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

Question

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 n

Answer 1

My attempt:

using System.IO;

static class PathUtils
{
  public static string IsValidFullPath([NotNull] string fullPath)
  {
    if (string.IsNullOrWhiteSpace(fullPath))
      return "Path is null, empty or white space.";

    bool pathContainsInvalidChars = fullPath.IndexOfAny(Path.GetInvalidPathChars()) != -1;
    if (pathContainsInvalidChars)
      return "Path contains invalid characters.";

    string fileName = Path.GetFileName(fullPath);
    if (fileName == "")
      return "Path must contain a file name.";

    bool fileNameContainsInvalidChars = fileName.IndexOfAny(Path.GetInvalidFileNameChars()) != -1;
    if (fileNameContainsInvalidChars)
      return "File name contains invalid characters.";

    if (!Path.IsPathRooted(fullPath))
      return "The path must be absolute.";

    return "";
  }
}

This is not perfect because Path.GetInvalidPathChars does not return the complete set of characters that are invalid in file and directory names and of course there's plenty more subtleties.

So I use this method as a complement:

public static bool TestIfFileCanBeCreated([NotNull] string fullPath)
{
  if (string.IsNullOrWhiteSpace(fullPath))
    throw new ArgumentException("Value cannot be null or whitespace.", "fullPath");

  string directoryName = Path.GetDirectoryName(fullPath);
  if (directoryName != null) Directory.CreateDirectory(directoryName);
  try
  {
    using (new FileStream(fullPath, FileMode.CreateNew)) { }
    File.Delete(fullPath);
    return true;
  }
  catch (IOException)
  {
    return false;
  }
}

It tries to create the file and return false if there is an exception. Of course, I need to create the file but I think it's the safest way to do that. Please also note that I am not deleting directories that have been created.

You can also use the first method to do basic validation, and then handle carefully the exceptions when the path is used.