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($"是否是線程池線程？{Thread.CurrentThread.IsThreadPoolThread}");
    await default (SkipToThreadPoolAwaiter);
    Console.WriteLine($"是否是線程池線程？{Thread.CurrentThread.IsThreadPoolThread}");
}

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

public struct SkipToThreadPoolAwaiter : INotifyCompletion
{
    public bool IsCompleted => false;
    public void GetResult() 
    {
        Console.WriteLine("調(diào)用GetResult以獲取結(jié)果");
    }
    public void OnCompleted(Action continuation)
    {
        Console.WriteLine("調(diào)用OnCompleted，把Await后面部分要執(zhí)行的代碼傳遞過來（傳遞MoveNext，以推動(dòng)狀態(tài)機(jī)流轉(zhuǎn)）");
        ThreadPool.QueueUserWorkItem(state =>
        {
            Console.WriteLine("開始執(zhí)行Await后面部分的代碼");
            continuation();
            Console.WriteLine("后面部分的代碼執(zhí)行完畢");
        });
        Console.WriteLine("返回調(diào)用線程");
    }

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

public static async Task AwaiterTest()
{
    Console.WriteLine($"是否是線程池線程？{Thread.CurrentThread.IsThreadPoolThread}");
    var res = await default (SkipToThreadPoolAwaiter);
    Console.WriteLine($"結(jié)果是{res}");
}

static void Main(string[] args)
    _ = AwaiterTest();
    Console.ReadLine();
public struct SkipToThreadPoolAwaiter : INotifyCompletion
    public bool IsCompleted => false;
    public int GetResult() 
    {
        Console.WriteLine("調(diào)用GetResult以獲取結(jié)果");
        return 1;
    }
    public void OnCompleted(Action continuation)
        Console.WriteLine("調(diào)用OnCompleted，把Await后面部分要執(zhí)行的代碼傳遞過來（傳遞MoveNext，以推動(dòng)狀態(tài)機(jī)流轉(zhuǎn)）");
        ThreadPool.QueueUserWorkItem(state =>
        {
            Console.WriteLine("開始執(zhí)行Await后面部分的代碼");
            continuation();
            Console.WriteLine("后面部分的代碼執(zhí)行完畢");
        });
        Console.WriteLine("返回調(diào)用線程");
    public SkipToThreadPoolAwaiter GetAwaiter()
        Console.WriteLine("獲得Awaiter");
        return this;