.Net C# async await 原理解析

.Net C# async await 原理解析,园子里的相关优秀文章已经有很多了,我写这篇文章完全是想以自己的思维方式来谈一谈自己的理解。(PS:文中涉及到了大量反编译源码,需要静下心来细细品味)

.Net C# async await 原理解析,一个老掉牙的话题,园子里的相关优秀文章已经有很多了,我写这篇文章完全是想以自己的思维方式来谈一谈自己的理解。(PS:文中涉及到了大量反编译源码,需要静下心来细细品味)

1|0从简单开始

为了更容易理解这个问题,我们举一个简单的例子:用异步的方式在控制台上分两步输出“Hello World!”,我这边使用的是Framework 4.5.2

class Program
{
    static async Task Main(string[] args)
    {
        Console.WriteLine("Let's Go!");
    
        await TestAsync();
        
        Console.Write(" World!");
    }

    static Task TestAsync()
    {
        return Task.Run(() =>
        {
            Console.Write("Hello");
        });
    }
}

2|0探究反编译后的源码

接下来我们使用 .NET reflector (也可使用 dnSpy 等) 反编译一下程序集,然后一步一步来探究 async await 内部的奥秘。

2|1Main方法

[DebuggerStepThrough]
private static void <Main>(string[] args)
{
    Main(args).GetAwaiter().GetResult();
}

[AsyncStateMachine(typeof(<Main>d__0)), DebuggerStepThrough]
private static Task Main(string[] args)
{
    <Main>d__0 stateMachine = new <Main>d__0 
    {
        <>t__builder = AsyncTaskMethodBuilder.Create(),
        args = args,
        <>1__state = -1
    };
    stateMachine.<>t__builder.Start<<Main>d__0>(ref stateMachine);
    return stateMachine.<>t__builder.Task;
}

// 实现了 IAsyncStateMachine 接口
[CompilerGenerated]
private sealed class <Main>d__0 : IAsyncStateMachine
{
    // Fields
    public int <>1__state;
    public AsyncTaskMethodBuilder <>t__builder;
    public string[] args;
    private TaskAwaiter <>u__1;

    // Methods
    private void MoveNext() { }
    [DebuggerHidden]
    private void SetStateMachine(IAsyncStateMachine stateMachine) { }
}

卧槽!竟然有两个 Main 方法:一个同步、一个异步。原来,虽然我们写代码时为了在 Main 方法中方便异步等待,将 void Main 改写成了async Task Main,但是实际上程序入口仍是我们熟悉的那个 void Main。

另外,我们可以看到异步 Main 方法被标注了AsyncStateMachine特性,这是因为在我们的源代码中,该方法带有修饰符async,表示该方法是一个异步方法。

好,我们先看一下异步Main方法内部实现,它主要做了三件事:

  1. 首先,创建了一个类型为<Main>d__0的状态机 stateMachine,并初始化了公共变量 <>t__builder、args、<>1__state = -1
    • <>t__builder:负责异步相关的操作,是实现异步 Main 方法异步的核心
    • <>1__state:状态机的当前状态
  2. 然后,调用Start方法,借助 stateMachine, 来执行我们在异步 Main 方法中写的代码
  3. 最后,将指示异步 Main 方法运行状态的Task对象返回出去

2|2Start

首先,我们先来看一下Start的内部实现

// 所属结构体:AsyncTaskMethodBuilder

[SecuritySafeCritical, DebuggerStepThrough, __DynamicallyInvokable]
public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine: IAsyncStateMachine
{
    if (((TStateMachine) stateMachine) == null)
    {
        throw new ArgumentNullException("stateMachine");
    }
    ExecutionContextSwitcher ecsw = new ExecutionContextSwitcher();
    RuntimeHelpers.PrepareConstrainedRegions();
    try
    {
        ExecutionContext.EstablishCopyOnWriteScope(ref ecsw);
        // 状态机状态流转
        stateMachine.MoveNext();
    }
    finally
    {
        ecsw.Undo();
    }
}

我猜,你只能看懂stateMachine.MoveNext(),对不对?好,那我们就来看看这个状态机类<Main>d__0,并且着重看它的方法MoveNext

2|3MoveNext

[CompilerGenerated]
private sealed class <Main>d__0 : IAsyncStateMachine
{
    // Fields
    public int <>1__state;
    public AsyncTaskMethodBuilder <>t__builder;
    public string[] args;
    private TaskAwaiter <>u__1;

    // Methods
    private void MoveNext()
    {
        // 在 Main 方法中,我们初始化 <>1__state = -1,所以此时 num = -1
        int num = this.<>1__state;
        try
        {
            TaskAwaiter awaiter;
            if (num != 0)
            {
                Console.WriteLine("Let's Go!");
                // 调用 TestAsync(),获取 awaiter,用于后续监控 TestAsync() 运行状态
                awaiter = Program.TestAsync().GetAwaiter();
                
                // 一般来说,异步任务不会很快就完成,所以大多数情况下都会进入该分支
                if (!awaiter.IsCompleted)
                {
                    // 状态机状态从 -1 流转为 0
                    this.<>1__state = num = 0;
                    this.<>u__1 = awaiter;
                    Program.<Main>d__0 stateMachine = this;
                    // 配置  TestAsync() 完成后的延续
                    this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<Main>d__0>(ref awaiter, ref stateMachine);
                    return;
                }
            }
            else
            {
                awaiter = this.<>u__1;
                this.<>u__1 = new TaskAwaiter();
                this.<>1__state = num = -1;
            }
            awaiter.GetResult();
            Console.Write(" World!");
        }
        catch (Exception exception)
        {
            this.<>1__state = -2;
            this.<>t__builder.SetException(exception);
            return;
        }
        this.<>1__state = -2;
        this.<>t__builder.SetResult();
    }

    [DebuggerHidden]
    private void SetStateMachine(IAsyncStateMachine stateMachine)
    {
    }
}

先简单理一下内部逻辑:

  1. 设置变量 num = -1,此时 num != 0,则会进入第一个if语句,
  2. 首先,执行Console.WriteLine("Let's Go!")
  3. 然后,调用异步方法TestAsyncTestAsync方法会在另一个线程池线程中执行,并获取指示该方法运行状态的 awaiter
  4. 如果此时TestAsync方法已执行完毕,则像没有异步一般:
    1. 继续执行接下来的Console.Write(" World!")
    2. 最后设置 <>1__state = -2,并设置异步 Main 方法的返回结果
  5. 如果此时TestAsync方法未执行完毕,则:
    1. 设置 <>1__state = num = 0
    2. 调用AwaitUnsafeOnCompleted方法,用于配置当TestAsync方法完成时的延续,即Console.Write(" World!")
    3. 返回指示异步 Main 方法执行状态的 Task 对象,由于同步 Main 方法中通过使用GetResult()同步阻塞主线程等待任务结束,所以不会释放主线程(废话,如果释放了程序就退出了)。不过对于其他子线程,一般会释放该线程

大部分逻辑我们都可以很容易的理解,唯一需要深入研究的就是AwaitUnsafeOnCompleted,那我们接下来就看看它的内部实现

2|4AwaitUnsafeOnCompleted

// 所属结构体:AsyncTaskMethodBuilder

[__DynamicallyInvokable]
public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter: ICriticalNotifyCompletion where TStateMachine: IAsyncStateMachine
{
    this.m_builder.AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref awaiter, ref stateMachine);
}

// 所属结构体:AsyncTaskMethodBuilder<TResult>

[SecuritySafeCritical, __DynamicallyInvokable]
public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter: ICriticalNotifyCompletion where TStateMachine: IAsyncStateMachine
{
    try
    {
        // 用于流转状态机状态的 runner
        AsyncMethodBuilderCore.MoveNextRunner runnerToInitialize = null;
        Action completionAction = this.m_coreState.GetCompletionAction(AsyncCausalityTracer.LoggingOn ? this.Task : null, ref runnerToInitialize);
        if (this.m_coreState.m_stateMachine == null)
        {
            // 此处构建指示异步 Main 方法执行状态的 Task 对象
            Task<TResult> builtTask = this.Task;
            this.m_coreState.PostBoxInitialization((TStateMachine) stateMachine, runnerToInitialize, builtTask);
        }
        awaiter.UnsafeOnCompleted(completionAction);
    }
    catch (Exception exception)
    {
        AsyncMethodBuilderCore.ThrowAsync(exception, null);
    }
}

咱们一步一步来,先看一下GetCompletionAction的实现:

// 所属结构体:AsyncMethodBuilderCore

[SecuritySafeCritical]
internal Action GetCompletionAction(Task taskForTracing, ref MoveNextRunner runnerToInitialize)
{
    Action defaultContextAction;
    MoveNextRunner runner;
    Debugger.NotifyOfCrossThreadDependency();
    // 
    ExecutionContext context = ExecutionContext.FastCapture();
    if ((context != null) && context.IsPreAllocatedDefault)
    {
        defaultContextAction = this.m_defaultContextAction;
        if (defaultContextAction != null)
        {
            return defaultContextAction;
        }
        
        // 构建 runner
        runner = new MoveNextRunner(context, this.m_stateMachine);
        // 返回值
        defaultContextAction = new Action(runner.Run);
        if (taskForTracing != null)
        {
            this.m_defaultContextAction = defaultContextAction = this.OutputAsyncCausalityEvents(taskForTracing, defaultContextAction);
        }
        else
        {
            this.m_defaultContextAction = defaultContextAction;
        }
    }
    else
    {
        runner = new MoveNextRunner(context, this.m_stateMachine);
        defaultContextAction = new Action(runner.Run);
        if (taskForTracing != null)
        {
            defaultContextAction = this.OutputAsyncCausalityEvents(taskForTracing, defaultContextAction);
        }
    }
    if (this.m_stateMachine == null)
    {
        runnerToInitialize = runner;
    }
    return defaultContextAction;
}

发现一个熟悉的家伙——ExecutionContext,它是用来给咱们延续方法(即Console.Write(" World!");)提供运行环境的,注意这里用的是FastCapture(),该内部方法并未捕获SynchronizationContext,因为不需要流动它。什么?你说你不认识它?大眼瞪小眼?那你应该好好看看《理解C#中的ExecutionContext vs SynchronizationContext》

接着来到new MoveNextRunner(context, this.m_stateMachine),这里初始化了 runner,我们看看构造函数中做了什么:

[SecurityCritical]
internal MoveNextRunner(ExecutionContext context, IAsyncStateMachine stateMachine)
{
    // 将 ExecutionContext 保存了下来
    this.m_context = context;
    
    // 将 stateMachine 保存了下来(不过此时为 null)
    this.m_stateMachine = stateMachine;
}

往下来到defaultContextAction = new Action(runner.Run),你可以发现,最终咱们返回的就是这个 defaultContextAction ,所以这个runner.Run至关重要,不过别着急,我们等用到它的时候我们再来看其内部实现。

最后,回到AwaitUnsafeOnCompleted方法,继续往下走。构建指示异步 Main 方法执行状态的 Task 对象,设置当前的状态机后,来到awaiter.UnsafeOnCompleted(completionAction);,要记住,入参 completionAction 就是刚才返回的runner.Run

// 所属结构体:TaskAwaiter

[SecurityCritical, __DynamicallyInvokable]
public void UnsafeOnCompleted(Action continuation)
{
    OnCompletedInternal(this.m_task, continuation, true, false);
}

[MethodImpl(MethodImplOptions.NoInlining), SecurityCritical]
internal static void OnCompletedInternal(Task task, Action continuation, bool continueOnCapturedContext, bool flowExecutionContext)
{
    if (continuation == null)
    {
        throw new ArgumentNullException("continuation");
    }
    StackCrawlMark lookForMyCaller = StackCrawlMark.LookForMyCaller;
    if (TplEtwProvider.Log.IsEnabled() || Task.s_asyncDebuggingEnabled)
    {
        continuation = OutputWaitEtwEvents(task, continuation);
    }
    
    // 配置延续方法
    task.SetContinuationForAwait(continuation, continueOnCapturedContext, flowExecutionContext, ref lookForMyCaller);
}

直接来到代码最后一行,看到延续方法的配置

// 所属类:Task

[SecurityCritical]
internal void SetContinuationForAwait(Action continuationAction, bool continueOnCapturedContext, bool flowExecutionContext, ref StackCrawlMark stackMark)
{
    TaskContinuation tc = null;
    if (continueOnCapturedContext)
    {
        // 这里我们用的是不进行流动的 SynchronizationContext
        SynchronizationContext currentNoFlow = SynchronizationContext.CurrentNoFlow;
        // 像 Winform、WPF 这种框架,实现了自定义的 SynchronizationContext,
        // 所以在 Winform、WPF 的 UI线程中进行异步等待时,一般 currentNoFlow 不会为 null
        if ((currentNoFlow != null) && (currentNoFlow.GetType() != typeof(SynchronizationContext)))
        {
            // 如果有 currentNoFlow,那么我就用它来执行延续方法
            tc = new SynchronizationContextAwaitTaskContinuation(currentNoFlow, continuationAction, flowExecutionContext, ref stackMark);
        }
        else
        {
            TaskScheduler internalCurrent = TaskScheduler.InternalCurrent;
            if ((internalCurrent != null) && (internalCurrent != TaskScheduler.Default))
            {
                tc = new TaskSchedulerAwaitTaskContinuation(internalCurrent, continuationAction, flowExecutionContext, ref stackMark);
            }
        }
    }
    if ((tc == null) & flowExecutionContext)
    {
        tc = new AwaitTaskContinuation(continuationAction, true, ref stackMark);
    }
    if (tc != null)
    {
        if (!this.AddTaskContinuation(tc, false))
        {
            tc.Run(this, false);
        }
    }
    // 这里会将 continuationAction 设置为 awaiter 中 task 对象的延续方法,所以当 TestAsync() 完成时,就会执行 runner.Run
    else if (!this.AddTaskContinuation(continuationAction, false))
    {
        AwaitTaskContinuation.UnsafeScheduleAction(continuationAction, this);
    }
}

对于我们的示例来说,既没有自定义 SynchronizationContext,也没有自定义 TaskScheduler,所以会直接来到最后一个else if (...),重点在于this.AddTaskContinuation(continuationAction, false),这个方法会将我们的延续方法添加到 Task 中,以便于当 TestAsync 方法执行完毕时,执行 runner.Run

2|5runner.Run

好,是时候让我们看看 runner.Run 的内部实现了:

[SecuritySafeCritical]
internal void Run()
{
    if (this.m_context != null)
    {
        try
        {
            // 我们并未给 s_invokeMoveNext 赋值,所以 callback == null
            ContextCallback callback = s_invokeMoveNext;
            if (callback == null)
            {
                // 将回调设置为下方的 InvokeMoveNext 方法
                s_invokeMoveNext = callback = new
                ContextCallback(AsyncMethodBuilderCore.MoveNextRunner.InvokeMoveNext);
            }
            ExecutionContext.Run(this.m_context, callback, this.m_stateMachine, true);
            return;
        }
        finally
        {
            this.m_context.Dispose();
        }
    }
    this.m_stateMachine.MoveNext();
}

[SecurityCritical]
private static void InvokeMoveNext(object stateMachine)
{
    ((IAsyncStateMachine) stateMachine).MoveNext();
}

来到ExecutionContext.Run(this.m_context, callback, this.m_stateMachine, true);,这里的 callback 是InvokeMoveNext方法。所以,当TestAsync执行完毕后,就会执行延续方法 runner.Run,也就会执行stateMachine.MoveNext()促使状态机继续进行状态流转,这样逻辑就打通了:

private void MoveNext()
{
    // num = 0
    int num = this.<>1__state;
    try
    {
        TaskAwaiter awaiter;
        if (num != 0)
        {
            Console.WriteLine("Let's Go!");
            awaiter = Program.TestAsync().GetAwaiter();

            if (!awaiter.IsCompleted)
            {
                this.<>1__state = num = 0;
                this.<>u__1 = awaiter;
                Program.<Main>d__0 stateMachine = this;
                this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<Main>d__0>(ref awaiter, ref stateMachine);
                return;
            }
        }
        else
        {
            awaiter = this.<>u__1;
            this.<>u__1 = new TaskAwaiter();
            // 状态机状态从 0 流转到 -1
            this.<>1__state = num = -1;
        }
        
        // 结束对 TestAsync() 的等待
        awaiter.GetResult();
        // 执行延续方法
        Console.Write(" World!");
    }
    catch (Exception exception)
    {
        this.<>1__state = -2;
        this.<>t__builder.SetException(exception);
        return;
    }
    
    // 状态机状态从 -1 流转到 -2
    this.<>1__state = -2;
    // 设置异步 Main 方法最终返回结果
    this.<>t__builder.SetResult();
}

至此,整个异步方法的执行就结束了,通过一张图总结一下:

最后,我们看一下各个线程的状态,看看和你的推理是否一致(如果有不清楚的联系我,我会通过文字补充):

3|0多个 async await 嵌套

理解了async await的简单使用,那你可曾想过,如果有多个 async await 嵌套,那会出现什么情况呢?接下来就改造一下我们的例子,来研究研究:

static Task TestAsync()
{
    return Task.Run(async () =>
    {
        // 增加了这行
        await Task.Run(() =>
        {
            Console.Write("Say: ");
        });

        Console.Write("Hello");
    });
}

反编译之后的代码,上面已经讲解的我就不再重复贴了,主要看看TestAsync()就行了:

private static Task TestAsync() => 
    Task.Run(delegate {
        <>c.<<TestAsync>b__1_0>d stateMachine = new <>c.<<TestAsync>b__1_0>d {
            <>t__builder = AsyncTaskMethodBuilder.Create(),
            <>4__this = this,
            <>1__state = -1
        };
        stateMachine.<>t__builder.Start<<>c.<<TestAsync>b__1_0>d>(ref stateMachine);
        return stateMachine.<>t__builder.Task;
    });

哦!原来,async await 的嵌套也就是状态机的嵌套,相信你通过上面的状态机状态流转,也能够梳理除真正的执行逻辑,那我们就只看一下线程状态吧:

这也印证了我上面所说的:当子线程完成执行任务时,会被释放,或回到线程池供其他线程使用。

4|0多个 async await 在同一方法中顺序执行

又可曾想过,如果有多个 async await 在同一方法中顺序执行,又会是何种景象呢?同样,先来个例子:

static async Task Main(string[] args)
{
    Console.WriteLine("Let's Go!");

    await Test1Async();

    await Test2Async();

    Console.Write(" World!");
}

static Task Test1Async()
{
    return Task.Run(() =>
    {
        Console.Write("Say: ");
    });
}

static Task Test2Async()
{
    return Task.Run(() =>
    {
        Console.Write("Hello");
    });
}

直接看状态机:

[CompilerGenerated]
private sealed class <Main>d__0 : IAsyncStateMachine
{
	// Fields
	public int <>1__state;
	public AsyncTaskMethodBuilder <>t__builder;
	public string[] args;
	private TaskAwaiter <>u__1;

    // Methods
	private void MoveNext()
	{
		int num = this.<>1__state;
		try
		{
			TaskAwaiter awaiter;
			TaskAwaiter awaiter2;
			if (num != 0)
			{
				if (num == 1)
				{
					awaiter = this.<>u__1;
					this.<>u__1 = default(TaskAwaiter);
					this.<>1__state = -1;
					goto IL_D8;
				}
				Console.WriteLine("Let's Go!");
				awaiter2 = Program.Test1Async().GetAwaiter();
				if (!awaiter2.IsCompleted)
				{
					this.<>1__state = 0;
					this.<>u__1 = awaiter2;
					Program.<Main>d__0 <Main>d__ = this;
					this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<Main>d__0>(ref awaiter2, ref <Main>d__);
					return;
				}
			}
			else
			{
				awaiter2 = this.<>u__1;
				this.<>u__1 = default(TaskAwaiter);
				this.<>1__state = -1;
			}
			awaiter2.GetResult();
			
			// 待 Test1Async 完成后,继续执行 Test2Async
			awaiter = Program.Test2Async().GetAwaiter();
			if (!awaiter.IsCompleted)
			{
				this.<>1__state = 1;
				this.<>u__1 = awaiter;
				Program.<Main>d__0 <Main>d__ = this;
				this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<Main>d__0>(ref awaiter, ref <Main>d__);
				return;
			}
			IL_D8:
			awaiter.GetResult();
			Console.Write(" World!");
		}
		catch (Exception exception)
		{
			this.<>1__state = -2;
			this.<>t__builder.SetException(exception);
			return;
		}
		this.<>1__state = -2;
		this.<>t__builder.SetResult();
	}

	[DebuggerHidden]
	private void SetStateMachine(IAsyncStateMachine stateMachine)
	{
	}
}

可见,就是一个状态机状态一直流转就完事了。我们就看看线程状态吧:

5|0WPF中使用 async await

上面我们都是通过控制台举的例子,这是没有任何SynchronizationContext的,但是WPF(Winform同理)就不同了,在UI线程中,它拥有属于自己的DispatcherSynchronizationContext

private async void Button_Click(object sender, RoutedEventArgs e)
{
    // UI 线程会一直保持 Running 状态,不会导致 UI 假死
    Show(Thread.CurrentThread);

    await TestAsync();

    Show(Thread.CurrentThread);
}

private Task TestAsync()
{
    return Task.Run(() =>
    {
        Show(Thread.CurrentThread);
    });
}

private static void Show(Thread thread)
{
    MessageBox.Show($"{nameof(thread.ManagedThreadId)}: {thread.ManagedThreadId}" +
        $"n{nameof(thread.ThreadState)}: {thread.ThreadState}");
}

通过使用DispatcherSynchronizationContext执行延续方法,又回到了 UI 线程中

原文出处:博客园
原文链接: https://www.cnblogs.com/xiaoxiaotank/p/14303803.html
文章标题:.Net C# async await 原理解析
文章链接:https://onebyone.icu/archives/1375
作者昵称:OneByOneDotNet
作者链接:https://onebyone.icu/user/1

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