                         for  var i   0  i  lt  outputLength  i                    output i    Convert ToByte new string new char 2     char sr Read     char sr Read       16                     return output          The revision avoids String Substring and uses a StringReader instead  The given reason is      Edit  you can improve performance for long strings by using a single   pass parser  like so    Well  looking at the reference code for String Substring  it s clearly  single-pass  already  and why shouldn t it be  It operates at byte-level  not on surrogate pairs   It does allocate a new string however  but then you need to allocate one to pass to Convert ToByte anyway  Furthermore  the solution provided in the revision allocates yet another object on every iteration  the two-char array   you can safely put that allocation outside the loop and reuse the array to avoid that       public static byte   HexadecimalStringToByteArray string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           using  var sr   new StringReader input                         for  var i   0  i  lt  outputLength  i                                  numeral 0     char sr Read                    numeral 1     char sr Read                    output i    Convert ToByte new string numeral   16                                   return output          Each hexadecimal numeral represents a single octet using two digits  symbols    But then  why call StringReader Read twice  Just call its second overload and ask it to read two characters in the two-char array at once  and reduce the amount of calls by two       public static byte   HexadecimalStringToByteArray string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           using  var sr   new StringReader input                         for  var i   0  i  lt  outputLength  i                                  var read   sr Read numeral  0  2                   Debug Assert read    2                   output i    Convert ToByte new string numeral   16                                   return output          What you re left with is a string reader whose only added  value  is a parallel index  internal  pos  which you could have declared yourself  as j for example   a redundant length variable  internal  length   and a redundant reference to the input string  internal  s   In other words  it s useless   If you wonder how Read  reads   just look at the code  all it does is call String CopyTo on the input string  The rest is just book-keeping overhead to maintain values we don t need   So  remove the string reader already  and call CopyTo yourself  it s simpler  clearer  and more efficient       public static byte   HexadecimalStringToByteArray string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           for  int i   0  j   0  i  lt  outputLength  i    j    2                        input CopyTo j  numeral  0  2               output i    Convert ToByte new string numeral   16                     return output          Do you really need a j index that increments in steps of two parallel to i  Of course not  just multiply i by two  which the compiler should be able to optimize to an addition        public static byte   HexadecimalStringToByteArray BestEffort string input                var outputLength   input Length   2          var output   new byte outputLength           var numeral   new char 2           for  int i   0  i  lt  outputLength  i                          input CopyTo i   2  numeral  0  2               output i    Convert ToByte new string numeral   16                     return output          What does the solution look like now  Exactly like it was at the beginning  only instead of using String Substring to allocate the string and copy the data to it  you re using an intermediary array to which you copy the hexadecimal numerals to  then allocate the string yourself and copy the data again from the array and into the string  when you pass it in the string constructor   The second copy might be optimized-out if the string is already in the intern pool  but then String Substring will also be able to avoid it in these cases   In fact  if you look at String Substring again  you see that it uses some low-level internal knowledge of how strings are constructed to allocate the string faster than you could normally do it  and it inlines the same code used by CopyTo directly in there to avoid the call overhead   String Substring   Worst-case  One fast allocation  one fast copy  Best-case  No allocation  no copy    Manual method   Worst-case  Two normal allocations  one normal copy  one fast copy  Best-case  One normal allocation  one normal copy    Conclusion  If you want to use Convert ToByte String  Int32   because you don t want to re-implement that functionality yourself   there doesn t seem to be a way to beat String Substring  all you do is run in circles  re-inventing the wheel  only with sub-optimal materials    Note that using Convert ToByte and String Substring is a perfectly valid choice if you don t need extreme performance  Remember  only opt for an alternative if you have the time and resources to investigate how it works properly   If there was a Convert ToByte char    Int32   things would be different of course  it would be possible to do what I described above and completely avoid String    I suspect that people who report better performance by  avoiding String Substring  also avoid Convert ToByte String  Int32   which you should really be doing if you need the performance anyway  Look at the countless other answers to discover all the different approaches to do that   Disclaimer  I haven t decompiled the latest version of the framework to verify that the reference source is up-to-date  I assume it is   Now  it all sounds good and logical  hopefully even obvious if you ve managed to get so far   But is it true   Intel R  Core TM  i7-3720QM CPU   2 60GHz     Cores  8     Current Clock Speed  2600     Max Clock Speed  2600 -------------------- Parsing hexadecimal string into an array of bytes -------------------- HexadecimalStringToByteArray Original  7 777 09 average ticks  over 10000 runs   1 2X HexadecimalStringToByteArray BestEffort  8 550 82 average ticks  over 10000 runs   1 1X HexadecimalStringToByteArray Rev4  9 218 03 average ticks  over 10000 runs   1 0X   Yes   Props to Partridge for the bench framework  it s easy to hack  The input used is the following SHA-1 hash repeated 5000 times to make a 100 000 bytes long string   209113288F93A9AB8E474EA78D899AFDBB874355   Have fun   But optimize with moderation

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With Java 8   we ca use Byte toUnsignedInt  public static String convertBytesToHex byte   bytes        StringBuilder result   new StringBuilder        for  byte byt   bytes            int decimal   Byte toUnsignedInt byt           String hex   Integer toHexString decimal           result append hex             return result toString

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I just encountered the very same problem today  and I came across this code   private static string ByteArrayToHex byte   barray        char   c   new char barray Length   2       byte b      for  int i   0  i  lt  barray Length    i                b     byte  barray i   gt  gt  4            c i   2     char  b  gt  9   b   0x37   b   0x30           b     byte  barray i   amp  0xF            c i   2   1     char  b  gt  9   b   0x37   b   0x30             return new string c       Source  Forum post byte   Array to Hex String  see the post by PZahra   I modified the code a little to remove the 0x prefix   I did some performance testing to the code and it was almost eight times faster than using BitConverter ToString    the fastest according to patridge s post

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Complement to answer by  CodesInChaos  reversed method   public static byte   HexToByteUsingByteManipulation string s        byte   bytes   new byte s Length   2       for  int i   0  i  lt  bytes Length  i                  int hi   s i 2  - 65          hi   hi   10     hi  gt  gt  31   amp  7            int lo   s i 2   1  - 65          lo   lo   10     lo  gt  gt  31   amp  7   amp  0x0f           bytes i     byte   lo   hi  lt  lt  4             return bytes      Explanation    amp  0x0f is to support  also lower case letters  hi   hi   10     hi  gt  gt  31   amp  7   is the same as   hi   ch-65   10      ch-65   gt  gt  31   amp  7    For  0    9  it is the same as hi   ch - 65   10   7  which is hi   ch - 48  this is because of 0xffffffff  amp  7    For  A    F  it is hi   ch - 65   10   this is because of 0x00000000  amp  7    For  a    f  we have to big numbers so we must subtract 32 from default version by making some bits 0 by using  amp  0x0f   65 is code for  A   48 is code for  0   7 is the number of letters between  9  and  A  in the ASCII table     456789   lt   gt   ABCD

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If performance matters  here s an optimized solution       static readonly char    hexDigits    0123456789abcdef  ToCharArray        public static string ToHexString this byte   bytes                char   digits   new char bytes Length   2           for  int i   0  i  lt  bytes Length  i                          int d1  d2              d1   Math DivRem bytes i   16  out d2               digits 2   i     hexDigits d1               digits 2   i   1     hexDigits d2                     return new string digits           It s about 2 5 times faster that BitConverter ToString  and about 7 times faster that BitConverter ToString   removal of the  -  chars

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Performance Analysis  Note  new leader as of 2015-08-20   I ran each of the various conversion methods through some crude Stopwatch performance testing  a run with a random sentence  n 61  1000 iterations  and a run with a Project Gutenburg text  n 1 238 957  150 iterations   Here are the results  roughly from fastest to slowest  All measurements are in ticks  10 000 ticks   1 ms  and all relative notes are compared to the  slowest  StringBuilder implementation  For the code used  see below or the test framework repo where I now maintain the code for running this   Disclaimer  WARNING  Do not rely on these stats for anything concrete  they are simply a sample run of sample data  If you really need top-notch performance  please test these methods in an environment representative of your production needs with data representative of what you will use   Results   Lookup by byte unsafe  via CodesInChaos   added to test repo by airbreather    Text  4 727 85  105 2X  Sentence  0 28  99 7X   Lookup by byte  via CodesInChaos    Text  10 853 96  45 8X faster  Sentence  0 65  42 7X faster   Byte Manipulation 2  via CodesInChaos    Text  12 967 69  38 4X faster  Sentence  0 73  37 9X faster   Byte Manipulation  via Waleed Eissa    Text  16 856 64  29 5X faster  Sentence  0 70  39 5X faster   Lookup Shift  via Nathan Moinvaziri    Text  23 201 23  21 4X faster  Sentence  1 24  22 3X faster   Lookup by nibble  via Brian Lambert    Text  23 879 41  20 8X faster  Sentence  1 15  23 9X faster   BitConverter  via Tomalak    Text  113 269 34  4 4X faster  Sentence  9 98  2 8X faster    SoapHexBinary  ToString  via Mykroft    Text  178 601 39  2 8X faster  Sentence  10 68  2 6X faster    byte  ToString  X2    using foreach   derived from Will Dean s answer    Text  308 805 38  2 4X faster  Sentence  16 89  2 4X faster    byte  ToString  X2    using  IEnumerable  Aggregate  requires System Linq   via Mark    Text  352 828 20  2 1X faster  Sentence  16 87  2 4X faster   Array ConvertAll  using string Join   via Will Dean    Text  675 451 57  1 1X faster  Sentence  17 95  2 2X faster   Array ConvertAll  using string Concat  requires  NET 4 0   via Will Dean    Text  752 078 70  1 0X faster  Sentence  18 28  2 2X faster    StringBuilder  AppendFormat  using foreach   via Tomalak    Text  672 115 77  1 1X faster  Sentence  36 82  1 1X faster    StringBuilder  AppendFormat  using  IEnumerable  Aggregate  requires System Linq   derived from Tomalak s answer    Text  718 380 63  1 0X faster  Sentence  39 71  1 0X faster     Lookup tables have taken the lead over byte manipulation  Basically  there is some form of precomputing what any given nibble or byte will be in hex  Then  as you rip through the data  you simply look up the next portion to see what hex string it would be  That value is then added to the resulting string output in some fashion  For a long time byte manipulation  potentially harder to read by some developers  was the top-performing approach   Your best bet is still going to be finding some representative data and trying it out in a production-like environment  If you have different memory constraints  you may prefer a method with fewer allocations to one that would be faster but consume more memory   Testing Code  Feel free to play with the testing code I used  A version is included here but feel free to clone the repo and add your own methods  Please submit a pull request if you find anything interesting or want to help improve the testing framework it uses    Add the new static method  Func lt byte    string gt   to  Tests ConvertByteArrayToHexString Test cs  Add that method s name to the TestCandidates return value in that same class  Make sure you are running the input version you want  sentence or text  by toggling the comments in GenerateTestInput in that same class  Hit F5 and wait for the output  an HTML dump is also generated in the  bin folder       static string ByteArrayToHexStringViaStringJoinArrayConvertAll byte   bytes        return string Join string Empty  Array ConvertAll bytes  b   gt  b ToString  X2        static string ByteArrayToHexStringViaStringConcatArrayConvertAll byte   bytes        return string Concat Array ConvertAll bytes  b   gt  b ToString  X2        static string ByteArrayToHexStringViaBitConverter byte   bytes        string hex   BitConverter ToString bytes       return hex Replace  -          static string ByteArrayToHexStringViaStringBuilderAggregateByteToString byte   bytes        return bytes Aggregate new StringBuilder bytes Length   2    sb  b    gt  sb Append b ToString  X2     ToString      static string ByteArrayToHexStringViaStringBuilderForEachByteToString byte   bytes        StringBuilder hex   new StringBuilder bytes Length   2       foreach  byte b in bytes          hex Append b ToString  X2         return hex ToString      static string ByteArrayToHexStringViaStringBuilderAggregateAppendFormat byte   bytes        return bytes Aggregate new StringBuilder bytes Length   2    sb  b    gt  sb AppendFormat   0 X2    b   ToString      static string ByteArrayToHexStringViaStringBuilderForEachAppendFormat byte   bytes        StringBuilder hex   new StringBuilder bytes Length   2       foreach  byte b in bytes          hex AppendFormat   0 X2    b       return hex ToString      static string ByteArrayToHexViaByteManipulation byte   bytes        char   c   new char bytes Length   2       byte b      for  int i   0  i  lt  bytes Length  i              b     byte  bytes i   gt  gt  4            c i   2     char  b  gt  9   b   0x37   b   0x30           b     byte  bytes i   amp  0xF            c i   2   1     char  b  gt  9   b   0x37   b   0x30             return new string c     static string ByteArrayToHexViaByteManipulation2 byte   bytes        char   c   new char bytes Length   2       int b      for  int i   0  i  lt  bytes Length  i              b   bytes i   gt  gt  4          c i   2     char  55   b      b - 10   gt  gt  31   amp  -7            b   bytes i   amp  0xF          c i   2   1     char  55   b      b - 10   gt  gt  31   amp  -7              return new string c     static string ByteArrayToHexViaSoapHexBinary byte   bytes        SoapHexBinary soapHexBinary   new SoapHexBinary bytes       return soapHexBinary ToString      static string ByteArrayToHexViaLookupAndShift byte   bytes        StringBuilder result   new StringBuilder bytes Length   2       string hexAlphabet    0123456789ABCDEF       foreach  byte b in bytes            result Append hexAlphabet  int  b  gt  gt  4             result Append hexAlphabet  int  b  amp  0xF               return result ToString      static readonly uint   lookup32UnsafeP    uint  GCHandle Alloc  Lookup32  GCHandleType Pinned  AddrOfPinnedObject    static string ByteArrayToHexViaLookup32UnsafeDirect byte   bytes        var lookupP    lookup32UnsafeP      var result   new string  char 0  bytes Length   2       fixed  byte  bytesP   bytes      fixed  char  resultP   result            uint  resultP2    uint  resultP          for  int i   0  i  lt  bytes Length  i                  resultP2 i    lookupP bytesP i                        return result    static uint    Lookup32   Enumerable Range 0  255  Select i   gt        string s   i ToString  X2        return   uint s 0       uint s 1   lt  lt  16      ToArray    static string ByteArrayToHexViaLookupPerByte byte   bytes        var result   new char bytes Length   2       for  int i   0  i  lt  bytes Length  i                  var val    Lookup32 bytes i            result 2 i     char val          result 2 i   1     char   val  gt  gt  16             return new string result     static string ByteArrayToHexViaLookup byte   bytes        string   hexStringTable   new string              00    01    02    03    04    05    06    07    08    09    0A    0B    0C    0D    0E    0F            10    11    12    13    14    15    16    17    18    19    1A    1B    1C    1D    1E    1F            20    21    22    23    24    25    26    27    28    29    2A    2B    2C    2D    2E    2F            30    31    32    33    34    35    36    37    38    39    3A    3B    3C    3D    3E    3F            40    41    42    43    44    45    46    47    48    49    4A    4B    4C    4D    4E    4F            50    51    52    53    54    55    56    57    58    59    5A    5B    5C    5D    5E    5F            60    61    62    63    64    65    66    67    68    69    6A    6B    6C    6D    6E    6F            70    71    72    73    74    75    76    77    78    79    7A    7B    7C    7D    7E    7F            80    81    82    83    84    85    86    87    88    89    8A    8B    8C    8D    8E    8F            90    91    92    93    94    95    96    97    98    99    9A    9B    9C    9D    9E    9F            A0    A1    A2    A3    A4    A5    A6    A7    A8    A9    AA    AB    AC    AD    AE    AF            B0    B1    B2    B3    B4    B5    B6    B7    B8    B9    BA    BB    BC    BD    BE    BF            C0    C1    C2    C3    C4    C5    C6    C7    C8    C9    CA    CB    CC    CD    CE    CF            D0    D1    D2    D3    D4    D5    D6    D7    D8    D9    DA    DB    DC    DD    DE    DF            E0    E1    E2    E3    E4    E5    E6    E7    E8    E9    EA    EB    EC    ED    EE    EF            F0    F1    F2    F3    F4    F5    F6    F7    F8    F9    FA    FB    FC    FD    FE    FF              StringBuilder result   new StringBuilder bytes Length   2       foreach  byte b in bytes            result Append hexStringTable b              return result ToString        Update  2010-01-13   Added Waleed s answer to analysis  Quite fast   Update  2011-10-05   Added string Concat Array ConvertAll variant for completeness  requires  NET 4 0   On par with string Join version   Update  2012-02-05   Test repo includes more variants such as StringBuilder Append b ToString  X2     None upset the results any  foreach is faster than  IEnumerable  Aggregate  for instance  but BitConverter still wins   Update  2012-04-03   Added Mykroft s SoapHexBinary answer to analysis  which took over third place   Update  2013-01-15   Added CodesInChaos s byte manipulation answer  which took over first place  by a large margin on large blocks of text    Update  2013-05-23   Added Nathan Moinvaziri s lookup answer and the variant from Brian Lambert s blog  Both rather fast  but not taking the lead on the test machine I used  AMD Phenom 9750    Update  2014-07-31   Added  CodesInChaos s new byte-based lookup answer  It appears to have taken the lead on both the sentence tests and the full-text tests   Update  2015-08-20   Added airbreather s optimizations and unsafe variant to this answer s repo  If you want to play in the unsafe game  you can get some huge performance gains over any of the prior top winners on both short strings and large texts

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There s a class called SoapHexBinary that does exactly what you want   using System Runtime Remoting Metadata W3cXsd2001   public static byte   GetStringToBytes string value        SoapHexBinary shb   SoapHexBinary Parse value       return shb Value     public static string GetBytesToString byte   value        SoapHexBinary shb   new SoapHexBinary value       return shb ToString

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This is a great post  I like Waleed s solution  I haven t run it through patridge s test but it seems to be quite fast  I also needed the reverse process  converting a hex string to a byte array  so I wrote it as a reversal of Waleed s solution  Not sure if it s any faster than Tomalak s original solution  Again  I did not run the reverse process through patridge s test either   private byte   HexStringToByteArray string hexString        int hexStringLength   hexString Length      byte   b   new byte hexStringLength   2       for  int i   0  i  lt  hexStringLength  i    2                int topChar    hexString i   gt  0x40   hexString i  - 0x37   hexString i  - 0x30   lt  lt  4          int bottomChar   hexString i   1   gt  0x40   hexString i   1  - 0x37   hexString i   1  - 0x30          b i   2    Convert ToByte topChar   bottomChar             return b

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There is also XmlWriter WriteBinHex  see the MSDN page   This is very useful if you need to put the hexadecimal string into an XML stream   Here is a standalone method to see how it works       public static string ToBinHex byte   bytes                XmlWriterSettings xmlWriterSettings   new XmlWriterSettings            xmlWriterSettings ConformanceLevel   ConformanceLevel Fragment          xmlWriterSettings CheckCharacters   false          xmlWriterSettings Encoding   ASCIIEncoding ASCII          MemoryStream memoryStream   new MemoryStream            using  XmlWriter xmlWriter   XmlWriter Create memoryStream  xmlWriterSettings                         xmlWriter WriteBinHex bytes  0  bytes Length                     return Encoding ASCII GetString memoryStream ToArray

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You can use the BitConverter ToString method   byte   bytes    0  1  2  4  8  16  32  64  128  256  Console WriteLine  BitConverter ToString bytes      Output      00-01-02-04-08-10-20-40-80-FF   More information  BitConverter ToString Method  Byte

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Another fast function     private static readonly byte   HexNibble   new byte         0x0  0x1  0x2  0x3  0x4  0x5  0x6  0x7      0x8  0x9  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0xA  0xB  0xC  0xD  0xE  0xF  0x0      0x0  0x0  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0x0  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0x0  0x0  0x0  0x0  0x0  0x0  0x0      0x0  0xA  0xB  0xC  0xD  0xE  0xF     public static byte   HexStringToByteArray  string str         int byteCount   str Length  gt  gt  1      byte   result   new byte byteCount    str Length  amp  1        for  int i   0  i  lt  byteCount  i             result i     byte   HexNibble str i  lt  lt  1  - 48   lt  lt  4   HexNibble str  i  lt  lt  1    1  - 48        if   str Length  amp  1     0           result byteCount     byte  HexNibble str str Length - 1  - 48       return result

[c#] How do you convert a byte array to a hexadecimal string, and vice versa?

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