Enable nullable reference types on old projects

gistfile1.txt

using System;
using System.IO;
using System.Linq;
using System.Windows.Forms;

namespace EnableNullableReferenceTypes
{
    /*

    Add the following to the .csproj file:

    <PropertyGroup>
        ...
        <LangVersion>8.0</LangVersion>
    </PropertyGroup>

    <Project>
        ...
        <PropertyGroup>
            <PreBuildEvent>"\Desktop\dev\cs\EnableNullableReferenceTypes\EnableNullableReferenceTypes\bin\Debug\EnableNullableReferenceTypes.exe" $(ProjectDir)</PreBuildEvent>
        </PropertyGroup>
    </Project>

    */

    class Program
    {
        static void Main(string[] args)
        {
            var files = Directory.EnumerateFiles(args[0], "*.cs", SearchOption.AllDirectories);

            files
                .ToList()
                .ForEach(filename =>
                {
                    if (Path.GetFileName(filename).StartsWith("TemporaryGeneratedFile")) return;
                    if (Path.GetFileName(filename).Contains(".Designer.cs")) return;

                    try
                    {
                        var firstLine = File.ReadLines(filename).First();

                        if (!firstLine.Equals("#nullable enable"))
                        {
                            var oldText = File.ReadAllText(filename);
                            var newText = "#nullable enable" + Environment.NewLine + oldText;
                            File.WriteAllText(filename, newText);
                        }
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show($"Error while processing: {filename}{Environment.NewLine}{Environment.NewLine}{ex}");
                    }
                });

        }
    }
}