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

Asked 2008-09-15T13:17:29.927
186

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?

c# windows file filesystems
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3 Answers

151

From MSDN's "Naming Files, Paths, and Namespaces," 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, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, COM¹, COM², COM³, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9, LPT¹, LPT², LPT³ (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)
answered 2008-09-15T13:30:30.127
5

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

answered 2008-09-15T13:24:18.503
2

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.

answered 2008-09-15T17:12:40.530

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