C#异步编程由浅入深(三)之详解Awaiter

namespace System.Runtime.CompilerServices
{
    /// <summary>
    /// Represents an operation that will schedule continuations when the operation completes.
    /// </summary>
    public interface INotifyCompletion
    {
        /// <summary>Schedules the continuation action to be invoked when the instance completes.</summary>
        /// <param name="continuation">The action to invoke when the operation completes.</param>
        /// <exception cref="System.ArgumentNullException">The <paramref name="continuation"/> argument is null (Nothing in Visual Basic).</exception>
        void OnCompleted(Action continuation);
    }
}

public static async Task AwaiterTest()
{
    Console.WriteLine($"是否是线程池线程？{Thread.CurrentThread.IsThreadPoolThread}");
    await default (SkipToThreadPoolAwaiter);
    Console.WriteLine($"是否是线程池线程？{Thread.CurrentThread.IsThreadPoolThread}");
}

static void Main(string[] args)
{
    _ = AwaiterTest();
    Console.ReadLine();
}

public struct SkipToThreadPoolAwaiter : INotifyCompletion
{
    public bool IsCompleted => false;
    public void GetResult() 
    {
        Console.WriteLine("调用GetResult以获取结果");
    }
    public void OnCompleted(Action continuation)
    {
        Console.WriteLine("调用OnCompleted，把Await后面部分要执行的代码传递过来（传递MoveNext，以推动状态机流转）");
        ThreadPool.QueueUserWorkItem(state =>
        {
            Console.WriteLine("开始执行Await后面部分的代码");
            continuation();
            Console.WriteLine("后面部分的代码执行完毕");
        });
        Console.WriteLine("返回调用线程");
    }

    public SkipToThreadPoolAwaiter GetAwaiter()
    {
        Console.WriteLine("获得Awaiter");
        return this;
    }
}

public static async Task AwaiterTest()
{
    Console.WriteLine($"是否是线程池线程？{Thread.CurrentThread.IsThreadPoolThread}");
    var res = await default (SkipToThreadPoolAwaiter);
    Console.WriteLine($"结果是{res}");
}

static void Main(string[] args)
    _ = AwaiterTest();
    Console.ReadLine();
public struct SkipToThreadPoolAwaiter : INotifyCompletion
    public bool IsCompleted => false;
    public int GetResult() 
    {
        Console.WriteLine("调用GetResult以获取结果");
        return 1;
    }
    public void OnCompleted(Action continuation)
        Console.WriteLine("调用OnCompleted，把Await后面部分要执行的代码传递过来（传递MoveNext，以推动状态机流转）");
        ThreadPool.QueueUserWorkItem(state =>
        {
            Console.WriteLine("开始执行Await后面部分的代码");
            continuation();
            Console.WriteLine("后面部分的代码执行完毕");
        });
        Console.WriteLine("返回调用线程");
    public SkipToThreadPoolAwaiter GetAwaiter()
        Console.WriteLine("获得Awaiter");
        return this;