自定义一个awaiter 代码来自网络

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

class TestAwaiter<T> : INotifyCompletion {
    private T result;
    private Action continuation;

    // INotifyCompletion Implement
    public void OnCompleted(Action continuation) { this.continuation = continuation; }

    // Compiler Call Methods
    public bool IsCompleted { get; private set; }
    public T GetResult() { return result; }
    public TestAwaiter<T> GetAwaiter() { return this; }

    // Self Call Methods
    public void SetResult(T ret) {
        result = ret;
        if (continuation != null) {
            continuation();
        }
    }
}

class Test {
    public static void Main(string[] args) {
        Task.Run(new Action(task1));
        Console.ReadLine();
    }

    private async static void task1() {
        Console.WriteLine("Begin await:");
        int ret = await testAwaiter();
        Console.WriteLine("Await Task Result:" + ret);
    }

    private static TestAwaiter<int> testAwaiter() {
        TestAwaiter<int> awaiter = new TestAwaiter<int>();
        ThreadPool.QueueUserWorkItem(_ => {
            Thread.Sleep(3000);
            awaiter.SetResult(100);
        });
        return awaiter;
    }
}