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

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.

Answer 2

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
{
    /// <summary>
    /// Characters allowed in a file name. Note that curly braces don't show up here
    /// becausee they are used for escaping invalid characters.
    /// </summary>
    private static readonly HashSet<char> CleanFileNameChars = new HashSet<char>
    {
        ' ', '!', '#', '$', '%', '&', '\'', '(', ')', '+', ',', '-', '.',
        '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',
    };

    /// <summary>
    /// Creates a clean file name from one that may contain invalid characters in 
    /// a way that will not collide.
    /// </summary>
    /// <param name="dirtyFileName">
    /// The file name that may contain invalid filename characters.
    /// </param>
    /// <returns>
    /// A file name that does not contain invalid filename characters.
    /// </returns>
    /// <remarks>
    /// <para>
    /// 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 '{' => "{{".
    /// </para>
    /// <para>
    /// Note that although NTFS allows unicode characters in file names, this
    /// method does not.
    /// </para>
    /// </remarks>
    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)));
    }
}

Answer 3

fileName = fileName.Replace(":", "-") 

However ":" is not the only illegal character for Windows. You will also have to handle:

/, \, :, *, ?, ", <, > and |

These are contained in System.IO.Path.GetInvalidFileNameChars();

Also (on Windows), "." cannot be the only character in the filename (both ".", "..", "...", and so on are invalid). Be careful when naming files with ".", for example:

echo "test" > .test.

Will generate a file named ".test"

Lastly, if you really want to do things correctly, there are some special file names you need to look out for. On Windows you can't create files named:

CON, PRN, AUX, CLOCK$, NUL
COM0, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9
LPT0, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.

Answer 4

This isn't more efficient, but it's more fun :)

var fileName = "foo:bar";
var invalidChars = System.IO.Path.GetInvalidFileNameChars();
var cleanFileName = new string(fileName.Where(m => !invalidChars.Contains(m)).ToArray<char>());

Answer 5

Cleaning a little my code and making a little refactoring... I created an extension for string type:

public static string ToValidFileName(this string s, char replaceChar = '_', char[] includeChars = null)
{
  var invalid = Path.GetInvalidFileNameChars();
  if (includeChars != null) invalid = invalid.Union(includeChars).ToArray();
  return string.Join(string.Empty, s.ToCharArray().Select(o => o.In(invalid) ? replaceChar : o));
}

Now it's easier to use with:

var name = "Any string you want using ? / \ or even +.zip";
var validFileName = name.ToValidFileName();

If you want to replace with a different char than "_" you can use:

var validFileName = name.ToValidFileName(replaceChar:'#');

And you can add chars to replace.. for example you dont want spaces or commas:

var validFileName = name.ToValidFileName(includeChars: new [] { ' ', ',' });

Hope it helps...

Cheers

Answer 6

Here's a version of the accepted answer using Linq which uses Enumerable.Aggregate:

string fileName = "something";

Path.GetInvalidFileNameChars()
    .Aggregate(fileName, (current, c) => current.Replace(c, '_'));