How do I check if a given string is a legal valid file name under Windows

Question

I want to include a batch file rename functionality in my application  A user can type a destination filename pattern and  after replacing some wildcards in the pattern  I need to check if it s going to be a legal filename under Windows  I ve tried to use regular expression like  a-zA-Z0-9    but it doesn t include many national-specific characters from various languages  e g  umlauts and so on   What is the best way to do such a check

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My attempt   using System IO   static class PathUtils     public static string IsValidFullPath  NotNull  string fullPath          if  string IsNullOrWhiteSpace fullPath         return  Path is null  empty or white space         bool pathContainsInvalidChars   fullPath IndexOfAny Path GetInvalidPathChars       -1      if  pathContainsInvalidChars        return  Path contains invalid characters         string fileName   Path GetFileName fullPath       if  fileName              return  Path must contain a file name         bool fileNameContainsInvalidChars   fileName IndexOfAny Path GetInvalidFileNameChars       -1      if  fileNameContainsInvalidChars        return  File name contains invalid characters         if   Path IsPathRooted fullPath         return  The path must be absolute         return             This is not perfect because Path GetInvalidPathChars does not return the complete set of characters that are invalid in file and directory names and of course there s plenty more subtleties   So I use this method as a complement   public static bool TestIfFileCanBeCreated  NotNull  string fullPath      if  string IsNullOrWhiteSpace fullPath       throw new ArgumentException  Value cannot be null or whitespace     fullPath       string directoryName   Path GetDirectoryName fullPath     if  directoryName    null  Directory CreateDirectory directoryName     try         using  new FileStream fullPath  FileMode CreateNew           File Delete fullPath       return true        catch  IOException          return false          It tries to create the file and return false if there is an exception  Of course  I need to create the file but I think it s the safest way to do that  Please also note that I am not deleting directories that have been created   You can also use the first method to do basic validation  and then handle carefully the exceptions when the path is used

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This class cleans filenames and paths  use it like   var myCleanPath   PathSanitizer SanitizeFilename myBadPath          Here s the code        lt summary gt      Cleans paths of invalid characters       lt  summary gt  public static class PathSanitizer            lt summary gt          The set of invalid filename characters  kept sorted for fast binary search          lt  summary gt      private readonly static char   invalidFilenameChars           lt summary gt          The set of invalid path characters  kept sorted for fast binary search          lt  summary gt      private readonly static char   invalidPathChars       static PathSanitizer                    set up the two arrays -- sorted once for speed          invalidFilenameChars   System IO Path GetInvalidFileNameChars            invalidPathChars   System IO Path GetInvalidPathChars            Array Sort invalidFilenameChars           Array Sort invalidPathChars                    lt summary gt          Cleans a filename of invalid characters          lt  summary gt           lt param name  input  gt the string to clean lt  param gt           lt param name  errorChar  gt the character which replaces bad characters lt  param gt           lt returns gt  lt  returns gt      public static string SanitizeFilename string input  char errorChar                return Sanitize input  invalidFilenameChars  errorChar                   lt summary gt          Cleans a path of invalid characters          lt  summary gt           lt param name  input  gt the string to clean lt  param gt           lt param name  errorChar  gt the character which replaces bad characters lt  param gt           lt returns gt  lt  returns gt      public static string SanitizePath string input  char errorChar                return Sanitize input  invalidPathChars  errorChar                   lt summary gt          Cleans a string of invalid characters           lt  summary gt           lt param name  input  gt  lt  param gt           lt param name  invalidChars  gt  lt  param gt           lt param name  errorChar  gt  lt  param gt           lt returns gt  lt  returns gt      private static string Sanitize string input  char   invalidChars  char errorChar                   null always sanitizes to null         if  input    null    return null            StringBuilder result   new StringBuilder            foreach  var characterToTest in input                           we binary search for the character in the invalid set  This should be lightning fast              if  Array BinarySearch invalidChars  characterToTest   gt   0                                   we found the character in the array of                  result Append errorChar                             else                                  the character was not found in invalid  so it is valid                  result Append characterToTest                                       we re done          return result ToString

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From MSDN s  Naming a File or Directory   here are the general conventions for what a legal file name is under Windows   You may use any character in the current code page  Unicode ANSI above 127   except     lt   gt                Characters whose integer representations are 0-31  less than ASCII space  Any other character that the target file system does not allow  say  trailing periods or spaces  Any of the DOS names  CON  PRN  AUX  NUL  COM0  COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  LPT0  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  and avoid AUX txt  etc  The file name is all periods   Some optional things to check    File paths  including the file name  may not have more than 260 characters  that don t use the     prefix  Unicode file paths  including the file name  with more than 32 000 characters when using      note that prefix may expand directory components and cause it to overflow the 32 000 limit

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This check  static bool IsValidFileName string name        return          string IsNullOrWhiteSpace name   amp  amp          name IndexOfAny Path GetInvalidFileNameChars     lt  0  amp  amp           Path GetFullPath name  StartsWith               filters out names with invalid chars   lt  gt         and ASCII 0-31   as well as reserved DOS devices  CON  NUL  COMx   It allows leading spaces and all-dot-names  consistent with Path GetFullPath   Creating file with leading spaces succeeds on my system      Used  NET Framework 4 7 1  tested on Windows 7

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One liner for verifying illigal chars in the string   public static bool IsValidFilename string testName    gt   Regex IsMatch testName        Regex Escape new string System IO Path InvalidPathChars

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many of these answers will not work if the filename is too long  amp  running on a pre Windows 10 environment  Similarly  have a think about what you want to do with periods - allowing leading or trailing is technically valid  but can create problems if you do not want the file to be difficult to see or delete respectively   This is a validation attribute I created to check for a valid filename    public class ValidFileNameAttribute   ValidationAttribute       public ValidFileNameAttribute                 RequireExtension   true          ErrorMessage     0  is an Invalid Filename           MaxLength   255    superseeded in modern windows environments           public override bool IsValid object value                  http   stackoverflow com questions 422090 in-c-sharp-check-that-filename-is-possibly-valid-not-that-it-exists         var fileName    string value          if  string IsNullOrEmpty fileName     return true             if  fileName IndexOfAny Path GetInvalidFileNameChars     gt  -1                  AllowHidden  amp  amp  fileName 0                         fileName fileName Length - 1                       fileName Length  gt  MaxLength                        return false                    string extension   Path GetExtension fileName           return   RequireExtension    extension    string Empty               amp  amp   ExtensionList  null    ExtensionList Contains extension              private const string  sepChar            private IEnumerable lt string gt  ExtensionList   get  set        public bool AllowHidden   get  set        public bool RequireExtension   get  set        public int MaxLength   get  set        public string AllowedExtensions           get   return string Join  sepChar  ExtensionList              set               if  string IsNullOrEmpty value                 ExtensionList   null                else                   ExtensionList   value Split new char      sepChar 0                          Select s   gt  s 0           s          s                        ToList                               public override bool RequiresValidationContext   gt  false      and the tests   TestMethod  public void TestFilenameAttribute         var rxa   new ValidFileNameAttribute        Assert IsFalse rxa IsValid  pptx          Assert IsFalse rxa IsValid  pp tx          Assert IsFalse rxa IsValid            Assert IsFalse rxa IsValid   pp tx         Assert IsFalse rxa IsValid   pptx         Assert IsFalse rxa IsValid  pptx         Assert IsFalse rxa IsValid  a abc pptx         Assert IsFalse rxa IsValid  a  abc pptx         Assert IsFalse rxa IsValid  c abc pptx         Assert IsFalse rxa IsValid  c lt abc pptx         Assert IsTrue rxa IsValid  abc pptx         rxa   new ValidFileNameAttribute   AllowedExtensions     pptx         Assert IsFalse rxa IsValid  abc docx         Assert IsTrue rxa IsValid  abc pptx

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Regular expressions are overkill for this situation   You can use the String IndexOfAny   method in combination with Path GetInvalidPathChars   and Path GetInvalidFileNameChars     Also note that both Path GetInvalidXXX   methods clone an internal array and return the clone   So if you re going to be doing this a lot  thousands and thousands of times  you can cache a copy of the invalid chars array for reuse

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One corner case to keep in mind  which surprised me when I first found out about it  Windows allows leading space characters in file names   For example  the following are all legal  and distinct  file names on Windows  minus the quotes     file txt    file txt     file txt    One takeaway from this  Use caution when writing code that trims leading trailing whitespace from a filename string

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You can get a list of invalid characters from Path GetInvalidPathChars and GetInvalidFileNameChars   UPD  See Steve Cooper s suggestion on how to use these in a regular expression   UPD2  Note that according to the Remarks section in MSDN  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 answer provided by sixlettervaliables goes into more details

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Regular expressions are overkill for this situation   You can use the String IndexOfAny   method in combination with Path GetInvalidPathChars   and Path GetInvalidFileNameChars     Also note that both Path GetInvalidXXX   methods clone an internal array and return the clone   So if you re going to be doing this a lot  thousands and thousands of times  you can cache a copy of the invalid chars array for reuse

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Simplifying the Eugene Katz s answer   bool IsFileNameCorrect string fileName       return  fileName Any f  gt Path GetInvalidFileNameChars   Contains f       Or  bool IsFileNameCorrect string fileName       return fileName All f  gt  Path GetInvalidFileNameChars   Contains f

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The question is are you trying to determine if a path name is a legal windows path  or if it s legal on the system where the code is running    I think the latter is more important  so personally  I d probably decompose the full path and try to use  mkdir to create the directory the file belongs in  then try to create the file   This way you know not only if the path contains only valid windows characters  but if it actually represents a path that can be written by this process

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Also CON  PRN  AUX  NUL  COM  and a few others are never legal filenames in any directory with any extension

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This is what I use       public static bool IsValidFileName this string expression  bool platformIndependent                string sPattern           PRN AUX CLOCK   NUL CON COM d LPT d                  x00- x1f                         if  platformIndependent                       sPattern         a-zA-Z                                        lt  gt                      lt  gt                       lt  gt                  lt  gt                             lt  gt                      lt  gt                       lt  gt                  lt  gt                              return  Regex IsMatch expression  sPattern  RegexOptions CultureInvariant            The first pattern creates a regular expression containing the invalid illegal file names and characters for Windows platforms only  The second one does the same but ensures that the name is legal for any platform

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The question is are you trying to determine if a path name is a legal windows path  or if it s legal on the system where the code is running    I think the latter is more important  so personally  I d probably decompose the full path and try to use  mkdir to create the directory the file belongs in  then try to create the file   This way you know not only if the path contains only valid windows characters  but if it actually represents a path that can be written by this process

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You can get a list of invalid characters from Path GetInvalidPathChars and GetInvalidFileNameChars   UPD  See Steve Cooper s suggestion on how to use these in a regular expression   UPD2  Note that according to the Remarks section in MSDN  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 answer provided by sixlettervaliables goes into more details

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Simplifying the Eugene Katz s answer   bool IsFileNameCorrect string fileName       return  fileName Any f  gt Path GetInvalidFileNameChars   Contains f       Or  bool IsFileNameCorrect string fileName       return fileName All f  gt  Path GetInvalidFileNameChars   Contains f

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I use this to get rid of invalid characters in filenames without throwing exceptions   private static readonly Regex InvalidFileRegex   new Regex      string Format    0     Regex Escape    lt  gt                public static string SanitizeFileName string fileName        return InvalidFileRegex Replace fileName  string Empty

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If you re only trying to check if a string holding your file name path has any invalid characters  the fastest method I ve found is to use Split   to break up the file name into an array of parts wherever there s an invalid character  If the result is only an array of 1  there are no invalid characters   -   var nameToTest    Best file name   ever   txt   bool isInvalidName   nameToTest Split System IO Path GetInvalidFileNameChars    Length  gt  1   var pathToTest    C   My Folder  lt secrets gt      bool isInvalidPath   pathToTest Split System IO Path GetInvalidPathChars    Length  gt  1    I tried running this and other methods mentioned above on a file path name 1 000 000 times in LinqPad   Using Split   is only  850ms   Using Regex       Regex Escape new string System IO Path GetInvalidPathChars            is around 6 seconds   The more complicated regular expressions fair MUCH worse  as do some of the other options  like using the various methods on the Path class to get file name and let their internal validation do the job  most likely due to the overhead of exception handling    Granted it s not very often you need to validation 1 million file names  so a single iteration is fine for most of these methods anyway  But it s still pretty efficient and effective if you re only looking for invalid characters

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Windows filenames are pretty unrestrictive  so really it might not even be that much of an issue  The characters that are disallowed by Windows are                lt   gt      You could easily write an expression to check if those characters are present  A better solution though would be to try and name the files as the user wants  and alert them when a filename doesn t stick

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This is what I use       public static bool IsValidFileName this string expression  bool platformIndependent                string sPattern           PRN AUX CLOCK   NUL CON COM d LPT d                  x00- x1f                         if  platformIndependent                       sPattern         a-zA-Z                                        lt  gt                      lt  gt                       lt  gt                  lt  gt                             lt  gt                      lt  gt                       lt  gt                  lt  gt                              return  Regex IsMatch expression  sPattern  RegexOptions CultureInvariant            The first pattern creates a regular expression containing the invalid illegal file names and characters for Windows platforms only  The second one does the same but ensures that the name is legal for any platform

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One corner case to keep in mind  which surprised me when I first found out about it  Windows allows leading space characters in file names   For example  the following are all legal  and distinct  file names on Windows  minus the quotes     file txt    file txt     file txt    One takeaway from this  Use caution when writing code that trims leading trailing whitespace from a filename string

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From MSDN s  Naming a File or Directory   here are the general conventions for what a legal file name is under Windows   You may use any character in the current code page  Unicode ANSI above 127   except     lt   gt                Characters whose integer representations are 0-31  less than ASCII space  Any other character that the target file system does not allow  say  trailing periods or spaces  Any of the DOS names  CON  PRN  AUX  NUL  COM0  COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  LPT0  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  and avoid AUX txt  etc  The file name is all periods   Some optional things to check    File paths  including the file name  may not have more than 260 characters  that don t use the     prefix  Unicode file paths  including the file name  with more than 32 000 characters when using      note that prefix may expand directory components and cause it to overflow the 32 000 limit

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From MSDN s  Naming a File or Directory   here are the general conventions for what a legal file name is under Windows   You may use any character in the current code page  Unicode ANSI above 127   except     lt   gt                Characters whose integer representations are 0-31  less than ASCII space  Any other character that the target file system does not allow  say  trailing periods or spaces  Any of the DOS names  CON  PRN  AUX  NUL  COM0  COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  LPT0  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  and avoid AUX txt  etc  The file name is all periods   Some optional things to check    File paths  including the file name  may not have more than 260 characters  that don t use the     prefix  Unicode file paths  including the file name  with more than 32 000 characters when using      note that prefix may expand directory components and cause it to overflow the 32 000 limit

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Windows filenames are pretty unrestrictive  so really it might not even be that much of an issue  The characters that are disallowed by Windows are                lt   gt      You could easily write an expression to check if those characters are present  A better solution though would be to try and name the files as the user wants  and alert them when a filename doesn t stick

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This is what I use       public static bool IsValidFileName this string expression  bool platformIndependent                string sPattern           PRN AUX CLOCK   NUL CON COM d LPT d                  x00- x1f                         if  platformIndependent                       sPattern         a-zA-Z                                        lt  gt                      lt  gt                       lt  gt                  lt  gt                             lt  gt                      lt  gt                       lt  gt                  lt  gt                              return  Regex IsMatch expression  sPattern  RegexOptions CultureInvariant            The first pattern creates a regular expression containing the invalid illegal file names and characters for Windows platforms only  The second one does the same but ensures that the name is legal for any platform

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My attempt   using System IO   static class PathUtils     public static string IsValidFullPath  NotNull  string fullPath          if  string IsNullOrWhiteSpace fullPath         return  Path is null  empty or white space         bool pathContainsInvalidChars   fullPath IndexOfAny Path GetInvalidPathChars       -1      if  pathContainsInvalidChars        return  Path contains invalid characters         string fileName   Path GetFileName fullPath       if  fileName              return  Path must contain a file name         bool fileNameContainsInvalidChars   fileName IndexOfAny Path GetInvalidFileNameChars       -1      if  fileNameContainsInvalidChars        return  File name contains invalid characters         if   Path IsPathRooted fullPath         return  The path must be absolute         return             This is not perfect because Path GetInvalidPathChars does not return the complete set of characters that are invalid in file and directory names and of course there s plenty more subtleties   So I use this method as a complement   public static bool TestIfFileCanBeCreated  NotNull  string fullPath      if  string IsNullOrWhiteSpace fullPath       throw new ArgumentException  Value cannot be null or whitespace     fullPath       string directoryName   Path GetDirectoryName fullPath     if  directoryName    null  Directory CreateDirectory directoryName     try         using  new FileStream fullPath  FileMode CreateNew           File Delete fullPath       return true        catch  IOException          return false          It tries to create the file and return false if there is an exception  Of course  I need to create the file but I think it s the safest way to do that  Please also note that I am not deleting directories that have been created   You can also use the first method to do basic validation  and then handle carefully the exceptions when the path is used

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The question is are you trying to determine if a path name is a legal windows path  or if it s legal on the system where the code is running    I think the latter is more important  so personally  I d probably decompose the full path and try to use  mkdir to create the directory the file belongs in  then try to create the file   This way you know not only if the path contains only valid windows characters  but if it actually represents a path that can be written by this process

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One liner for verifying illigal chars in the string   public static bool IsValidFilename string testName    gt   Regex IsMatch testName        Regex Escape new string System IO Path InvalidPathChars

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From MSDN  here s a list of characters that aren t allowed                           Use almost any character in the current code page for a name  including Unicode characters and characters in the extended character set  128   255   except for the following                            The following reserved characters are not allowed            lt                          Characters whose integer representations are in the range from zero through 31 are not allowed          Any other character that the target file system does not allow

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Also CON  PRN  AUX  NUL  COM  and a few others are never legal filenames in any directory with any extension

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In my opinion  the only proper answer to this question is to try to use the path and let the OS and filesystem validate it  Otherwise you are just reimplementing  and probably poorly  all the validation rules that the OS and filesystem already use and if those rules are changed in the future you will have to change your code to match them

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I suggest just use the Path GetFullPath    string tagetFileFullNameToBeChecked  try     Path GetFullPath tagetFileFullNameToBeChecked    catch AugumentException ex         invalid chars found

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If you re only trying to check if a string holding your file name path has any invalid characters  the fastest method I ve found is to use Split   to break up the file name into an array of parts wherever there s an invalid character  If the result is only an array of 1  there are no invalid characters   -   var nameToTest    Best file name   ever   txt   bool isInvalidName   nameToTest Split System IO Path GetInvalidFileNameChars    Length  gt  1   var pathToTest    C   My Folder  lt secrets gt      bool isInvalidPath   pathToTest Split System IO Path GetInvalidPathChars    Length  gt  1    I tried running this and other methods mentioned above on a file path name 1 000 000 times in LinqPad   Using Split   is only  850ms   Using Regex       Regex Escape new string System IO Path GetInvalidPathChars            is around 6 seconds   The more complicated regular expressions fair MUCH worse  as do some of the other options  like using the various methods on the Path class to get file name and let their internal validation do the job  most likely due to the overhead of exception handling    Granted it s not very often you need to validation 1 million file names  so a single iteration is fine for most of these methods anyway  But it s still pretty efficient and effective if you re only looking for invalid characters

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Microsoft Windows  Windows kernel forbids the use of characters in range 1-31  i e   0x01-0x1F  and characters        lt           Although NTFS allows each path component  directory or filename  to be 255 characters long and paths up to about 32767 characters long  the Windows kernel only supports paths up to 259 characters long  Additionally  Windows forbids the use of the MS-DOS device names AUX  CLOCK   COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  CON  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  NUL and PRN  as well as these names with any extension  for example  AUX txt   except when using Long UNC paths  ex     C  nul txt or    D  aux con    In fact  CLOCK  may be used if an extension is provided   These restrictions only apply to Windows - Linux  for example  allows use of        lt          even in NTFS   Source  http   en wikipedia org wiki Filename

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Rather than explicitly include all possible characters  you could do a regex to check for the presence of illegal characters  and report an error then  Ideally your application should name the files exactly as the user wishes  and only cry foul if it stumbles across an error

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From MSDN  here s a list of characters that aren t allowed                           Use almost any character in the current code page for a name  including Unicode characters and characters in the extended character set  128   255   except for the following                            The following reserved characters are not allowed            lt                          Characters whose integer representations are in the range from zero through 31 are not allowed          Any other character that the target file system does not allow

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For  Net Frameworks prior to 3 5 this should work   Regular expression matching should get you some of the way  Here s a snippet using the System IO Path InvalidPathChars constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                  Regex Escape System IO Path InvalidPathChars              if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      For  Net Frameworks after 3 0 this should work   http   msdn microsoft com en-us library system io path getinvalidpathchars v vs 90  aspx  Regular expression matching should get you some of the way  Here s a snippet using the System IO Path GetInvalidPathChars   constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                 Regex Escape new string System IO Path GetInvalidPathChars                 if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      Once you know that  you should also check for different formats  eg c  my drive and   server share dir file ext

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This is an already answered question  but just for the sake of  Other options   here s a non-ideal one    non-ideal because using Exceptions as flow control is a  Bad Thing   generally   public static bool IsLegalFilename string name        try                var fileInfo   new FileInfo name           return true            catch               return false

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For  Net Frameworks prior to 3 5 this should work   Regular expression matching should get you some of the way  Here s a snippet using the System IO Path InvalidPathChars constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                  Regex Escape System IO Path InvalidPathChars              if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      For  Net Frameworks after 3 0 this should work   http   msdn microsoft com en-us library system io path getinvalidpathchars v vs 90  aspx  Regular expression matching should get you some of the way  Here s a snippet using the System IO Path GetInvalidPathChars   constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                 Regex Escape new string System IO Path GetInvalidPathChars                 if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      Once you know that  you should also check for different formats  eg c  my drive and   server share dir file ext

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Try to use it  and trap for the error  The allowed set may change across file systems  or across different versions of Windows  In other words  if you want know if Windows likes the name  hand it the name and let it tell you

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I got this idea from someone  - don t know who  Let the OS do the heavy lifting   public bool IsPathFileNameGood string fname        bool rc   Constants Fail      try               this  stream   new StreamWriter fname  true           rc   Constants Pass            catch  Exception ex                MessageBox Show ex Message   Problem opening file            rc   Constants Fail            return rc

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Windows filenames are pretty unrestrictive  so really it might not even be that much of an issue  The characters that are disallowed by Windows are                lt   gt      You could easily write an expression to check if those characters are present  A better solution though would be to try and name the files as the user wants  and alert them when a filename doesn t stick

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In my opinion  the only proper answer to this question is to try to use the path and let the OS and filesystem validate it  Otherwise you are just reimplementing  and probably poorly  all the validation rules that the OS and filesystem already use and if those rules are changed in the future you will have to change your code to match them

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Rather than explicitly include all possible characters  you could do a regex to check for the presence of illegal characters  and report an error then  Ideally your application should name the files exactly as the user wishes  and only cry foul if it stumbles across an error

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Microsoft Windows  Windows kernel forbids the use of characters in range 1-31  i e   0x01-0x1F  and characters        lt           Although NTFS allows each path component  directory or filename  to be 255 characters long and paths up to about 32767 characters long  the Windows kernel only supports paths up to 259 characters long  Additionally  Windows forbids the use of the MS-DOS device names AUX  CLOCK   COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  CON  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  NUL and PRN  as well as these names with any extension  for example  AUX txt   except when using Long UNC paths  ex     C  nul txt or    D  aux con    In fact  CLOCK  may be used if an extension is provided   These restrictions only apply to Windows - Linux  for example  allows use of        lt          even in NTFS   Source  http   en wikipedia org wiki Filename

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Rather than explicitly include all possible characters  you could do a regex to check for the presence of illegal characters  and report an error then  Ideally your application should name the files exactly as the user wishes  and only cry foul if it stumbles across an error

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From MSDN s  Naming a File or Directory   here are the general conventions for what a legal file name is under Windows   You may use any character in the current code page  Unicode ANSI above 127   except     lt   gt                Characters whose integer representations are 0-31  less than ASCII space  Any other character that the target file system does not allow  say  trailing periods or spaces  Any of the DOS names  CON  PRN  AUX  NUL  COM0  COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  LPT0  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  and avoid AUX txt  etc  The file name is all periods   Some optional things to check    File paths  including the file name  may not have more than 260 characters  that don t use the     prefix  Unicode file paths  including the file name  with more than 32 000 characters when using      note that prefix may expand directory components and cause it to overflow the 32 000 limit

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This is what I use       public static bool IsValidFileName this string expression  bool platformIndependent                string sPattern           PRN AUX CLOCK   NUL CON COM d LPT d                  x00- x1f                         if  platformIndependent                       sPattern         a-zA-Z                                        lt  gt                      lt  gt                       lt  gt                  lt  gt                             lt  gt                      lt  gt                       lt  gt                  lt  gt                              return  Regex IsMatch expression  sPattern  RegexOptions CultureInvariant            The first pattern creates a regular expression containing the invalid illegal file names and characters for Windows platforms only  The second one does the same but ensures that the name is legal for any platform

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I got this idea from someone  - don t know who  Let the OS do the heavy lifting   public bool IsPathFileNameGood string fname        bool rc   Constants Fail      try               this  stream   new StreamWriter fname  true           rc   Constants Pass            catch  Exception ex                MessageBox Show ex Message   Problem opening file            rc   Constants Fail            return rc

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many of these answers will not work if the filename is too long  amp  running on a pre Windows 10 environment  Similarly  have a think about what you want to do with periods - allowing leading or trailing is technically valid  but can create problems if you do not want the file to be difficult to see or delete respectively   This is a validation attribute I created to check for a valid filename    public class ValidFileNameAttribute   ValidationAttribute       public ValidFileNameAttribute                 RequireExtension   true          ErrorMessage     0  is an Invalid Filename           MaxLength   255    superseeded in modern windows environments           public override bool IsValid object value                  http   stackoverflow com questions 422090 in-c-sharp-check-that-filename-is-possibly-valid-not-that-it-exists         var fileName    string value          if  string IsNullOrEmpty fileName     return true             if  fileName IndexOfAny Path GetInvalidFileNameChars     gt  -1                  AllowHidden  amp  amp  fileName 0                         fileName fileName Length - 1                       fileName Length  gt  MaxLength                        return false                    string extension   Path GetExtension fileName           return   RequireExtension    extension    string Empty               amp  amp   ExtensionList  null    ExtensionList Contains extension              private const string  sepChar            private IEnumerable lt string gt  ExtensionList   get  set        public bool AllowHidden   get  set        public bool RequireExtension   get  set        public int MaxLength   get  set        public string AllowedExtensions           get   return string Join  sepChar  ExtensionList              set               if  string IsNullOrEmpty value                 ExtensionList   null                else                   ExtensionList   value Split new char      sepChar 0                          Select s   gt  s 0           s          s                        ToList                               public override bool RequiresValidationContext   gt  false      and the tests   TestMethod  public void TestFilenameAttribute         var rxa   new ValidFileNameAttribute        Assert IsFalse rxa IsValid  pptx          Assert IsFalse rxa IsValid  pp tx          Assert IsFalse rxa IsValid            Assert IsFalse rxa IsValid   pp tx         Assert IsFalse rxa IsValid   pptx         Assert IsFalse rxa IsValid  pptx         Assert IsFalse rxa IsValid  a abc pptx         Assert IsFalse rxa IsValid  a  abc pptx         Assert IsFalse rxa IsValid  c abc pptx         Assert IsFalse rxa IsValid  c lt abc pptx         Assert IsTrue rxa IsValid  abc pptx         rxa   new ValidFileNameAttribute   AllowedExtensions     pptx         Assert IsFalse rxa IsValid  abc docx         Assert IsTrue rxa IsValid  abc pptx

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You can get a list of invalid characters from Path GetInvalidPathChars and GetInvalidFileNameChars   UPD  See Steve Cooper s suggestion on how to use these in a regular expression   UPD2  Note that according to the Remarks section in MSDN  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 answer provided by sixlettervaliables goes into more details

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Try to use it  and trap for the error  The allowed set may change across file systems  or across different versions of Windows  In other words  if you want know if Windows likes the name  hand it the name and let it tell you

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The question is are you trying to determine if a path name is a legal windows path  or if it s legal on the system where the code is running    I think the latter is more important  so personally  I d probably decompose the full path and try to use  mkdir to create the directory the file belongs in  then try to create the file   This way you know not only if the path contains only valid windows characters  but if it actually represents a path that can be written by this process

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Microsoft Windows  Windows kernel forbids the use of characters in range 1-31  i e   0x01-0x1F  and characters        lt           Although NTFS allows each path component  directory or filename  to be 255 characters long and paths up to about 32767 characters long  the Windows kernel only supports paths up to 259 characters long  Additionally  Windows forbids the use of the MS-DOS device names AUX  CLOCK   COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  CON  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  NUL and PRN  as well as these names with any extension  for example  AUX txt   except when using Long UNC paths  ex     C  nul txt or    D  aux con    In fact  CLOCK  may be used if an extension is provided   These restrictions only apply to Windows - Linux  for example  allows use of        lt          even in NTFS   Source  http   en wikipedia org wiki Filename

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Windows filenames are pretty unrestrictive  so really it might not even be that much of an issue  The characters that are disallowed by Windows are                lt   gt      You could easily write an expression to check if those characters are present  A better solution though would be to try and name the files as the user wants  and alert them when a filename doesn t stick

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From MSDN  here s a list of characters that aren t allowed                           Use almost any character in the current code page for a name  including Unicode characters and characters in the extended character set  128   255   except for the following                            The following reserved characters are not allowed            lt                          Characters whose integer representations are in the range from zero through 31 are not allowed          Any other character that the target file system does not allow

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Also the destination file system is important   Under NTFS  some files can not be created in specific directories  E G   Boot in root

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This check  static bool IsValidFileName string name        return          string IsNullOrWhiteSpace name   amp  amp          name IndexOfAny Path GetInvalidFileNameChars     lt  0  amp  amp           Path GetFullPath name  StartsWith               filters out names with invalid chars   lt  gt         and ASCII 0-31   as well as reserved DOS devices  CON  NUL  COMx   It allows leading spaces and all-dot-names  consistent with Path GetFullPath   Creating file with leading spaces succeeds on my system      Used  NET Framework 4 7 1  tested on Windows 7

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Try to use it  and trap for the error  The allowed set may change across file systems  or across different versions of Windows  In other words  if you want know if Windows likes the name  hand it the name and let it tell you

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You can get a list of invalid characters from Path GetInvalidPathChars and GetInvalidFileNameChars   UPD  See Steve Cooper s suggestion on how to use these in a regular expression   UPD2  Note that according to the Remarks section in MSDN  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 answer provided by sixlettervaliables goes into more details

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This class cleans filenames and paths  use it like   var myCleanPath   PathSanitizer SanitizeFilename myBadPath          Here s the code        lt summary gt      Cleans paths of invalid characters       lt  summary gt  public static class PathSanitizer            lt summary gt          The set of invalid filename characters  kept sorted for fast binary search          lt  summary gt      private readonly static char   invalidFilenameChars           lt summary gt          The set of invalid path characters  kept sorted for fast binary search          lt  summary gt      private readonly static char   invalidPathChars       static PathSanitizer                    set up the two arrays -- sorted once for speed          invalidFilenameChars   System IO Path GetInvalidFileNameChars            invalidPathChars   System IO Path GetInvalidPathChars            Array Sort invalidFilenameChars           Array Sort invalidPathChars                    lt summary gt          Cleans a filename of invalid characters          lt  summary gt           lt param name  input  gt the string to clean lt  param gt           lt param name  errorChar  gt the character which replaces bad characters lt  param gt           lt returns gt  lt  returns gt      public static string SanitizeFilename string input  char errorChar                return Sanitize input  invalidFilenameChars  errorChar                   lt summary gt          Cleans a path of invalid characters          lt  summary gt           lt param name  input  gt the string to clean lt  param gt           lt param name  errorChar  gt the character which replaces bad characters lt  param gt           lt returns gt  lt  returns gt      public static string SanitizePath string input  char errorChar                return Sanitize input  invalidPathChars  errorChar                   lt summary gt          Cleans a string of invalid characters           lt  summary gt           lt param name  input  gt  lt  param gt           lt param name  invalidChars  gt  lt  param gt           lt param name  errorChar  gt  lt  param gt           lt returns gt  lt  returns gt      private static string Sanitize string input  char   invalidChars  char errorChar                   null always sanitizes to null         if  input    null    return null            StringBuilder result   new StringBuilder            foreach  var characterToTest in input                           we binary search for the character in the invalid set  This should be lightning fast              if  Array BinarySearch invalidChars  characterToTest   gt   0                                   we found the character in the array of                  result Append errorChar                             else                                  the character was not found in invalid  so it is valid                  result Append characterToTest                                       we re done          return result ToString

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This is an already answered question  but just for the sake of  Other options   here s a non-ideal one    non-ideal because using Exceptions as flow control is a  Bad Thing   generally   public static bool IsLegalFilename string name        try                var fileInfo   new FileInfo name           return true            catch               return false

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Microsoft Windows  Windows kernel forbids the use of characters in range 1-31  i e   0x01-0x1F  and characters        lt           Although NTFS allows each path component  directory or filename  to be 255 characters long and paths up to about 32767 characters long  the Windows kernel only supports paths up to 259 characters long  Additionally  Windows forbids the use of the MS-DOS device names AUX  CLOCK   COM1  COM2  COM3  COM4  COM5  COM6  COM7  COM8  COM9  CON  LPT1  LPT2  LPT3  LPT4  LPT5  LPT6  LPT7  LPT8  LPT9  NUL and PRN  as well as these names with any extension  for example  AUX txt   except when using Long UNC paths  ex     C  nul txt or    D  aux con    In fact  CLOCK  may be used if an extension is provided   These restrictions only apply to Windows - Linux  for example  allows use of        lt          even in NTFS   Source  http   en wikipedia org wiki Filename

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For  Net Frameworks prior to 3 5 this should work   Regular expression matching should get you some of the way  Here s a snippet using the System IO Path InvalidPathChars constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                  Regex Escape System IO Path InvalidPathChars              if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      For  Net Frameworks after 3 0 this should work   http   msdn microsoft com en-us library system io path getinvalidpathchars v vs 90  aspx  Regular expression matching should get you some of the way  Here s a snippet using the System IO Path GetInvalidPathChars   constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                 Regex Escape new string System IO Path GetInvalidPathChars                 if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      Once you know that  you should also check for different formats  eg c  my drive and   server share dir file ext

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Also CON  PRN  AUX  NUL  COM  and a few others are never legal filenames in any directory with any extension

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Also CON  PRN  AUX  NUL  COM  and a few others are never legal filenames in any directory with any extension

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Try to use it  and trap for the error  The allowed set may change across file systems  or across different versions of Windows  In other words  if you want know if Windows likes the name  hand it the name and let it tell you

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To complement the other answers  here are a couple of additional edge cases that you might want to consider    Excel can have problems if you save a workbook in a file whose name contains the     or     characters   See http   support microsoft com kb 215205 for details  Sharepoint has a whole additional set of restrictions   See http   support microsoft com kb 905231 for details

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Regular expressions are overkill for this situation   You can use the String IndexOfAny   method in combination with Path GetInvalidPathChars   and Path GetInvalidFileNameChars     Also note that both Path GetInvalidXXX   methods clone an internal array and return the clone   So if you re going to be doing this a lot  thousands and thousands of times  you can cache a copy of the invalid chars array for reuse

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Regular expressions are overkill for this situation   You can use the String IndexOfAny   method in combination with Path GetInvalidPathChars   and Path GetInvalidFileNameChars     Also note that both Path GetInvalidXXX   methods clone an internal array and return the clone   So if you re going to be doing this a lot  thousands and thousands of times  you can cache a copy of the invalid chars array for reuse

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Also the destination file system is important   Under NTFS  some files can not be created in specific directories  E G   Boot in root

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I suggest just use the Path GetFullPath    string tagetFileFullNameToBeChecked  try     Path GetFullPath tagetFileFullNameToBeChecked    catch AugumentException ex         invalid chars found

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To complement the other answers  here are a couple of additional edge cases that you might want to consider    Excel can have problems if you save a workbook in a file whose name contains the     or     characters   See http   support microsoft com kb 215205 for details  Sharepoint has a whole additional set of restrictions   See http   support microsoft com kb 905231 for details

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I use this to get rid of invalid characters in filenames without throwing exceptions   private static readonly Regex InvalidFileRegex   new Regex      string Format    0     Regex Escape    lt  gt                public static string SanitizeFileName string fileName        return InvalidFileRegex Replace fileName  string Empty

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For  Net Frameworks prior to 3 5 this should work   Regular expression matching should get you some of the way  Here s a snippet using the System IO Path InvalidPathChars constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                  Regex Escape System IO Path InvalidPathChars              if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      For  Net Frameworks after 3 0 this should work   http   msdn microsoft com en-us library system io path getinvalidpathchars v vs 90  aspx  Regular expression matching should get you some of the way  Here s a snippet using the System IO Path GetInvalidPathChars   constant   bool IsValidFilename string testName        Regex containsABadCharacter   new Regex                 Regex Escape new string System IO Path GetInvalidPathChars                 if  containsABadCharacter IsMatch testName     return false             other checks for UNC  drive-path format  etc      return true      Once you know that  you should also check for different formats  eg c  my drive and   server share dir file ext

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Rather than explicitly include all possible characters  you could do a regex to check for the presence of illegal characters  and report an error then  Ideally your application should name the files exactly as the user wishes  and only cry foul if it stumbles across an error

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From MSDN  here s a list of characters that aren t allowed                           Use almost any character in the current code page for a name  including Unicode characters and characters in the extended character set  128   255   except for the following                            The following reserved characters are not allowed            lt                          Characters whose integer representations are in the range from zero through 31 are not allowed          Any other character that the target file system does not allow

[c#] How do I check if a given string is a legal/valid file name under Windows?

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