async/await 源码解析
这篇文章主要是分析 async/await 这个语法糖，分析一下 async 和 await 是如何做到异步的。首先，我先抛出两个问题，各位可以先想一下。
- await 之后的方法是何时执行，如何执行的？
- 为什么 await 之后的代码会在不同的线程执行？
demo
要想知道 async/await 是怎么运行的，需要先写一个demo，然后进行一下反编译，就可以得到 async/await 编译后的代码，然后就可以开始分析了。
下面是简单使用 async/await 的demo:
static async Task Main(string[] args)
{
Console.WriteLine("1"+Thread.CurrentThread.ManagedThreadId.ToString());
await GetThreadID2();
Console.WriteLine("2"+Thread.CurrentThread.ManagedThreadId.ToString());
}
public async static Task GetThreadID2()
{
Console.WriteLine("3"+Thread.CurrentThread.ManagedThreadId.ToString());
await Task.Run(() => Console.WriteLine("4"+Thread.CurrentThread.ManagedThreadId.ToString()));
Console.WriteLine("5"+Thread.CurrentThread.ManagedThreadId.ToString());
}
然后我们来反编译一下这段代码(可以使用 dnSpy 进行反编译):
namespace AsyncTest
{
internal class Program
{
[DebuggerStepThrough]
private static Task Main(string[] args)
{
Program.<Main>d__0 <Main>d__ = new Program.<Main>d__0();
<Main>d__.<>t__builder = AsyncTaskMethodBuilder.Create();
<Main>d__.args = args;
<Main>d__.<>1__state = -1;
<Main>d__.<>t__builder.Start<Program.<Main>d__0>(ref <Main>d__);
return <Main>d__.<>t__builder.Task;
}
[DebuggerStepThrough]
public static Task GetThreadID2()
{
Program.<GetThreadID2>d__2 <GetThreadID2>d__ = new Program.<GetThreadID2>d__2();
<GetThreadID2>d__.<>t__builder = AsyncTaskMethodBuilder.Create();
<GetThreadID2>d__.<>1__state = -1;
<GetThreadID2>d__.<>t__builder.Start<Program.<GetThreadID2>d__2>(ref <GetThreadID2>d__);
return <GetThreadID2>d__.<>t__builder.Task;
}
[DebuggerStepThrough]
private static void <Main>(string[] args)
{
Program.Main(args).GetAwaiter().GetResult();
}
[CompilerGenerated]
private sealed class <Main>d__0 : IAsyncStateMachine
{
void IAsyncStateMachine.MoveNext()
{
int num = this.<>1__state;
try
{
TaskAwaiter awaiter;
if (num != 0)
{
Console.WriteLine("1" + Thread.CurrentThread.ManagedThreadId.ToString());
awaiter = Program.GetThreadID2().GetAwaiter();
if (!awaiter.IsCompleted)
{
this.<>1__state = 0;
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;
}
}
else
{
awaiter = this.<>u__1;
this.<>u__1 = default(TaskAwaiter);
this.<>1__state = -1;
}
awaiter.GetResult();
Console.WriteLine("2" + Thread.CurrentThread.ManagedThreadId.ToString());
Console.ReadLine();
}
catch (Exception exception)
{
this.<>1__state = -2;
this.<>t__builder.SetException(exception);
return;
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
}
[CompilerGenerated]
[Serializable]
private sealed class <>c
{
internal void <GetThreadID2>b__2_0()
{
Console.WriteLine("4" + Thread.CurrentThread.ManagedThreadId.ToString());
}
public static readonly Program.<>c <>9 = new Program.<>c();
public static Func<string> <>9__1_0;
public static Action <>9__2_0;
}
[CompilerGenerated]
private sealed class <GetThreadID2>d__2 : IAsyncStateMachine
{
void IAsyncStateMachine.MoveNext()
{
int num = this.<>1__state;
try
{
TaskAwaiter awaiter;
if (num != 0)
{
Console.WriteLine("3" + Thread.CurrentThread.ManagedThreadId.ToString());
awaiter = Task.Run(new Action(Program.<>c.<>9.<GetThreadID2>b__2_0)).GetAwaiter();
if (!awaiter.IsCompleted)
{
this.<>1__state = 0;
this.<>u__1 = awaiter;
Program.<GetThreadID2>d__2 <GetThreadID2>d__ = this;
this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<GetThreadID2>d__2>(ref awaiter, ref <GetThreadID2>d__);
return;
}
}
else
{
awaiter = this.<>u__1;
this.<>u__1 = default(TaskAwaiter);
this.<>1__state = -1;
}
awaiter.GetResult();
Console.WriteLine("5" + Thread.CurrentThread.ManagedThreadId.ToString());
}
catch (Exception exception)
{
this.<>1__state = -2;
this.<>t__builder.SetException(exception);
return;
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
[DebuggerHidden]
void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine){}
public int <>1__state;
public AsyncTaskMethodBuilder <>t__builder;
private TaskAwaiter <>u__1;
}
}
}
上面的代码是反编译出来的所有代码，如果各位选择太长不看的话，这里还有个简化版本:
private static Task Main(string[] args)
<Main>d__.<>t__builder.Start<Program.<Main>d__0>(ref <Main>d__);
<Main>d__0.MoveNext()
<GetThreadID2>d__.<>t__builder.Start<Program.<GetThreadID2>d__2>(ref <GetThreadID2>d__);
<GetThreadID2>d__2.MoveNext()
<GetThreadID2>d__2.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<GetThreadID2>d__2>(ref awaiter, ref <GetThreadID2>d__);
<GetThreadID2>d__2.MoveNext()
<Main>d__0.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<Main>d__0>(ref awaiter, ref <Main>d__);
<Main>d__0.MoveNext()
入口
我们先来看一下 Main 方法。
private static Task Main(string[] args)
{
Program.<Main>d__0 <Main>d__ = new Program.<Main>d__0();
<Main>d__.<>t__builder = AsyncTaskMethodBuilder.Create();
<Main>d__.args = args;
<Main>d__.<>1__state = -1;
<Main>d__.<>t__builder.Start<Program.<Main>d__0>(ref <Main>d__);
return <Main>d__.<>t__builder.Task;
}
首先第一行，构建了一个 Program.<Main>d__0 类型实例，嗯？ 这个类哪里来的，没写过呀。
哎嘿，这个类就是编译器帮我们实现的。每一个 async/await 方法，编译器都会帮我们实现一个这样的类。
简单分析一下这个方法，实例化一个 Program.<Main>d__0()， 初始化 1__state 值为-1，调用 Program.<Main>d__0() 的 Start 方法。
Start
public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
{
ExecutionContextSwitcher executionContextSwitcher = default(ExecutionContextSwitcher);
RuntimeHelpers.PrepareConstrainedRegions();
try
{
ExecutionContext.EstablishCopyOnWriteScope(ref executionContextSwitcher);
stateMachine.MoveNext();
}
finally
{
executionContextSwitcher.Undo();
}
}
Start 代码的主要作用就是启动 stateMachine，即 stateMachine.MoveNext()。
MoveNext
MoveNext 是 async/await 里非常重要的一段代码，这个方法控制了整个异步任务的执行步骤。
我们可以将其分解成三个部分，即
- await 之前
- await 执行
- await 之后的代码
具体的分析各位请看注释。
void IAsyncStateMachine.MoveNext()
{
// 这里 <>1__state 之前初始化的时候是 -1，所以 num 就是-1
int num = this.<>1__state;
try
{
TaskAwaiter awaiter;
// 第一次进来，num 是 -1 所以接下来的逻辑
if (num != 0)
{
// 这里执行 await 之前的代码
Console.WriteLine("1" + Thread.CurrentThread.ManagedThreadId.ToString());
// 这里就是执行 await 方法
awaiter = Program.GetThreadID2().GetAwaiter();
// 判断是否执行完成，通常第一次进来 IsCompleted 都是 false
if (!awaiter.IsCompleted)
{
// 修改状态，下次再执行 MoveNext 就不会继续走这段逻辑来了
this.<>1__state = 0;
this.<>u__1 = awaiter;
Program.<Main>d__0 <Main>d__ = this;
// 这里其实是往 Task 里添加了一个 任务结束时的回调，在任务结束时会再次调用 MoveNext
// 这就解释了为什么 await 之后的方法是另外一个线程，因为await 之后的方法是在 下一个 MoveNext 的里调用的
// 而那个 MoveNext 是由线程池挑选一个线程进行执行的
this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Program.<Main>d__0>(ref awaiter, ref <Main>d__);
return;
}
}
else
{
awaiter = this.<>u__1;
this.<>u__1 = default(TaskAwaiter);
this.<>1__state = -1;
}
// GetResult 里有一个 死循环 等待结果
awaiter.GetResult();
// await 之后的方法
Console.WriteLine("2" + Thread.CurrentThread.ManagedThreadId.ToString());
Console.ReadLine();
}
catch (Exception exception)
{
this.<>1__state = -2;
this.<>t__builder.SetException(exception);
return;
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
await 之前的代码
// 这里执行 await 之前的代码
Console.WriteLine("1" + Thread.CurrentThread.ManagedThreadId.ToString());
await 之前的代码很简单，就是正常依次执行。
await 的代码
await 的代码大都以 Task 形式执行，主要分为两步。
第一步，将 Task 推入线程池中。
awaiter = Task.Run(new Action(Program.<>c.<>9.<GetThreadID2>b__3_0)).GetAwaiter();
public static Task Run(Action action)
{
StackCrawlMark stackCrawlMark = StackCrawlMark.LookForMyCaller;
return Task.InternalStartNew(null, action, null, default(CancellationToken), TaskScheduler.Default, TaskCreationOptions.DenyChildAttach, InternalTaskOptions.None, ref stackCrawlMark);
}
internal static Task InternalStartNew(Task creatingTask, Delegate action, object state, CancellationToken cancellationToken, TaskScheduler scheduler, TaskCreationOptions options, InternalTaskOptions internalOptions, ref StackCrawlMark stackMark)
{
Task task = new Task(action, state, creatingTask, cancellationToken, options, internalOptions | InternalTaskOptions.QueuedByRuntime, scheduler);
task.ScheduleAndStart(false);
return task;
}
internal void ScheduleAndStart(bool needsProtection)
{
try
{
this.m_taskScheduler.InternalQueueTask(this);
}
catch (ThreadAbortException exceptionObject){}
}
internal void InternalQueueTask(Task task)
{
this.QueueTask(task);
}
第二步，线程池执行 Task。
void IThreadPoolWorkItem.ExecuteWorkItem()
{
this.ExecuteEntry(false);
}
internal bool ExecuteEntry(bool bPreventDoubleExecution)
{
if (!this.IsCancellationRequested && !this.IsCanceled)
{
this.ExecuteWithThreadLocal(ref Task.t_currentTask);
}
return true;
}
private void ExecuteWithThreadLocal(ref Task currentTaskSlot)
{
Task task = currentTaskSlot;
TplEtwProvider log = TplEtwProvider.Log;
Guid currentThreadActivityId = default(Guid);
bool flag = log.IsEnabled();
try
{
currentTaskSlot = this;
ExecutionContext capturedContext = this.CapturedContext;
if (capturedContext == null)
{
this.Execute();
}
else
{
if (this.IsSelfReplicatingRoot || this.IsChildReplica)
{
this.CapturedContext = Task.CopyExecutionContext(capturedContext);
}
ContextCallback contextCallback = Task.s_ecCallback;
if (contextCallback == null)
{
contextCallback = (Task.s_ecCallback = new ContextCallback(Task.ExecutionContextCallback));
}
// 这里是执行 Task 方法的地方
ExecutionContext.Run(capturedContext, contextCallback, this, true);
}
this.Finish(true);
}
finally
{
currentTaskSlot = task;
}
}
await 之后的代码
那么 await 之后的代码是如何执行的呢？
大家回头看一下 MoveNext 方法，可以看到首次执行 MoveNext 的时候，代码的逻辑是这样的
if(num!=0) -> if(!awaiter.IsCompleted) -> AwaitUnsafeOnCompleted;return;
关键就在于 AwaitUnsafeOnCompleted 这个方法，来看一下 AwaitUnsafeOnCompleted 方法。
public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine
{
try
{
AsyncMethodBuilderCore.MoveNextRunner runner = null;
// 这里是准备一个 任务执行完的回调，简单理解成是将 MoveNext 包装成一个 action
Action completionAction = this.m_coreState.GetCompletionAction(AsyncCausalityTracer.LoggingOn ? this.Task : null, ref runner);
if (this.m_coreState.m_stateMachine == null)
{
Task<TResult> task = this.Task;
this.m_coreState.PostBoxInitialization(stateMachine, runner, task);
}
// 将 completionAction 推到 task里，具体看一下接下来的代码
awaiter.UnsafeOnCompleted(completionAction);
}
catch (Exception exception)
{
AsyncMethodBuilderCore.ThrowAsync(exception, null);
}
}
public void UnsafeOnCompleted(Action continuation)
{
TaskAwaiter.OnCompletedInternal(this.m_task, continuation, true, false);
}
internal static void OnCompletedInternal(Task task, Action continuation, bool continueOnCapturedContext, bool flowExecutionContext)
{
task.SetContinuationForAwait(continuation, continueOnCapturedContext, flowExecutionContext, ref stackCrawlMark);
}
internal void SetContinuationForAwait(Action continuationAction, bool continueOnCapturedContext, bool flowExecutionContext, ref StackCrawlMark stackMark)
{
if (!this.AddTaskContinuation(continuationAction, false))
{
AwaitTaskContinuation.UnsafeScheduleAction(continuationAction, this);
}
}
private bool AddTaskContinuation(object tc, bool addBeforeOthers)
{
//这里就把completionAction 放到了 Task 的 m_continuationObject 里
return !this.IsCompleted && ((this.m_continuationObject == null && Interlocked.CompareExchange(ref this.m_continuationObject, tc, null) == null) || this.AddTaskContinuationComplex(tc, addBeforeOthers));
}
至此为止，准备工作(将 MoveNext 放到 task 的回调里)已经做完了，接下来就是如何来触发这个方法。
简单分析一下，MoveNext 肯定是在 task 执行完触发，让我们回头看看 Task 执行的代码。会发现，哎嘿，在ExecutionContext.Run(capturedContext, contextCallback, this, true); 方法执行完后还有一句代码 this.Finish(true)。
internal void Finish(bool bUserDelegateExecuted)
{
this.FinishStageTwo();
}
internal void FinishStageTwo()
{
this.FinishStageThree();
}
internal void FinishStageThree()
{
this.FinishContinuations();
}
internal void FinishContinuations()
{
// 获取之前注册的所有回调 最简单的就可以理解成 MoveNext
object obj = Interlocked.Exchange(ref this.m_continuationObject, Task.s_taskCompletionSentinel);
if (obj != null)
{
bool flag = (this.m_stateFlags & 134217728) == 0 && Thread.CurrentThread.ThreadState != ThreadState.AbortRequested && (this.m_stateFlags & 64) == 0;
Action action = obj as Action;
if (action != null)
{
AwaitTaskContinuation.RunOrScheduleAction(action, flag, ref Task.t_currentTask);
return;
}
}
}
internal static void RunOrScheduleAction(Action action, bool allowInlining, ref Task currentTask)
{
// 这里调用了 MoveNextRunner.Run ,简单理解 就是 MoveNext 方法
action();
}
void IAsyncStateMachine.MoveNext()
{
// 此时 num 就是 0 了，就会执行 await 之后的代码了。
int num = this.<>1__state;
try
{
TaskAwaiter awaiter;
if (num != 0)
{}
else
{
awaiter = this.<>u__1;
this.<>u__1 = default(TaskAwaiter);
this.<>1__state = -1;
}
awaiter.GetResult();
Console.WriteLine("2" + Thread.CurrentThread.ManagedThreadId.ToString());
Console.ReadLine();
}
catch (Exception exception)
{
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
这一段代码我删掉了很多内容，但大致的执行顺序就是如此，如果想要了解更多细节，可以查看 Task 这个类的源码。
总结
回头看看开头的两个问题，现在就可以回答了。
- await 之后的方法是何时执行，如何执行的？
await 的方法在 Task 执行完成之后，通过调用Finish方法执行的。
具体的执行步骤是先将MoveNext方法注册到 Task 的回调里，然后在 Task 执行完后调用这个方法。 - 为什么 await 之后的代码会在不同的线程执行？
这个其实是因为 Task 的机制，Task 会被推到线程池里，由线程池挑选一个线程去执行，await 之后的代码其实是由这个线程去执行的，自然就跟 await 的之前的代码不是一个线程。
PS: 补一张图
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文章来源: 博客园
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