首先我们来看一段控制台应用代码：
class Program
{
static async Task Main(string[] args)
{
System.Console.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
var result = await ExampleTask(2);
System.Console.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
System.Console.WriteLine(result);
Console.WriteLine("Async Completed");
}
private static async Task<string> ExampleTask(int Second)
{
await Task.Delay(TimeSpan.FromSeconds(Second));
return $"It's Async Completed in {Second} seconds";
}
}
输出结果
Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:4,Is Thread Pool:True
It's Async Completed in 2 seconds
Async Completed
如果这段代码在WPF运行，猜猜会输出啥？
private async void Async_Click(object sender, RoutedEventArgs e)
{
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
var result= await ExampleTask(2);
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
Debug.WriteLine(result);
Debug.WriteLine("Async Completed");
}
private async Task<string> ExampleTask(int Second)
{
await Task.Delay(TimeSpan.FromSeconds(Second));
return $"It's Async Completed in {Second} seconds";
}
输出结果：
Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:1,Is Thread Pool:False
It's Async Completed in 2 seconds
Async Completed
这时候你肯定是想说，小朋友，你是否有很多问号？？？？，我们接下看下去
一.SynchronizationContext(同步上下文)
首先我们知道async await 异步函数本质是状态机,我们通过反编译工具dnspy，看看反编译的两段代码是否有不同之处：
控制台应用：
internal class Program
{
[DebuggerStepThrough]
private static Task Main(string[] args)
{
Program.<Main>d__0 <Main>d__ = new Program.<Main>d__0();
<Main>d__.args = args;
<Main>d__.<>t__builder = AsyncTaskMethodBuilder.Create();
<Main>d__.<>1__state = -1;
<Main>d__.<>t__builder.Start<Program.<Main>d__0>(ref <Main>d__);
return <Main>d__.<>t__builder.Task;
}
[DebuggerStepThrough]
private static Task<string> ExampleTask(int Second)
{
Program.<ExampleTask>d__1 <ExampleTask>d__ = new Program.<ExampleTask>d__1();
<ExampleTask>d__.Second = Second;
<ExampleTask>d__.<>t__builder = AsyncTaskMethodBuilder<string>.Create();
<ExampleTask>d__.<>1__state = -1;
<ExampleTask>d__.<>t__builder.Start<Program.<ExampleTask>d__1>(ref <ExampleTask>d__);
return <ExampleTask>d__.<>t__builder.Task;
}
[DebuggerStepThrough]
private static void <Main>(string[] args)
{
Program.Main(args).GetAwaiter().GetResult();
}
}
WPF:
public class MainWindow : Window, IComponentConnector
{
public MainWindow()
{
this.InitializeComponent();
}
[DebuggerStepThrough]
private void Async_Click(object sender, RoutedEventArgs e)
{
MainWindow.<Async_Click>d__1 <Async_Click>d__ = new MainWindow.<Async_Click>d__1();
<Async_Click>d__.<>4__this = this;
<Async_Click>d__.sender = sender;
<Async_Click>d__.e = e;
<Async_Click>d__.<>t__builder = AsyncVoidMethodBuilder.Create();
<Async_Click>d__.<>1__state = -1;
<Async_Click>d__.<>t__builder.Start<MainWindow.<Async_Click>d__1>(ref <Async_Click>d__);
}
[DebuggerStepThrough]
private Task<string> ExampleTask(int Second)
{
MainWindow.<ExampleTask>d__3 <ExampleTask>d__ = new MainWindow.<ExampleTask>d__3();
<ExampleTask>d__.<>4__this = this;
<ExampleTask>d__.Second = Second;
<ExampleTask>d__.<>t__builder = AsyncTaskMethodBuilder<string>.Create();
<ExampleTask>d__.<>1__state = -1;
<ExampleTask>d__.<>t__builder.Start<MainWindow.<ExampleTask>d__3>(ref <ExampleTask>d__);
return <ExampleTask>d__.<>t__builder.Task;
}
[DebuggerNonUserCode]
[GeneratedCode("PresentationBuildTasks", "4.8.1.0")]
public void InitializeComponent()
{
bool contentLoaded = this._contentLoaded;
if (!contentLoaded)
{
this._contentLoaded = true;
Uri resourceLocater = new Uri("/WpfApp1;component/mainwindow.xaml", UriKind.Relative);
Application.LoadComponent(this, resourceLocater);
}
}
private bool _contentLoaded;
}
我们可以看到完全是一致的，没有任何区别，为什么编译器生成的代码是一致的，却会产生不一样的结果，我们看看创建和启动状态机代码部分的实现：
public static AsyncVoidMethodBuilder Create()
{
SynchronizationContext synchronizationContext = SynchronizationContext.Current;
if (synchronizationContext != null)
{
synchronizationContext.OperationStarted();
}
return new AsyncVoidMethodBuilder
{
_synchronizationContext = synchronizationContext
};
}
[DebuggerStepThrough]
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public void Start<[Nullable(0)] TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
{
AsyncMethodBuilderCore.Start<TStateMachine>(ref stateMachine);
}
[DebuggerStepThrough]
public static void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
{
if (stateMachine == null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.stateMachine);
}
Thread currentThread = Thread.CurrentThread;
Thread thread = currentThread;
ExecutionContext executionContext = currentThread._executionContext;
ExecutionContext executionContext2 = executionContext;
SynchronizationContext synchronizationContext = currentThread._synchronizationContext;
try
{
stateMachine.MoveNext();//状态机执行代码
}
finally
{
SynchronizationContext synchronizationContext2 = synchronizationContext;
Thread thread2 = thread;
if (synchronizationContext2 != thread2._synchronizationContext)
{
thread2._synchronizationContext = synchronizationContext2;
}
ExecutionContext executionContext3 = executionContext2;
ExecutionContext executionContext4 = thread2._executionContext;
if (executionContext3 != executionContext4)
{
ExecutionContext.RestoreChangedContextToThread(thread2, executionContext3, executionContext4);
}
}
}
在这里总结下：
- 创建状态机的Create函数通过SynchronizationContext.Current获取到当前同步执行上下文
- 启动状态机的Start函数之后通过MoveNext函数执行我们的异步方法
- 这里还有一个小提示，不管async函数里面有没有await，都会生成状态机，只是MoveNext函数执行同步方法,因此没await的情况下避免将函数标记为async，会损耗性能
同样的这里貌似没能获取到原因，但是有个很关键的地方，就是Create函数为啥要获取当前同步执行上下文，之后我从MSDN找到关于SynchronizationContext
的介绍，有兴趣的朋友可以去阅读以下，以下是各个.NET框架使用的SynchronizationContext：
| SynchronizationContext | 默认 |
|---|---|
| WindowsFormsSynchronizationContext | WindowsForm |
| DispatcherSynchronizationContext | WPF/Silverlight |
| AspNetSynchronizationContext | ASP.NET |
我们貌似已经一步步接近真相了，接下来我们来看看DispatcherSynchronizationContext
二.DispatcherSynchronizationContext
首先来看看DispatcherSynchronizationContext类的比较关键的几个函数实现：
public DispatcherSynchronizationContext(Dispatcher dispatcher, DispatcherPriority priority)
{
if (dispatcher == null)
{
throw new ArgumentNullException("dispatcher");
}
Dispatcher.ValidatePriority(priority, "priority");
_dispatcher = dispatcher;
_priority = priority;
SetWaitNotificationRequired();
}
//同步执行
public override void Send(SendOrPostCallback d, object state)
{
if (BaseCompatibilityPreferences.GetInlineDispatcherSynchronizationContextSend() && _dispatcher.CheckAccess())
{
_dispatcher.Invoke(DispatcherPriority.Send, d, state);
}
else
{
_dispatcher.Invoke(_priority, d, state);
}
}
//异步执行
public override void Post(SendOrPostCallback d, object state)
{
_dispatcher.BeginInvoke(_priority, d, state);
}
我们貌似看到了熟悉的东西了，Send函数调用Dispatcher的Invoke函数，Post函数调用Dispatcher的BeginInvoke函数，那么是否WPF执行异步函数之后会调用这里的函数吗？我用dnspy进行了调试：
我通过调试之后发现，当等待执行完整个状态机的之后，也就是两秒后跳转到该Post函数，那么，我们可以将之前的WPF那段代码大概可以改写成如此：
private async void Async_Click(object sender, RoutedEventArgs e)
{
//async生成状态机的Create函数。获取到UI主线程的同步执行上下文
DispatcherSynchronizationContext synchronizationContext = (DispatcherSynchronizationContext)SynchronizationContext.Current;
//UI主线程执行
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
//开始在状态机的MoveNext执行该异步操作
var result= await ExampleTask(2);
//等待两秒，异步执行完成，再在同步上下文异步执行
synchronizationContext.Post((state) =>
{
//模仿_dispatcher.BeginInvoke
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
Debug.WriteLine(result);
Debug.WriteLine("Async Completed");
},"Post");
}
输出结果：
Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:1,Is Thread Pool:False
It's Async Completed in 2 seconds
Async Completed
也就是asyn负责生成状态机和执行状态机，await将代码分为两部分，一部分是异步执行状态机部分，一部分是异步执行完之后，通过之前拿到的DispatcherSynchronizationContext，再去异步执行接下来的部分。我们可以通过dnspy调试DispatcherSynchronizationContext的 _dispatcher字段的Thread属性，知道Thread为UI主线程，而同步界面UI控件的时候，也就是通过Dispatcher的BeginInvoke函数去执行同步的
三.Task.ConfigureAwait
Task有个ConfigureAwait方法，是可以设置是否对Task的awaiter的延续任务执行原始上下文，也就是为true时，是以一开始那个UI主线程的DispatcherSynchronizationContext执行Post方法,而为false，则以await那个Task里面的DispatcherSynchronizationContext执行Post方法，我们来验证下：
我们将代码改为以下：
private async void Async_Click(object sender, RoutedEventArgs e)
{
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
var result= await ExampleTask(2).ConfigureAwait(false);
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
Debug.WriteLine(result);
Debug.WriteLine($"Async Completed");
}
输出：
Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:4,Is Thread Pool:True
It's Async Completed in 2 seconds
Async Completed
结果和控制台输出的一模一样，且通过dnspy断点调试依旧进入到DispatcherSynchronizationContext的Post方法,因此我们也可以证明我们上面的猜想，而且默认ConfigureAwait的参数是为true的,我们还可以将异步结果赋值给UI界面的Text block：
private async void Async_Click(object sender, RoutedEventArgs e)
{
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
var result= await ExampleTask(2).ConfigureAwait(false);
Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
this.txt.Text = result;//修改部分
Debug.WriteLine($"Async Completed");
}
抛出异常：
调用线程无法访问此对象，因为另一个线程拥有该对象
补充
推荐林大佬的一篇文章，也讲的也简洁透彻C# dotnet 自己实现一个线程同步上下文
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文章来源: 博客园
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