Monitor.Enter pumps some messages including WM_PAINT

WinFormsApp.cs

// https://github.com/mono/SkiaSharp/issues/1383
// based on https://gist.github.com/retran/b57e4db1a173048c2cee49ac6d523fc2

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();
        }

        [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) =>
            {
                var start = System.Environment.TickCount;

                Task.Run(() =>
                {
                    Debug.Assert(Thread.CurrentThread.ManagedThreadId != mainThreadId);
                    lock (sync)
                    {
                        // Will send WM_PAINT if called from a non-main thread.
                        Thread.Sleep(250);
                        form.Invalidate();
                        Thread.Sleep(2000);
                    }
                });

                throwOnPaint = true;
                try
                {
                    Thread.Sleep(500);
                    // acquiring the lock shouldn't be pumping,
                    // but it does!
                    lock (sync)  
                    {
                        Debug.Assert(System.Environment.TickCount - start >= 2250);
                    }
                }
                finally
                {
                    throwOnPaint = false;
                }
            };

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