Shows how to make any class "awaitable".

Awaitable.cs

using System;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;

namespace AwaitTest
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			MainAsync().Wait();

			Console.ReadLine();
		}

		async static Task MainAsync()
		{
			var foo = new Foo();

			// We can "await" foo - but it's not a Task, nor does it implement some "IAwaiter" or "IAwaitable" interface.
			// It simply follows some conventions which make Foo implicitly awaitable.
			await foo;
		}
	}


	// To be "awaitable":
	// - Interface INotifyCompletion must be implemented
	// - A GetResult() method must be there (returning void or an actual result)
	// - it must have a bool IsCompleted property.
	//
	// The implementation is just meant to demonstrate how something can be made awaitable!
	// It is not doing anything async!
	//
	// Good reference with more information: http://weblogs.asp.net/dixin/understanding-c-sharp-async-await-2-awaitable-awaiter-pattern
	public class Foo : INotifyCompletion
	{
		#region INotifyCompletion implementation

		public void OnCompleted (Action continuation)
		{
			this.IsCompleted = true;
			continuation?.Invoke();
		}

		#endregion

		public bool IsCompleted
		{
			get;
			private set;
		}

		public void GetResult()
		{
			Console.WriteLine("We're done!");
		}

		public Foo GetAwaiter()
		{
			// Foo itself is the awaitable.
			return this;
		}
	}
}