C# Sanitize File Name

I recently have been moving a bunch of MP3s from various locations into a repository. I had been constructing the new file names using the ID3 tags (thanks, TagLib-Sharp!), and I noticed that I was getting a System.NotSupportedException:

> "The given path's format is not supported."

This was generated by either File.Copy() or Directory.CreateDirectory().

It didn't take long to realize that my file names needed to be sanitized. So I did the obvious thing:

public static string SanitizePath_(string path, char replaceChar)
{
    string dir = Path.GetDirectoryName(path);
    foreach (char c in Path.GetInvalidPathChars())
        dir = dir.Replace(c, replaceChar);
    
    string name = Path.GetFileName(path);
    foreach (char c in Path.GetInvalidFileNameChars())
        name = name.Replace(c, replaceChar);

    return dir + name;
}

To my surprise, I continued to get exceptions. It turned out that ':' is not in the set of Path.GetInvalidPathChars(), because it is valid in a path root. I suppose that makes sense - but this has to be a pretty common problem. Does anyone have some short code that sanitizes a path? The most thorough I've come up with this, but it feels like it is probably overkill.

    // replaces invalid characters with replaceChar
    public static string SanitizePath(string path, char replaceChar)
    {
        // construct a list of characters that can't show up in filenames.
        // need to do this because ":" is not in InvalidPathChars
        if (_BadChars == null)
        {
            _BadChars = new List<char>(Path.GetInvalidFileNameChars());
            _BadChars.AddRange(Path.GetInvalidPathChars());
            _BadChars = Utility.GetUnique<char>(_BadChars);
        }
        
        // remove root
        string root = Path.GetPathRoot(path);
        path = path.Remove(0, root.Length);
        
        // split on the directory separator character. Need to do this
        // because the separator is not valid in a filename.
        List<string> parts = new List<string>(path.Split(new char[]{Path.DirectorySeparatorChar}));

        // check each part to make sure it is valid.
        for (int i = 0; i < parts.Count; i++)
        {
            string part = parts[i];
            foreach (char c in _BadChars)
            {
                part = part.Replace(c, replaceChar);
            }
            parts[i] = part;
        }

        return root + Utility.Join(parts, Path.DirectorySeparatorChar.ToString());
    }

Any improvements to make this function faster and less baroque would be much appreciated.

To clean up a file name you could do this

private static string MakeValidFileName( string name )
{
   string invalidChars = System.Text.RegularExpressions.Regex.Escape( new string( System.IO.Path.GetInvalidFileNameChars() ) );
   string invalidRegStr = string.Format( @"([{0}]*\.+$)|([{0}]+)", invalidChars );

   return System.Text.RegularExpressions.Regex.Replace( name, invalidRegStr, "_" );
}

A shorter solution:

var invalids = System.IO.Path.GetInvalidFileNameChars();
var newName = String.Join("_", origFileName.Split(invalids, StringSplitOptions.RemoveEmptyEntries) ).TrimEnd('.');

Based on Andre's excellent answer but taking into account Spud's comment on reserved words, I made this version:

/// <summary>
/// Strip illegal chars and reserved words from a candidate filename (should not include the directory path)
/// </summary>
/// <remarks>
/// http://stackoverflow.com/questions/309485/c-sharp-sanitize-file-name
/// </remarks>
public static string CoerceValidFileName(string filename)
{
    var invalidChars = Regex.Escape(new string(Path.GetInvalidFileNameChars()));
    var invalidReStr = string.Format(@"[{0}]+", invalidChars);

    var reservedWords = new []
    {
        "CON", "PRN", "AUX", "CLOCK$", "NUL", "COM0", "COM1", "COM2", "COM3", "COM4",
        "COM5", "COM6", "COM7", "COM8", "COM9", "LPT0", "LPT1", "LPT2", "LPT3", "LPT4",
        "LPT5", "LPT6", "LPT7", "LPT8", "LPT9"
    };

    var sanitisedNamePart = Regex.Replace(filename, invalidReStr, "_");
    foreach (var reservedWord in reservedWords)
    {
        var reservedWordPattern = string.Format("^{0}\\.", reservedWord);
        sanitisedNamePart = Regex.Replace(sanitisedNamePart, reservedWordPattern, "_reservedWord_.", RegexOptions.IgnoreCase);
    }

    return sanitisedNamePart;
}

And these are my unit tests

[Test]
public void CoerceValidFileName_SimpleValid()
{
    var filename = @"thisIsValid.txt";
    var result = PathHelper.CoerceValidFileName(filename);
    Assert.AreEqual(filename, result);
}

[Test]
public void CoerceValidFileName_SimpleInvalid()
{
    var filename = @"thisIsNotValid\3\\_3.txt";
    var result = PathHelper.CoerceValidFileName(filename);
    Assert.AreEqual("thisIsNotValid_3__3.txt", result);
}

[Test]
public void CoerceValidFileName_InvalidExtension()
{
    var filename = @"thisIsNotValid.t\xt";
    var result = PathHelper.CoerceValidFileName(filename);
    Assert.AreEqual("thisIsNotValid.t_xt", result);
}

[Test]
public void CoerceValidFileName_KeywordInvalid()
{
    var filename = "aUx.txt";
    var result = PathHelper.CoerceValidFileName(filename);
    Assert.AreEqual("_reservedWord_.txt", result);
}

[Test]
public void CoerceValidFileName_KeywordValid()
{
    var filename = "auxillary.txt";
    var result = PathHelper.CoerceValidFileName(filename);
    Assert.AreEqual("auxillary.txt", result);
}

string clean = String.Concat(dirty.Split(Path.GetInvalidFileNameChars()));

there are a lot of working solutions here. just for the sake of completeness, here's an approach that doesn't use regex, but uses LINQ:

var invalids = Path.GetInvalidFileNameChars();
filename = invalids.Aggregate(filename, (current, c) => current.Replace(c, '_'));

Also, it's a very short solution ;)

I'm using the System.IO.Path.GetInvalidFileNameChars() method to check invalid characters and I've got no problems.

I'm using the following code:

foreach( char invalidchar in System.IO.Path.GetInvalidFileNameChars())
{
    filename = filename.Replace(invalidchar, '_');
}

I wanted to retain the characters in some way, not just simply replace the character with an underscore.

One way I thought was to replace the characters with similar looking characters which are (in my situation), unlikely to be used as regular characters. So I took the list of invalid characters and found look-a-likes.

The following are functions to encode and decode with the look-a-likes.

This code does not include a complete listing for all System.IO.Path.GetInvalidFileNameChars() characters. So it is up to you to extend or utilize the underscore replacement for any remaining characters.

private static Dictionary<string, string> EncodeMapping()
{
	//-- Following characters are invalid for windows file and folder names.
	//-- \/:*?"<>|
	Dictionary<string, string> dic = new Dictionary<string, string>();
	dic.Add(@"\", "Ì"); // U+OOCC
	dic.Add("/", "Í"); // U+OOCD
	dic.Add(":", "¦"); // U+00A6
	dic.Add("*", "¤"); // U+00A4
	dic.Add("?", "¿"); // U+00BF
	dic.Add(@"""", "ˮ"); // U+02EE
	dic.Add("<", "«"); // U+00AB
	dic.Add(">", "»"); // U+00BB
	dic.Add("|", "│"); // U+2502
	return dic;
}

public static string Escape(string name)
{
	foreach (KeyValuePair<string, string> replace in EncodeMapping())
	{
		name = name.Replace(replace.Key, replace.Value);
	}

	//-- handle dot at the end
	if (name.EndsWith(".")) name = name.CropRight(1) + "°";

	return name;
}

public static string UnEscape(string name)
{
	foreach (KeyValuePair<string, string> replace in EncodeMapping())
	{
		name = name.Replace(replace.Value, replace.Key);
	}

	//-- handle dot at the end
	if (name.EndsWith("°")) name = name.CropRight(1) + ".";

	return name;
}

You can select your own look-a-likes. I used the Character Map app in windows to select mine %windir%\system32\charmap.exe

As I make adjustments through discovery, I will update this code.

I think the problem is that you first call Path.GetDirectoryName on the bad string. If this has non-filename characters in it, .Net can't tell which parts of the string are directories and throws. You have to do string comparisons.

Assuming it's only the filename that is bad, not the entire path, try this:

public static string SanitizePath(string path, char replaceChar)
{
int filenamePos = path.LastIndexOf(Path.DirectorySeparatorChar) + 1;
var sb = new System.Text.StringBuilder();
sb.Append(path.Substring(0, filenamePos));
for (int i = filenamePos; i < path.Length; i++)
{
char filenameChar = path[i];
foreach (char c in Path.GetInvalidFileNameChars())
if (filenameChar.Equals(c))
{
filenameChar = replaceChar;
break;
}



    sb.Append(filenameChar);
}

return sb.ToString();




}
}

I have had success with this in the past.

Nice, short and static :-)

    public static string returnSafeString(string s)
    {
        foreach (char character in Path.GetInvalidFileNameChars())
        {
            s = s.Replace(character.ToString(),string.Empty);
        }

        foreach (char character in Path.GetInvalidPathChars())
        {
            s = s.Replace(character.ToString(), string.Empty);
        }

        return (s);
    }

Here's an efficient lazy loading extension method based on Andre's code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LT
{
    public static class Utility
    {
        static string invalidRegStr;

        public static string MakeValidFileName(this string name)
        {
            if (invalidRegStr == null)
            {
                var invalidChars = System.Text.RegularExpressions.Regex.Escape(new string(System.IO.Path.GetInvalidFileNameChars()));
                invalidRegStr = string.Format(@"([{0}]*\.+$)|([{0}]+)", invalidChars);
            }

            return System.Text.RegularExpressions.Regex.Replace(name, invalidRegStr, "_");
        }
    }
}

Your code would be cleaner if you appended the directory and filename together and sanitized that rather than sanitizing them independently. As for sanitizing away the :, just take the 2nd character in the string. If it is equal to "replacechar", replace it with a colon. Since this app is for your own use, such a solution should be perfectly sufficient.

using System;
using System.IO;
using System.Linq;
using System.Text;

public class Program
{
    public static void Main()
    {
        try
        {
            var badString = "ABC\\DEF/GHI<JKL>MNO:PQR\"STU\tVWX|YZA*BCD?EFG";
            Console.WriteLine(badString);
            Console.WriteLine(SanitizeFileName(badString, '.'));
            Console.WriteLine(SanitizeFileName(badString));
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }
    }

    private static string SanitizeFileName(string fileName, char? replacement = null)
    {
        if (fileName == null) { return null; }
        if (fileName.Length == 0) { return ""; }

        var sb = new StringBuilder();
        var badChars = Path.GetInvalidFileNameChars().ToList();

        foreach (var @char in fileName)
        {
            if (badChars.Contains(@char)) 
            {
                if (replacement.HasValue)
                {
                    sb.Append(replacement.Value);
                }
                continue; 
            }
            sb.Append(@char);
        }
        return sb.ToString();
    }
}

Based @fiat's and @Andre's approach, I'd like to share my solution too. Main difference:

• its an extension method
• regex is compiled at first use to save some time with a lot executions
• reserved words are preserved

public static class StringPathExtensions
{
	private static Regex _invalidPathPartsRegex;
	
	static StringPathExtensions()
	{
		var invalidReg = System.Text.RegularExpressions.Regex.Escape(new string(Path.GetInvalidFileNameChars()));
		_invalidPathPartsRegex = new Regex($"(?<reserved>^(CON|PRN|AUX|CLOCK\\$|NUL|COM0|COM1|COM2|COM3|COM4|COM5|COM6|COM7|COM8|COM9|LPT0|LPT1|LPT2|LPT3|LPT4|LPT5|LPT6|LPT7|LPT8|LPT9))|(?<invalid>[{invalidReg}:]+|\\.$)", RegexOptions.Compiled);
	}

	public static string SanitizeFileName(this string path)
	{
		return _invalidPathPartsRegex.Replace(path, m =>
		{
			if (!string.IsNullOrWhiteSpace(m.Groups["reserved"].Value))
				return string.Concat("_", m.Groups["reserved"].Value);
			return "_";
		});
	}
}