为什么调用者方法和异步方法具有相同的线程ID

Question

请参考我下面的代码。

public MainViewModel()
    {
        LongRunningOperationCommand = new RelayCommand(ExecuteLongRunningOperationCommand);
    }

    private void ExecuteLongRunningOperationCommand()
    {
        Test();
    }

    private async Task Test()
    {
        Log += "Command begin: " + DateTime.Now + "\r\n";
        Log += "Command thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
        var getStringAsync = GetStringAsync();
        Log += "Work in Command...\r\n";
        Log += "Work in Command which not related to the result of async method will complete: " + DateTime.Now + "\r\n";
        Log += "Work in Command which not related to the result of async method will complete, thread: " +
               Thread.CurrentThread.ManagedThreadId + "\r\n";
        string result = await getStringAsync;
        Log += "Command will complete: " + DateTime.Now + "\r\n";
        Log += "Command will complete, thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
        Log += result + "\r\n";
    }

    private async Task<string> GetStringAsync()
    {
        Log += "Async method begin: " + DateTime.Now + "\r\n";
        Log += "Async method thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
        Log += "Work in Async method... \r\n";
        await Task.Delay(10000);
        Log += "Async method will complete: " + DateTime.Now + "\r\n";
        Log += "Async method will complete, thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
        return "GetStringAsync method completed!";
    }

结果如下

Command begin: 1/6/2016 11:58:37 PM
Command thread: 8
Async method begin: 1/6/2016 11:58:37 PM
Async method thread: 8
Work in Async method... 
Work in Command...
Work in Command which not related to the result of async method will complete: 1/6/2016 11:58:37 PM
Work in Command which not related to the result of async method will complete, thread: 8
Async method will complete: 1/6/2016 11:58:47 PM
Async method will complete, thread: 8
Command will complete: 1/6/2016 11:58:47 PM
Command will complete, thread: 8
GetStringAsync method completed!

GetStringAsync 方法中 await Task.Delay 之后的线程 id 应该与之前不同。为什么结果是一样的？在控制台应用程序中，线程 ID 不同，但在 WPF 应用程序中，它们是相同的。有人可以帮忙吗？

Answer 1

async/await 的一大要点是，如果您有一个 SynchronizationContext，例如 WPF 的 DispatcherSynchronizationContext，则在该线程上开始的工作将在等待之后继续在该线程上，除非您告诉它不要这样做。

控制台应用程序没有 SynchronizationContext，因此它使用在线程池中调度线程的默认上下文，这就是您看到 WPF 和控制台应用程序的不同行为的原因。

要告诉 async/await 它不需要保持在相同的同步上下文中，您可以在等待时使用 .ConfigureAwait(false)，然后它会在需要时使用默认线程池上下文来执行回调。

private async Task Test()
{
    Log += "Command begin: " + DateTime.Now + "\r\n";
    Log += "Command thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
    var getStringAsync = GetStringAsync();
    Log += "Work in Command...\r\n";
    Log += "Work in Command which not related to the result of async method will complete: " + DateTime.Now + "\r\n";
    Log += "Work in Command which not related to the result of async method will complete, thread: " +
           Thread.CurrentThread.ManagedThreadId + "\r\n";
    string result = await getStringAsync.ConfigureAwait(false);
    Log += "Command will complete: " + DateTime.Now + "\r\n";
    Log += "Command will complete, thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
    Log += result + "\r\n";
}

private async Task<string> GetStringAsync()
{
    Log += "Async method begin: " + DateTime.Now + "\r\n";
    Log += "Async method thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
    Log += "Work in Async method... \r\n";
    await Task.Delay(10000).ConfigureAwait(false);
    Log += "Async method will complete: " + DateTime.Now + "\r\n";
    Log += "Async method will complete, thread: " + Thread.CurrentThread.ManagedThreadId + "\r\n";
    return "GetStringAsync method completed!";
}

注意，执行.ConfigureAwait(false) 并不能保证其余代码将在线程池中，如果任务处于Completed 状态，代码将同步执行并停留在最初称为await 的任何线程上.

有关详细信息，请参阅文章“It's All About the SynchronizationContext”。