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

Try something like this:

string fileName = "something";
foreach (char c in System.IO.Path.GetInvalidFileNameChars())
{
   fileName = fileName.Replace(c, '_');
}

Edit:

Since GetInvalidFileNameChars() will return 10 or 15 chars, it’s better to use a StringBuilder instead of a simple string; the original version will take longer and consume more memory.