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

I needed a system that couldn't create collisions so I couldn't map multiple characters to one. I ended up with:

public static class Extension
{
    /// 

    /// Characters allowed in a file name. Note that curly braces don't show up here
    /// becausee they are used for escaping invalid characters.
    /// 
    private static readonly HashSet CleanFileNameChars = new HashSet
    {
        ' ', '!', '#', '$', '%', '&', '\'', '(', ')', '+', ',', '-', '.',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '=', '@',
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
        'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
        '[', ']', '^', '_', '`',
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
        'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
    };

    /// 

    /// Creates a clean file name from one that may contain invalid characters in 
    /// a way that will not collide.
    /// 
    /// 
    /// The file name that may contain invalid filename characters.
    /// 
    /// 
    /// A file name that does not contain invalid filename characters.
    /// 
    /// 
    /// 
    /// Escapes invalid characters by converting their ASCII values to hexadecimal
    /// and wrapping that value in curly braces. Curly braces are escaped by doubling
    /// them, for example '{' => "{{".
    /// 
    /// 
    /// Note that although NTFS allows unicode characters in file names, this
    /// method does not.
    /// 
    /// 
    public static string CleanFileName(this string dirtyFileName)
    {
        string EscapeHexString(char c) =>
            "{" + (c > 255 ? $"{(uint)c:X4}" : $"{(uint)c:X2}") + "}";

        return string.Join(string.Empty,
                           dirtyFileName.Select(
                               c =>
                                   c == '{' ? "{{" :
                                   c == '}' ? "}}" :
                                   CleanFileNameChars.Contains(c) ? $"{c}" :
                                   EscapeHexString(c)));
    }
}