TaskExtensions.cs

using System;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Windows.Forms;

public static class TaskExtensions
{
	/// <summary>
	/// Creates an awaitable task that asynchronously yields back to the current context of the control.
	/// </summary>
	/// <param name="control"></param>
	/// <returns>A context that, when awaited, will asynchronously transition back into the
	///     context of the control.</returns>
	public static ControlYieldAwaitable Yield(this Control control)
	{
		return new ControlYieldAwaitable(control);
	}

	public struct ControlYieldAwaitable
	{
		private readonly Control control;

		public ControlYieldAwaitable(Control control)
		{
			this.control = control;
		}

		public YieldAwaiter GetAwaiter()
		{
			return new YieldAwaiter(this.control);
		}

		public readonly struct YieldAwaiter : ICriticalNotifyCompletion, INotifyCompletion
		{
			private readonly SynchronizationContext syncContext;
			private static readonly SendOrPostCallback sendOrPostCallbackRunAction = RunAction;

			public YieldAwaiter(Control control)
			{
				this.syncContext = GetSynchronizationContext(control);
			}

			public bool IsCompleted => this.syncContext == SynchronizationContext.Current;

			public void OnCompleted(Action continuation)
			{
				this.syncContext.Post(sendOrPostCallbackRunAction, continuation);
			}

			public void UnsafeOnCompleted(Action continuation)
			{
				this.syncContext.Post(sendOrPostCallbackRunAction, continuation);
			}

			public void GetResult()
			{
			}

			private static SynchronizationContext GetSynchronizationContext(Control control)
			{
				if (control.InvokeRequired)
					return (SynchronizationContext)control.Invoke(new Func<Control, SynchronizationContext>(GetSynchronizationContext), control);

				return SynchronizationContext.Current;
			}

			private static void RunAction(object state) => ((Action)state).Invoke();
		}
	}
}