In C# check that filename is *possibly* valid (not that it exists)

Just do;

System.IO.FileInfo fi = null;
try {
  fi = new System.IO.FileInfo(fileName);
}
catch (ArgumentException) { }
catch (System.IO.PathTooLongException) { }
catch (NotSupportedException) { }
if (ReferenceEquals(fi, null)) {
  // file name is not valid
} else {
  // file name is valid... May check for existence by calling fi.Exists.
}

For creating a FileInfo instance the file does not need to exist.

You can get a list of invalid characters from Path.GetInvalidPathChars and GetInvalidFileNameChars as discussed in this question.

As noted by jberger, there some other characters which are not included in the response from this method. For much more details of the windows platform, take a look at Naming Files, Paths and Namespaces on MSDN.

As Micah points out, there is Directory.GetLogicalDrives to get a list of valid drives.

There are several methods you could use that exist in the System.IO namespace:

Directory.GetLogicalDrives() // Returns an array of strings like "c:\"
Path.GetInvalidFileNameChars() // Returns an array of characters that cannot be used in a file name
Path.GetInvalidPathChars() // Returns an array of characters that cannot be used in a path.

As suggested you could then do this:

bool IsValidFilename(string testName) {
    string regexString = "[" + Regex.Escape(Path.GetInvalidPathChars()) + "]";
    Regex containsABadCharacter = new Regex(regexString);
    if (containsABadCharacter.IsMatch(testName)) {
        return false;
    }

    // Check for drive
    string pathRoot = Path.GetPathRoot(testName);
    if (Directory.GetLogicalDrives().Contains(pathRoot)) {
        // etc
    }

    // other checks for UNC, drive-path format, etc

    return true;
}

Even if the filename is valid, you may still want to touch it to be sure the user has permission to write.

If you won't be thrashing the disk with hundreds of files in a short period of time, I think creating an empty file is a reasonable approach.

If you really want something lighter, like just checking for invalid chars, then compare your filename against Path.GetInvalidFileNameChars().

Use the static GetInvalidFileNameChars method on the Path class in the System.IO namespace to determine what characters are illegal in a file name.

To do so in a path, call the static GetInvalidPathChars method on the same class.

To determine if the root of a path is valid, you would call the static GetPathRoot method on the Path class to get the root, then use the Directory class to determine if it is valid. Then you can validate the rest of the path normally.

Several of the System.IO.Path methods will throw exceptions if the path or filename is invalid:

• Path.IsPathRooted()
• Path.GetFileName()

http://msdn.microsoft.com/en-us/library/system.io.path_methods.aspx

I don't know of anything out of the box that can just validate all of that for you, however the Path class in .NET can help you out tremendously.

For starters, it has:

char[] invalidChars = Path.GetInvalidFileNameChars(); //returns invalid charachters

or:

Path.GetPathRoot(string); // will return the root.

I've had luck using regular expressions as others have shown.

One thing to keep in mind is that Windows at least prohibits some filenames that otherwise containlegal characters. A few come to mind: com, nul, prn.

I don't have it with me now, but I have a regex that takes these filename into consideration. If you want I can post it, otherwise I'm sure you can find it the same way I did: Google.

-Jay

Probably the bast way is to build a custom method mixing a combination of regex and small look up on your file system (to see the drives, for example)

This will get you the drives on the machine:

System.IO.DriveInfo.GetDrives()

These two methods will get you the bad characters to check:

System.IO.Path.GetInvalidFileNameChars();
System.IO.Path.GetInvalidPathChars();