How to make a valid Windows filename from an arbitrary string?

Question

I\'ve got a string like \"Foo: Bar\" that I want to use as a filename, but on Windows the \":\" char isn\'t allowed in a filename.

Is there a method that will turn \

Answer 1

Here's a slight twist on Diego's answer.

If you're not afraid of Unicode, you can retain a bit more fidelity by replacing the invalid characters with valid Unicode symbols that resemble them. Here's the code I used in a recent project involving lumber cutlists:

static string MakeValidFilename(string text) {
  text = text.Replace('\'', '’'); // U+2019 right single quotation mark
  text = text.Replace('"',  '”'); // U+201D right double quotation mark
  text = text.Replace('/', '⁄');  // U+2044 fraction slash
  foreach (char c in System.IO.Path.GetInvalidFileNameChars()) {
    text = text.Replace(c, '_');
  }
  return text;
}

This produces filenames like 1⁄2” spruce.txt instead of 1_2_ spruce.txt

Yes, it really works:

Caveat Emptor

I knew this trick would work on NTFS but was surprised to find it also works on FAT and FAT32 partitions. That's because long filenames are stored in Unicode, even as far back as Windows 95/NT. I tested on Win7, XP, and even a Linux-based router and they showed up OK. Can't say the same for inside a DOSBox.

That said, before you go nuts with this, consider whether you really need the extra fidelity. The Unicode look-alikes could confuse people or old programs, e.g. older OS's relying on codepages.