Hi, I'm Ray

C#: Handling invalid characters and duplicate filenames

Programming .NET C#

Posted on Thu 8th Jul 2021 and modified on Sat 12th Mar 2022

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This is a quick one. Whenever I’m saving a file, I’m checking for two things: invalid characters in the filename and whether that filename already exists. You might be relying on one of the dialog components to help you, which is fine. I find myself often writing bulk-exports or filename suggestions, so this helps me.

These are a couple of functions that help me out.

I can never seem to find them when I need them, so I’m continually writing them again. But enough is enough, I’m putting it down on paper.

Let’s get started.

1. Replacing invalid characters
This replaces invalid characters with a “-” and returns “untitled”, if the filename is blank.

public string CleanFilename(string value, string defaultValue = "untitled", string replaceChar = "-")
    if (string.IsNullOrWhiteSpace(value)) return defaultValue;

    return string.Join(replaceChar, value.Replace(".", replaceChar).Split(Path.GetInvalidFileNameChars()));


2. Auto-naming for duplicate filenames
This adds a number (in brackets) at the end of duplicate filenames, starting at one. So, you’ll geta series of: “filename”, “filename (1)” and “filename (2)”.

public string AutoFilename(string filename)
    if (!File.Exists(filename)) return filename;

    string dir = Path.GetDirectoryName(filename);
    string name = Path.GetFileNameWithoutExtension(filename);
    string ext = Path.GetExtension(filename);
    int n = 0;

    string newFilename = null;

    while (true)
        newFilename = string.Concat(dir, "\\", name, " (", n.ToString(), ")", ext);

        if (!File.Exists(newFilename))

    return newFilename;


Well, that’s that.

I hope someone finds this useful.