WM_PAINT reentrancy

Program.cs

using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WutExample
{

    static class Program
    {
        class MyVeryCustomException : Exception { }

        private static readonly object sync = new object();

        static void Throw()
        {
            throw new MyVeryCustomException();
        }

        static void Lock()
        {
            lock (sync) { }
        }

        [STAThread]
        static void Main()
        {
            var mainThreadId = Thread.CurrentThread.ManagedThreadId;
            bool throwOnPaint = false;
            var form = new Form();
            form.Paint += (sender, args) =>
            {
                Debug.Assert(Thread.CurrentThread.ManagedThreadId == mainThreadId);

                if (throwOnPaint)
                {
                    Throw();
                }
            };

            var button = new Button { Text = "Click Me" };
            button.Click += (sender, args) =>
            {
                Task.Run(() =>
                {
                    Debug.Assert(Thread.CurrentThread.ManagedThreadId != mainThreadId);
                    lock (sync)
                    {
                        Thread.CurrentThread.Join(500);
                        // Will send WM_PAINT if called from a non-main thread.
                        form.Invalidate();
                    }
                });

                try
                {
                    Debug.Assert(Thread.CurrentThread.ManagedThreadId == mainThreadId);
                    throwOnPaint = true;
                    Thread.CurrentThread.Join(100);
                    Lock();
                }
                finally
                {
                    throwOnPaint = false;
                }
            };

            form.Controls.Add(button);
            Application.Run(form);
        }
    }
}