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

Solution 1:

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:

• < > : " / \ | ? *
• 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)

Solution 2:

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.