How to remove illegal characters from path and filenames

Question

I need a robust and simple way to remove illegal path and file characters from a simple string  I ve used the below code but it doesn t seem to do anything  what am I missing   using System  using System IO   namespace ConsoleApplication1       class Program               static void Main string   args                        string illegal      M lt  gt     a ry  h  ad  gt  gt  a         lt  gt   li tt le   la  mb                  illegal   illegal Trim Path GetInvalidFileNameChars                 illegal   illegal Trim Path GetInvalidPathChars                  Console WriteLine illegal               Console ReadLine

User · Answer

One liner to cleanup string from any illegal chars for windows file naming   public static string CleanIllegalName string p testName    gt  new Regex string Format    0     Regex Escape new string Path GetInvalidFileNameChars      new string Path GetInvalidPathChars       Replace p testName

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Scanning over the answers here  they all   seem to involve using a char array of invalid filename characters    Granted  this may be micro-optimising - but for the benefit of anyone who might be looking to check a large number of values for being valid filenames  it s worth noting that building a hashset of invalid chars will bring about notably better performance    I have been very surprised  shocked  in the past just how quickly a hashset  or dictionary  outperforms iterating over a list  With strings  it s a ridiculously low number  about 5-7 items from memory   With most other simple data  object references  numbers etc  the magic crossover seems to be around 20 items    There are 40 invalid characters in the Path InvalidFileNameChars  list   Did a search today and there s quite a good benchmark here on StackOverflow that shows the hashset will take a little over half the time of an array list for 40 items  https   stackoverflow com a 10762995 949129  Here s the helper class I use for sanitising paths  I forget now why I had the fancy replacement option in it  but it s there as a cute bonus   Additional bonus method  IsValidLocalPath  too         those which don t use regular expressions   public static class PathExtensions       private static HashSet lt char gt   invalidFilenameChars      private static HashSet lt char gt  InvalidFilenameChars               get   return  invalidFilenameChars      invalidFilenameChars   new HashSet lt char gt  Path GetInvalidFileNameChars                         lt summary gt Replaces characters in  lt c gt text lt  c gt  that are not allowed in file names with the          specified replacement character  lt  summary gt           lt param name  text  gt Text to make into a valid filename  The same string is returned if          it is valid already  lt  param gt           lt param name  replacement  gt Replacement character  or NULL to remove bad characters  lt  param gt           lt param name  fancyReplacements  gt TRUE to replace quotes and slashes with the non-ASCII characters     and    lt  param gt           lt returns gt A string that can be used as a filename  If the output string would otherwise be empty      is returned  lt  returns gt      public static string ToValidFilename this string text  char  replacement        bool fancyReplacements   false                StringBuilder sb   new StringBuilder text Length           HashSet lt char gt  invalids   InvalidFilenameChars          bool changed   false           for  int i   0  i  lt  text Length  i                          char c   text i               if  invalids Contains c                                 changed   true                  char repl   replacement      0                   if  fancyReplacements                                        if  c         repl             U 201D right double quotation mark                     else if  c          repl             U 2019 right single quotation mark                     else if  c         repl           U 2044 fraction slash                                   if  repl      0                       sb Append repl                             else                 sb Append c                      if  sb Length    0              return               return changed   sb ToString     text                   lt summary gt          Returns TRUE if the specified path is a valid  local filesystem path           lt  summary gt           lt param name  pathString  gt  lt  param gt           lt returns gt  lt  returns gt      public static bool IsValidLocalPath this string pathString                   From solution at https   stackoverflow com a 11636052 949129         Uri pathUri          Boolean isValidUri   Uri TryCreate pathString  UriKind Absolute  out pathUri           return isValidUri  amp  amp  pathUri    null  amp  amp  pathUri IsLoopback

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Here s a code snippet that should help for  NET 3 and higher   using System IO  using System Text RegularExpressions   public static class PathValidation       private static string pathValidatorExpression           string Join     Array ConvertAll Path GetInvalidPathChars    x   gt  Regex Escape x ToString                   private static Regex pathValidator   new Regex pathValidatorExpression  RegexOptions Compiled        private static string fileNameValidatorExpression           string Join     Array ConvertAll Path GetInvalidFileNameChars    x   gt  Regex Escape x ToString                   private static Regex fileNameValidator   new Regex fileNameValidatorExpression  RegexOptions Compiled        private static string pathCleanerExpression         string Join     Array ConvertAll Path GetInvalidPathChars    x   gt  Regex Escape x ToString                 private static Regex pathCleaner   new Regex pathCleanerExpression  RegexOptions Compiled        private static string fileNameCleanerExpression         string Join     Array ConvertAll Path GetInvalidFileNameChars    x   gt  Regex Escape x ToString                 private static Regex fileNameCleaner   new Regex fileNameCleanerExpression  RegexOptions Compiled        public static bool ValidatePath string path                return pathValidator IsMatch path              public static bool ValidateFileName string fileName                return fileNameValidator IsMatch fileName              public static string CleanPath string path                return pathCleaner Replace path                  public static string CleanFileName string fileName                return fileNameCleaner Replace fileName

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I use regular expressions to achieve this  First  I dynamically build the regex   string regex   string Format                        0                        Regex Escape new string Path GetInvalidFileNameChars       Regex removeInvalidChars   new Regex regex  RegexOptions Singleline   RegexOptions Compiled   RegexOptions CultureInvariant     Then I just call removeInvalidChars Replace to do the find and replace  This can obviously be extended to cover path chars as well

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I absolutely prefer the idea of Jeff Yates  It will work perfectly  if you slightly modify it   string regex   String Format    0     Regex Escape new string Path GetInvalidFileNameChars       Regex removeInvalidChars   new Regex regex  RegexOptions Singleline   RegexOptions Compiled   RegexOptions CultureInvariant     The improvement is just to escape the automaticially generated regex

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File name can not contain characters from Path GetInvalidPathChars      and   symbols  and other specific names  We combined all checks into one class   public static class FileNameExtensions       private static readonly Lazy lt string   gt  InvalidFileNameChars           new Lazy lt string   gt       gt  Path GetInvalidPathChars                Union Path GetInvalidFileNameChars                Union new                  Select c   gt  c ToString CultureInfo InvariantCulture   ToArray           private static readonly HashSet lt string gt  ProhibitedNames   new HashSet lt string gt                  aux             con             clock              nul             prn              com1             com2             com3             com4             com5             com6             com7             com8             com9              lpt1             lpt2             lpt3             lpt4             lpt5             lpt6             lpt7             lpt8             lpt9              public static bool IsValidFileName string fileName                return  string IsNullOrWhiteSpace fileName               amp  amp  fileName All o   gt   IsInvalidFileNameChar o                amp  amp   IsProhibitedName fileName              public static bool IsProhibitedName string fileName                return ProhibitedNames Contains fileName ToLower CultureInfo InvariantCulture               private static string ReplaceInvalidFileNameSymbols  CanBeNull  this string value  string replacementValue                if  value    null                        return null                     return InvalidFileNameChars Value Aggregate new StringBuilder value                sb  currentChar    gt  sb Replace currentChar  replacementValue   ToString               public static bool IsInvalidFileNameChar char value                return InvalidFileNameChars Value Contains value ToString CultureInfo InvariantCulture               public static string GetValidFileName  NotNull  this string value                return GetValidFileName value                    public static string GetValidFileName  NotNull  this string value  string replacementValue                if  string IsNullOrWhiteSpace value                         throw new ArgumentException   value should be non empty   nameof value                       if  IsProhibitedName value                         return  string IsNullOrWhiteSpace replacementValue           replacementValue    value                      return ReplaceInvalidFileNameSymbols value  replacementValue              public static string GetFileNameError string fileName                if  string IsNullOrWhiteSpace fileName                         return CommonResources SelectReportNameError                     if  IsProhibitedName fileName                         return CommonResources FileNameIsProhibited                     var invalidChars   fileName Where IsInvalidFileNameChar  Distinct   ToArray             if invalidChars Length  gt  0                        return string Format CultureInfo CurrentCulture                  invalidChars Length    1   CommonResources InvalidCharacter   CommonResources InvalidCharacters                  StringExtensions JoinQuoted             invalidChars Select c   gt  c ToString CultureInfo CurrentCulture                         return string Empty            Method GetValidFileName replaces all incorrect data to

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Here is my small contribution  A method to replace within the same string without creating new strings or stringbuilders  It s fast  easy to understand and a good alternative to all mentions in this post  private static HashSet lt char gt   invalidCharsHash  private static HashSet lt char gt  InvalidCharsHash     get   return  invalidCharsHash      invalidCharsHash   new HashSet lt char gt  Path GetInvalidFileNameChars           private static string ReplaceInvalidChars string fileName  string newValue      char newChar   newValue 0      char   chars   fileName ToCharArray      for  int i   0  i  lt  chars Length  i            char c   chars i       if  InvalidCharsHash Contains c         chars i    newChar         return new string chars      You can call it like this  string illegal    quot   quot M lt  gt   quot   a ry  h  ad  gt  gt  a        quot  lt  gt   li tt le   la  quot mb   quot   string legal   ReplaceInvalidChars illegal    and returns   M     a ry  h  ad    a          li tt le   la mb    It s worth to note that this method will always replace invalid chars with a given value  but will not remove them  If you want to remove invalid chars  this alternative will do the trick  private static string RemoveInvalidChars string fileName  string newValue      char newChar   string IsNullOrEmpty newValue    char MinValue   newValue 0     bool remove   newChar    char MinValue     char   chars   fileName ToCharArray      char   newChars   new char chars Length     int i2   0    for  int i   0  i  lt  chars Length  i            char c   chars i       if  InvalidCharsHash Contains c               if   remove          newChars i2      newChar            else       newChars i2      c          return new string newChars  0  i2      BENCHMARK I executed timed test runs with most methods found in this post  if performance is what you are after  Some of these methods don t replace with a given char  since OP was asking to clean the string  I added tests replacing with a given char  and some others replacing with an empty char if your intended scenario only needs to remove the unwanted chars  Code used for this benchmark is at the end  so you can run your own tests  Note  Methods Test1 and Test2 are both proposed in this post  First Run replacing with      1000000 iterations Results              Test1                Elapsed 00 00 01 6665595 Result  M     a ry  h  ad    a          li tt le   la mb                Test2                Elapsed 00 00 01 7526835 Result  M     a ry  h  ad    a          li tt le   la mb                Test3                Elapsed 00 00 05 2306227 Result  M     a ry  h  ad    a          li tt le   la mb                Test4                Elapsed 00 00 14 8203696 Result  M     a ry  h  ad    a          li tt le   la mb                Test5                Elapsed 00 00 01 8273760 Result  M     a ry  h  ad    a          li tt le   la mb                Test6                Elapsed 00 00 05 4249985 Result  M     a ry  h  ad    a          li tt le   la mb                Test7                Elapsed 00 00 07 5653833 Result  M     a ry  h  ad    a          li tt le   la mb                Test8                Elapsed 00 12 23 1410106 Result  M     a ry  h  ad    a          li tt le   la mb                Test9                Elapsed 00 00 02 1016708 Result  M     a ry  h  ad    a          li tt le   la mb                Test10                Elapsed 00 00 05 0987225 Result M ary had a little lamb               Test11                Elapsed 00 00 06 8004289 Result M ary had a little lamb   Second Run removing invalid chars  1000000 iterations Note  Test1 will not remove  only replace  Results              Test1                Elapsed 00 00 01 6945352 Result  M     a ry  h  ad    a          li tt le   la mb               Test2                Elapsed 00 00 01 4798049 Result M ary had a little lamb               Test3                Elapsed 00 00 04 0415688 Result M ary had a little lamb               Test4                Elapsed 00 00 14 3397960 Result M ary had a little lamb               Test5                Elapsed 00 00 01 6782505 Result M ary had a little lamb               Test6                Elapsed 00 00 04 9251707 Result M ary had a little lamb               Test7                Elapsed 00 00 07 9562379 Result M ary had a little lamb               Test8                Elapsed 00 12 16 2918943 Result M ary had a little lamb               Test9                Elapsed 00 00 02 0770277 Result M ary had a little lamb               Test10                Elapsed 00 00 05 2721232 Result M ary had a little lamb               Test11                Elapsed 00 00 05 2802903 Result M ary had a little lamb   BENCHMARK RESULTS Methods Test1  Test2 and Test5 are the fastest  Method Test8 is the slowest  CODE Here s the complete code of the benchmark  private static HashSet lt char gt   invalidCharsHash  private static HashSet lt char gt  InvalidCharsHash     get   return  invalidCharsHash      invalidCharsHash   new HashSet lt char gt  Path GetInvalidFileNameChars           private static string  invalidCharsValue  private static string InvalidCharsValue     get   return  invalidCharsValue      invalidCharsValue   new string Path GetInvalidFileNameChars           private static char    invalidChars  private static char   InvalidChars     get   return  invalidChars      invalidChars   Path GetInvalidFileNameChars          static void Main string   args      string testPath    quot   quot M  lt  gt   quot   a ry  h  ad  gt  gt  a        quot  lt  gt   li tt le   la  quot mb   quot      int max   1000000    string newValue    quot  quot      TimeBenchmark max  Test1  testPath  newValue     TimeBenchmark max  Test2  testPath  newValue     TimeBenchmark max  Test3  testPath  newValue     TimeBenchmark max  Test4  testPath  newValue     TimeBenchmark max  Test5  testPath  newValue     TimeBenchmark max  Test6  testPath  newValue     TimeBenchmark max  Test7  testPath  newValue     TimeBenchmark max  Test8  testPath  newValue     TimeBenchmark max  Test9  testPath  newValue     TimeBenchmark max  Test10  testPath  newValue     TimeBenchmark max  Test11  testPath  newValue      Console Read       private static void TimeBenchmark int maxLoop  Func lt string  string  string gt  func  string testString  string newValue      var sw   new Stopwatch      sw Start      string result   string Empty     for  int i   0  i  lt  maxLoop  i        result   func  Invoke testString  newValue      sw Stop       Console WriteLine   quot              func Method Name                 quot      Console WriteLine  quot Elapsed  0  quot   sw Elapsed     Console WriteLine  quot Result  0  quot   result     Console WriteLine  quot  quot       private static string Test1 string fileName  string newValue      char newChar   string IsNullOrEmpty newValue    char MinValue   newValue 0      char   chars   fileName ToCharArray      for  int i   0  i  lt  chars Length  i            if  InvalidCharsHash Contains chars i          chars i    newChar         return new string chars      private static string Test2 string fileName  string newValue      char newChar   string IsNullOrEmpty newValue    char MinValue   newValue 0     bool remove   newChar    char MinValue     char   chars   fileName ToCharArray      char   newChars   new char chars Length     int i2   0    for  int i   0  i  lt  chars Length  i            char c   chars i       if  InvalidCharsHash Contains c               if   remove          newChars i2      newChar            else       newChars i2      c          return new string newChars  0  i2      private static string Test3 string filename  string newValue      foreach  char c in InvalidCharsValue          filename   filename Replace c ToString    newValue          return filename     private static string Test4 string filename  string newValue      Regex r   new Regex string Format  quot   0   quot   Regex Escape InvalidCharsValue       filename   r Replace filename  newValue     return filename     private static string Test5 string filename  string newValue      return string Join newValue  filename Split InvalidChars       private static string Test6 string fileName  string newValue      return InvalidChars Aggregate fileName   current  c    gt  current Replace c ToString    newValue       private static string Test7 string fileName  string newValue      string regex   string Format  quot   0   quot   Regex Escape InvalidCharsValue      return Regex Replace fileName  regex  newValue  RegexOptions Compiled      private static string Test8 string fileName  string newValue      string regex   string Format  quot   0   quot   Regex Escape InvalidCharsValue      Regex removeInvalidChars   new Regex regex  RegexOptions Singleline   RegexOptions Compiled   RegexOptions CultureInvariant     return removeInvalidChars Replace fileName  newValue      private static string Test9 string fileName  string newValue      StringBuilder sb   new StringBuilder fileName Length     bool changed   false     for  int i   0  i  lt  fileName Length  i            char c   fileName i       if  InvalidCharsHash Contains c               changed   true        sb Append newValue             else       sb Append c          if  sb Length    0      return newValue     return changed   sb ToString     fileName     private static string Test10 string fileName  string newValue      if   fileName Any c   gt  InvalidChars Contains c            return fileName         return new string fileName Where c   gt   InvalidChars Contains c   ToArray        private static string Test11 string fileName  string newValue      string invalidCharsRemoved   new string fileName      Where x   gt   InvalidChars Contains x        ToArray        return invalidCharsRemoved

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public static class StringExtensions                 public static string RemoveUnnecessary this string source                        string result   string Empty              string regex   new string Path GetInvalidFileNameChars      new string Path GetInvalidPathChars                 Regex reg   new Regex string Format    0     Regex Escape regex                 result   reg Replace source                   return result                    You can use method clearly

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I created an extension method that combines several suggestions    Holding illegal characters in a hash set Filtering out characters below ascii 127  Since Path GetInvalidFileNameChars does not include all invalid characters possible with ascii codes from 0 to 255  See here and MSDN Possiblity to define the replacement character   Source   public static class FileNameCorrector       private static HashSet lt char gt  invalid   new HashSet lt char gt  Path GetInvalidFileNameChars          public static string ToValidFileName this string name  char replacement     0                 var builder   new StringBuilder            foreach  var cur in name                        if  cur  gt  31  amp  amp  cur  lt  128  amp  amp   invalid Contains cur                                 builder Append cur                             else if  replacement      0                                 builder Append replacement                                    return builder ToString

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If you remove or replace with a single character the invalid characters  you can have collisions    lt abc - gt  abc  gt abc - gt  abc   Here is a simple method to avoid this   public static string ReplaceInvalidFileNameChars string s        char   invalidFileNameChars   System IO Path GetInvalidFileNameChars        foreach  char c in invalidFileNameChars          s   s Replace c ToString          Array IndexOf invalidFileNameChars  c              return s      The result     lt abc - gt   1 abc   gt abc - gt   2 abc

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Try something like this instead   string illegal      M    a ry  h  ad  gt  gt  a          li tt le   la  mb     string invalid   new string Path GetInvalidFileNameChars      new string Path GetInvalidPathChars      foreach  char c in invalid        illegal   illegal Replace c ToString              But I have to agree with the comments  I d probably try to deal with the source of the illegal paths  rather than try to mangle an illegal path into a legitimate but probably unintended one   Edit  Or a potentially  better  solution  using Regex s   string illegal      M    a ry  h  ad  gt  gt  a          li tt le   la  mb     string regexSearch   new string Path GetInvalidFileNameChars      new string Path GetInvalidPathChars     Regex r   new Regex string Format    0     Regex Escape regexSearch     illegal   r Replace illegal         Still  the question begs to be asked  why you re doing this in the first place

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I think it is much easier to validate using a regex and specifiing which characters are allowed  instead of trying to check for all bad characters  See these links  http   www c-sharpcorner com UploadFile prasad 1 RegExpPSD12062005021717AM RegExpPSD aspx http   www windowsdevcenter com pub a oreilly windows news csharp 0101 html  Also  do a search for  regular expression editor s  they help a lot  There are some around which even output the code in c  for you

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The original question asked to  remove illegal characters    public string RemoveInvalidChars string filename        return string Concat filename Split Path GetInvalidFileNameChars          You may instead want to replace them   public string ReplaceInvalidChars string filename        return string Join      filename Split Path GetInvalidFileNameChars              This answer was on another thread by Ceres  I really like it neat and simple

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Throw an exception   if   fileName IndexOfAny Path GetInvalidFileNameChars     gt  -1                                 throw new ArgumentException

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This seems to be O n  and does not spend too much memory on strings       private static readonly HashSet lt char gt  invalidFileNameChars   new HashSet lt char gt  Path GetInvalidFileNameChars          public static string RemoveInvalidFileNameChars string name                if   name Any c   gt  invalidFileNameChars Contains c                  return name                     return new string name Where c   gt   invalidFileNameChars Contains c   ToArray

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Or you can just do   YOUR STRING  Replace            Replace           Replace           Replace           Replace           Replace           Replace   lt         Replace   gt         Replace           Trim

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Here is a function which replaces all illegal characters in a file name by a replacement character   public static string ReplaceIllegalFileChars string FileNameWithoutPath  char ReplacementChar      const string IllegalFileChars           lt  gt         StringBuilder sb   new StringBuilder FileNameWithoutPath Length     char c     for  int i   0  i  lt  FileNameWithoutPath Length  i            c   FileNameWithoutPath i       if  IllegalFileChars IndexOf c   gt   0              c   ReplacementChar            sb Append c         return  sb ToString         For example the underscore can be used as a replacement character   NewFileName   ReplaceIllegalFileChars FileName

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The best way to  remove illegal character from user input is to replace illegal character using Regex class  create method in code behind or also it validate at client side using RegularExpression control   public string RemoveSpecialCharacters string str        return Regex Replace str     a-zA-Z0-9           RegexOptions Compiled       OR   lt asp RegularExpressionValidator ID  regxFolderName                                   runat  server                                   ErrorMessage  Enter folder name with  a-z A-Z0-9                                    ControlToValidate  txtFolderName                                   Display  Dynamic                                   ValidationExpression    a-zA-Z0-9                                       ForeColor  Red  gt

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For starters  Trim only removes characters from the beginning or end of the string  Secondly  you should evaluate if you really want to remove the offensive characters  or fail fast and let the user know their filename is invalid  My choice is the latter  but my answer should at least show you how to do things the right AND wrong way   StackOverflow question showing how to check if a given string is a valid file name  Note you can use the regex from this question to remove characters with a regular expression replacement  if you really need to do this

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You can remove illegal chars using Linq like this   var invalidChars   Path GetInvalidFileNameChars     var invalidCharsRemoved   stringWithInvalidChars  Where x   gt   invalidChars Contains x    ToArray      EDIT This is how it looks with the required edit mentioned in the comments   var invalidChars   Path GetInvalidFileNameChars     string invalidCharsRemoved   new string stringWithInvalidChars    Where x   gt   invalidChars Contains x      ToArray

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For file names   var cleanFileName   string Join     fileName Split Path GetInvalidFileNameChars        For full paths   var cleanPath   string Join     path Split Path GetInvalidPathChars        Note that if you intend to use this as a security feature  a more robust approach would be to expand all paths and then verify that the user supplied path is indeed a child of a directory the user should have access to

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These are all great solutions  but they all rely on Path GetInvalidFileNameChars  which may not be as reliable as you d think  Notice the following remark in the MSDN documentation on Path GetInvalidFileNameChars      The array returned from this method is not guaranteed to contain the complete set of characters that are invalid in file and directory names  The full set of invalid characters can vary by file system  For example  on Windows-based desktop platforms  invalid path characters might include ASCII Unicode characters 1 through 31  as well as quote      less than   lt    greater than      pipe      backspace   b   null   0  and tab   t     It s not any better with Path GetInvalidPathChars method  It contains the exact same remark

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I think the question already not full answered    The answers only describe clean filename OR path    not both  Here is my solution   private static string CleanPath string path        string regexSearch   new string Path GetInvalidFileNameChars      new string Path GetInvalidPathChars         Regex r   new Regex string Format    0     Regex Escape regexSearch         List lt string gt  split   path Split       ToList        string returnValue   split Aggregate string Empty   current  s    gt  current    r Replace s                    returnValue   returnValue TrimEnd            return returnValue

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public static bool IsValidFilename string testName        return  new Regex       Regex Escape new String System IO Path GetInvalidFileNameChars            IsMatch testName

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I ve rolled my own method  which seems to be a lot faster of other posted here  especially the regex which is so sloooooow  but I didn t tested all methods posted  https   dotnetfiddle net haIXiY The first method  mine  and second  also mine  but old one  also do an added check on backslashes  so the benchmark are not perfect  but anyways it s just to give you an idea  Result on my laptop  for 100 000 iterations   StringHelper RemoveInvalidCharacters 1  451 ms   StringHelper RemoveInvalidCharacters 2  7139 ms   StringHelper RemoveInvalidCharacters 3  2447 ms   StringHelper RemoveInvalidCharacters 4  3733 ms   StringHelper RemoveInvalidCharacters 5  11689 ms      gt  Regex    The fastest method  public static string RemoveInvalidCharacters string content  char replace        bool doNotReplaceBackslashes   false        if  string IsNullOrEmpty content           return content       var idx   content IndexOfAny InvalidCharacters       if  idx  gt   0                var sb   new StringBuilder content           while  idx  gt   0                        if  sb idx              doNotReplaceBackslashes                  sb idx    replace              idx   content IndexOfAny InvalidCharacters  idx 1                     return sb ToString              return content     Method doesn t compile  quot as is quot  dur to InvalidCharacters property  check the fiddle for full code

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This will do want you want  and avoid collisions   static string SanitiseFilename string key                var invalidChars   Path GetInvalidFileNameChars            var sb   new StringBuilder            foreach  var c in key                        var invalidCharIndex   -1              for  var i   0  i  lt  invalidChars Length  i                                  if  c    invalidChars i                                         invalidCharIndex   i                                              if  invalidCharIndex  gt  -1                                sb Append      Append invalidCharIndex                   continue                             if  c                                       sb Append                        continue                             sb Append c                     return sb ToString

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If you have to use the method in many places in a project  you could also make an extension method and call it anywhere in the project for strings   public static class StringExtension               public static string RemoveInvalidChars this string originalString                                    string finalString string Empty              if   string IsNullOrEmpty originalString                                 return string Concat originalString Split Path GetInvalidFileNameChars                                return finalString                               You can call the above extension method as  string illegal    quot   quot M lt  gt   quot   a ry  h  ad  gt  gt  a        quot  lt  gt   li tt le   la  quot mb   quot   string afterIllegalChars   illegal RemoveInvalidChars

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I wrote this monster for fun  it lets you roundtrip   public static class FileUtility       private const char PrefixChar            private static readonly int MaxLength      private static readonly Dictionary lt char char   gt  Illegals      static FileUtility                 List lt char gt  illegal   new List lt char gt    PrefixChar            illegal AddRange Path GetInvalidFileNameChars             MaxLength   illegal Select x   gt    int x  ToString   Length  Max            Illegals   illegal ToDictionary x   gt  x  x   gt    int x  ToString  D    MaxLength  ToCharArray                public static string FilenameEncode string s                var builder   new StringBuilder            char   replacement          using  var reader   new StringReader s                         while  true                                int read   reader Read                    if  read    -1                      break                  char c    char read                  if Illegals TryGetValue c out replacement                                         builder Append PrefixChar                       builder Append replacement                                     else                                       builder Append c                                                     return builder ToString               public static string FilenameDecode string s                var builder   new StringBuilder            char   buffer   new char MaxLength           using  var reader   new StringReader s                         while  true                                int read   reader Read                    if  read    -1                      break                  char c    char read                  if  c    PrefixChar                                        reader Read buffer  0  MaxLength                       var encoded   char  ParseCharArray buffer                       builder Append encoded                                     else                                       builder Append c                                                     return builder ToString               public static int ParseCharArray char   buffer                int result   0          foreach  char t in buffer                        int digit   t -  0               if   digit  lt  0      digit  gt  9                                 throw new ArgumentException  Input string was not in the correct format                              result    10              result    digit                    return result

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I use Linq to clean up filenames  You can easily extend this to check for valid paths as well   private static string CleanFileName string fileName        return Path GetInvalidFileNameChars   Aggregate fileName   current  c    gt  current Replace c ToString    string Empty        Update  Some comments indicate this method is not working for them so I ve included a link to a DotNetFiddle snippet so you may validate the method    https   dotnetfiddle net nw1SWY

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Most solutions above combine illegal chars for both path and filename which is wrong  even when both calls currently return the same set of chars   I would first split the path filename in path and filename  then apply the appropriate set to either if them and then combine the two again   wvd vegt

[c#] How to remove illegal characters from path and filenames?

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