IAsyncStateMachine 如何管理 MethodBuilder 上的多个等待者？

Question

我想了解这 4 个部分是如何组合在一起的：

IAwaiter
IAwaitable
IAsyncMethodBuilder
IAsyncStateMachine

我不明白 IAsyncMethodBuilder 等待者和状态机之间的关系。 如果我的方法生成器接收到让我们说 2 个等待者，为什么状态机在它的肚子里只有一个它使用 Get/Set 结果的等待者？ 我说的是 methodbuilder 上的等待者：

var a= await MyMethod(); 
var b=await MyMethod(); 

其中 MyMethod定义为：

async  Task<T> MyMethod(){
await f1()   ->don't care about thsese
await f2()   -----/-----
await f3()   -----/------
........
}

我会post迪欣博客的两段代码https://weblogs.asp.net/dixin/functional-csharp-asynchronous-function:

用户编写的代码：

internal static async Task<T> Async<T>(T value)
{
    T value1 = Start(value);
    T result1 = await Async1(value1);
    T value2 = Continuation1(result1);
    T result2 = await Async2(value2);
    T value3 = Continuation2(result2);
    T result3 = await Async3(value3);
    T result = Continuation3(result3);
    return result;
}

internal static T Start<T>(T value) => value;

internal static Task<T> Async1<T>(T value) => Task.Run(() => value);

internal static T Continuation1<T>(T value) => value;

internal static Task<T> Async2<T>(T value) => Task.FromResult(value);

internal static T Continuation2<T>(T value) => value;

internal static Task<T> Async3<T>(T value) => Task.Run(() => value);

internal static T Continuation3<T>(T value) => value;

编译器生成的内容：

[CompilerGenerated]
[StructLayout(LayoutKind.Auto)]
private struct AsyncStateMachine<TResult> : IAsyncStateMachine
{
    public int State;

    public AsyncTaskMethodBuilder<TResult> Builder;

    public TResult Value;

    private TaskAwaiter<TResult> awaiter;  //Why only one?

    void IAsyncStateMachine.MoveNext()
    {
        TResult result;
        try
        {
            switch (this.State)
            {
                case -1: // Start code from the beginning to the 1st await.
                    // Workflow begins.
                    TResult value1 = Start(this.Value);
                    this.awaiter = Async1(value1).GetAwaiter();
                    if (this.awaiter.IsCompleted)
                    {
                        // If the task returned by Async1 is already completed, immediately execute the continuation.
                        goto case 0;
                    }
                    else
                    {
                        this.State = 0;
                        // If the task returned by Async1 is not completed, specify the continuation as its callback.
                        this.Builder.AwaitUnsafeOnCompleted(ref this.awaiter, ref this);
                        // Later when the task returned by Async1 is completed, it calls back MoveNext, where State is 0.
                        return;
                    }
                case 0: // Continuation code from after the 1st await to the 2nd await.
                    // The task returned by Async1 is completed. The result is available immediately through GetResult.
                    TResult result1 = this.awaiter.GetResult();
                    TResult value2 = Continuation1(result1);
                    this.awaiter = Async2(value2).GetAwaiter();
                    if (this.awaiter.IsCompleted)
                    {
                        // If the task returned by Async2 is already completed, immediately execute the continuation.
                        goto case 1;
                    }
                    else
                    {
                        this.State = 1;
                        // If the task returned by Async2 is not completed, specify the continuation as its callback.
                        this.Builder.AwaitUnsafeOnCompleted(ref this.awaiter, ref this);
                        // Later when the task returned by Async2 is completed, it calls back MoveNext, where State is 1.
                        return;
                    }
                case 1: // Continuation code from after the 2nd await to the 3rd await.
                    // The task returned by Async2 is completed. The result is available immediately through GetResult.
                    TResult result2 = this.awaiter.GetResult();
                    TResult value3 = Continuation2(result2);
                    this.awaiter = Async3(value3).GetAwaiter();
                    if (this.awaiter.IsCompleted)
                    {
                        // If the task returned by Async3 is already completed, immediately execute the continuation.
                        goto case 2;
                    }
                    else
                    {
                        this.State = 2;
                        // If the task returned by Async3 is not completed, specify the continuation as its callback.
                        this.Builder.AwaitUnsafeOnCompleted(ref this.awaiter, ref this);
                        // Later when the task returned by Async3 is completed, it calls back MoveNext, where State is 1.
                        return;
                    }
                case 2: // Continuation code from after the 3rd await to the end.
                    // The task returned by Async3 is completed. The result is available immediately through GetResult.
                    TResult result3 = this.awaiter.GetResult();
                    result = Continuation3(result3);
                    this.State = -2; // -2 means end.
                    this.Builder.SetResult(result);
                    // Workflow ends.
                    return;
            }
        }
        catch (Exception exception)
        {
            this.State = -2; // -2 means end.
            this.Builder.SetException(exception);
        }
    }

    [DebuggerHidden]
    void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine asyncStateMachine) =>
        this.Builder.SetStateMachine(asyncStateMachine);
}

鉴于我在开头编写的示例，AsyncStateMachine 不应该有一个等待者列表吗？如果我在 methodbuilder 上有 N 个等待者，机器如何将 SetResult 传播给它的所有等待者？

Answer 1

状态机一次只能等待一个可等待的操作。可能涉及多个操作，但一次只能在一个等待点。因此，如果这些等待者是同一类型，则只需要一个字段来等待它。

如果同一个方法中有不同类型的等待者，我相信您会看到每种等待者类型一个字段。 （编译器可能会为所有等待者使用单个 object 字段，并在继续触发时将其适当地转换回去，但这会带来不同的问题，特别是当等待者是值类型时。）

这是一个例子：

using System;
using System.Threading.Tasks;

class Test
{
    static async Task FooAsync()
    {
        await Bar<int>();
        await Bar<string>();
        await Task.Delay(1000);
        await Bar<string>();
        await Task.Yield();
    }

    static Task<T> Bar<T>() => Task.FromResult(default(T));
}

这里我们在以下状态机中得到等待者字段：

TaskAwaiter<int> <>u__1;            // From the Bar<int> call
TaskAwaiter<string> <>u__2;         // From both Bar<string> calls
TaskAwaiter <>u__3;                 // From Task.Delay
YieldAwaitable.YieldAwaiter <>u__4; // From Task.Yield