相关疑难解决方法(0)

阅读太久了.使用Task.ConfigureAwait(continueOnCapturedContext: false)可能会引入冗余线程切换.我正在寻找一致的解决方案.

长版.隐藏的主要设计目标ConfigureAwait(false)是在可能的情况下减少冗余的SynchronizationContext.Post延续回调await.这通常意味着更少的线程切换和更少的UI线程工作.但是,它并不总是如何运作.

例如,有一个实现SomeAsyncApiAPI的第三方库.请注意ConfigureAwait(false),由于某些原因,此库中的任何位置都不使用:

// some library, SomeClass class
public static async Task<int> SomeAsyncApi()
{
    TaskExt.Log("X1");

    // await Task.Delay(1000) without ConfigureAwait(false);
    // WithCompletionLog only shows the actual Task.Delay completion thread
    // and doesn't change the awaiter behavior

    await Task.Delay(1000).WithCompletionLog(step: "X1.5");

    TaskExt.Log("X2");

    return 42;
}

// logging helpers
public static partial class TaskExt
{
    public static void Log(string step)
    {
        Debug.WriteLine(new { step, thread = Environment.CurrentManagedThreadId }); …

我刚刚注意到,使用.NET 4.5,每个Dispatcher.BeginInvoke/ InvokeAsync回调都是在它自己非常独特的同步上下文(一个实例DispatcherSynchronizationContext)上执行的.这种变化背后的原因是什么？

以下简单的WPF应用程序说明了这一点:

using System;
using System.Diagnostics;
using System.Threading;
using System.Windows;
using System.Windows.Threading;

namespace WpfApplication
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            Action test = null;
            var i = 0;

            test = () =>
            {
                var sc = SynchronizationContext.Current;

                Dispatcher.CurrentDispatcher.InvokeAsync(() => 
                {
                    Debug.Print("same context #" + i + ": " +
                        (sc == SynchronizationContext.Current));
                    if ( i < 10 ) 
                    {
                        i++;
                        test();
                    }
                });
            };

            this.Loaded += …

这是一个WinForms代码:

async void Form1_Load(object sender, EventArgs e)
{
    // on the UI thread
    Debug.WriteLine(new { where = "before", 
        Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread });

    var tcs = new TaskCompletionSource<bool>();

    this.BeginInvoke(new MethodInvoker(() => tcs.SetResult(true)));

    await tcs.Task.ContinueWith(t => { 
        // still on the UI thread
        Debug.WriteLine(new { where = "ContinueWith", 
            Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread });
    }, TaskContinuationOptions.ExecuteSynchronously).ConfigureAwait(false);

    // on a pool thread
    Debug.WriteLine(new { where = "after", 
        Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread });
}

输出:

{ where = before, ManagedThreadId = 10, IsThreadPoolThread = False }
{ where = ContinueWith, …

为什么以下异步递归失败StackOverflowException,为什么当计数器变为零时,它恰好发生在最后一步？

static async Task<int> TestAsync(int c)
{
    if (c < 0)
        return c;

    Console.WriteLine(new { c, where = "before", Environment.CurrentManagedThreadId });

    await Task.Yield();

    Console.WriteLine(new { c, where = "after", Environment.CurrentManagedThreadId });

    return await TestAsync(c-1);
}

static void Main(string[] args)
{
    Task.Run(() => TestAsync(5000)).GetAwaiter().GetResult();
}

输出:

...
{ c = 10, where = before, CurrentManagedThreadId = 4 }
{ c = 10, where = after, CurrentManagedThreadId = 4 }
{ c = 9, where = before, CurrentManagedThreadId = 4 …