C# Helper To See If There Is A Directory Traversal Attempt on a File

IsDirectoryTraversing.cs

/// <summary>
/// This is to guage whether someone is trying a directory traversal attack.
/// I.e. they should be requesting 'somefilename.pdf'
/// but they request '../anotherlocation/someotherfilename.pdf
/// </summary>
/// <param name="fileName">The file name to check</param>
/// <returns></returns>
public bool IsDirectoryTraversing(string fileName)
{
    bool isTraversing = false;

    if(String.IsNullOrWhiteSpace(fileName))
    {
        return isTraversing;
    }

    // Url decode to reveal sneaky encoded attempts e.g. '%2f' (/) or '%2e%2e%2f' (../)
    var decodedFileName = HttpUtility.UrlDecode(fileName);

    if(decodedFileName.Contains("/") || 
        decodedFileName.Contains(@"\") ||
        decodedFileName.Contains("$") ||
        decodedFileName.Contains("..") ||
        decodedFileName.Contains("?"))
    {
        isTraversing = true;
    }

    return isTraversing;
}