CodeSigner/Program.cs

const string AuthorName = "Syntriax";
const string AuthorEmail = "Syntriax@gmail.com";

const string FileSearchPattern = "*.cs";
const string ProgramSignature = $"Signed by an automated program written by Syntriax";
const string CreationDateTag = "<CreationDate>";
const string Signature =
$@"/*
    Author: {AuthorName}
    Email: {AuthorEmail}
    Creation Date: {CreationDateTag} UTC
    {ProgramSignature}
*/
";

void ExploreDirectory(string path)
{
    string[] files = Directory.GetFiles(path, FileSearchPattern);
    string[] directories = Directory.GetDirectories(path);

    foreach (string filePath in files)
        Sign(filePath);

    foreach (string directoryPath in directories)
        ExploreDirectory(directoryPath);
}

void Sign(string filePath)
{
    string[] lines = File.ReadAllLines(filePath);

    foreach (string line in lines)
        if (line.Contains(ProgramSignature))
            return;

    Console.WriteLine($"Signing -> {filePath}");
    DateTime dateTime = File.GetCreationTimeUtc(filePath);
    string newContents = Signature.Replace(CreationDateTag, $"{dateTime.ToLongDateString()} - {dateTime.ToLongTimeString()}");
    newContents += File.ReadAllText(filePath);
    File.WriteAllText(filePath, newContents);
}

ExploreDirectory(Environment.GetCommandLineArgs()[1]);
Console.ReadKey();