How do I check for illegal characters in a path?

InvalidPathChars is deprecated. Use GetInvalidPathChars() instead:

    public static bool FilePathHasInvalidChars(string path)
    {

        return (!string.IsNullOrEmpty(path) && path.IndexOfAny(System.IO.Path.GetInvalidPathChars()) >= 0);
    }

Edit: Slightly longer, but handles path vs file invalid chars in one function:

    // WARNING: Not tested
    public static bool FilePathHasInvalidChars(string path)
    {
        bool ret = false;
        if(!string.IsNullOrEmpty(path))
        {
            try
            {
                // Careful!
                //    Path.GetDirectoryName("C:\Directory\SubDirectory")
                //    returns "C:\Directory", which may not be what you want in
                //    this case. You may need to explicitly add a trailing \
                //    if path is a directory and not a file path. As written, 
                //    this function just assumes path is a file path.
                string fileName = System.IO.Path.GetFileName(path);
                string fileDirectory = System.IO.Path.GetDirectoryName(path);

                // we don't need to do anything else,
                                    // if we got here without throwing an 
                                    // exception, then the path does not
                                    // contain invalid characters
            }
            catch (ArgumentException)
            {
                                    // Path functions will throw this 
                                    // if path contains invalid chars
                ret = true;
            }
        }
        return ret;
    }