为什么每个Dispatcher.BeginInvoke回调都有一个唯一的同步上下文？

Question

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 += (s, e) => test();
        }
    }
}

输出:

same context #0: False
same context #1: False
same context #2: False
...

设置BaseCompatibilityPreferences.ReuseDispatcherSynchronizationContextInstance以true恢复.NET 4.0行为:

public partial class App : Application
{
    static App()
    {
        BaseCompatibilityPreferences.ReuseDispatcherSynchronizationContextInstance = true;
    }
}

same context #0: True
same context #1: True
same context #2: True
...

研究.NET源代码可以DispatcherOperation看出:

[SecurityCritical]
private void InvokeImpl() 
{
    SynchronizationContext oldSynchronizationContext = SynchronizationContext.Current;

    try 
    {
        // We are executing under the "foreign" execution context, but the 
        // SynchronizationContext must be for the correct dispatcher. 
        SynchronizationContext.SetSynchronizationContext(new DispatcherSynchronizationContext(_dispatcher));

        // Invoke the delegate that does the work for this operation.
        _result = _dispatcher.WrappedInvoke(_method, _args, _isSingleParameter);
    }
    finally 
    {
        SynchronizationContext.SetSynchronizationContext(oldSynchronizationContext); 
    } 
}

我不明白为什么可能需要这个,用Dispatcher.BeginInvoke/ 排队的回调InvokeAsync无论如何都要在已经DispatcherSynchronizationContext安装了实例的正确线程上执行.

这种变化的一个有趣的副作用是,在.NET 4.5 WPF中,await TaskCompletionSource.Taskcontinuation(触发TaskCompletionSource.SetResult)几乎总是异步的,与WinForms或v4.0 WPF(更多细节)不同.

Answer 1

它在源代码中用很长的注释来解释.引用wpf\src\Base\System\Windows\BaseCompatibilityPreferences.cs中的4.5.1引用源:

    ///     WPF 4.0 had a performance optimization where it would
    ///     frequently reuse the same instance of the
    ///     DispatcherSynchronizationContext when preparing the
    ///     ExecutionContext for invoking a DispatcherOperation.  This
    ///     had observable impacts on behavior.
    ///
    ///     1) Some task-parallel implementations check the reference
    ///         equality of the SynchronizationContext to determine if the
    ///         completion can be inlined - a significant performance win.
    ///
    ///     2) But, the ExecutionContext would flow the
    ///         SynchronizationContext which could result in the same
    ///         instance of the DispatcherSynchronizationContext being the
    ///         current SynchronizationContext on two different threads.
    ///         The continuations would then be inlined, resulting in code
    ///         running on the wrong thread.
    ///
    ///     In 4.5 we changed this behavior to use a new instance of the
    ///     DispatcherSynchronizationContext for every operation, and
    ///     whenever SynchronizationContext.CreateCopy is called - such
    ///     as when the ExecutionContext is being flowed to another thread.
    ///     This has its own observable impacts:
    ///
    ///     1) Some task-parallel implementations check the reference
    ///         equality of the SynchronizationContext to determine if the
    ///         completion can be inlined - since the instances are
    ///         different, this causes them to resort to the slower
    ///         path for potentially cross-thread completions.
    ///
    ///     2) Some task-parallel implementations implement potentially
    ///         cross-thread completions by callling
    ///         SynchronizationContext.Post and Wait() and an event to be
    ///         signaled.  If this was not a true cross-thread completion,
    ///         but rather just two seperate instances of
    ///         DispatcherSynchronizationContext for the same thread, this
    ///         would result in a deadlock.

或者换句话说,他们修复了代码中的错误:)

Answer 2

我认为主要原因是4.5 DispatcherSynchronizationContext还捕获了操作DispatcherPriority,因此无法重用(此行为也可以通过配置BaseCompatibilityPreferences.FlowDispatcherSynchronizationContextPriority).

关于await- 由异步方法捕获的同步上下文与当前的同步上下文(由返回方式)SynchronizationContextAwaitTaskContinuation存在参考相等性,如果不重用上下文,这当然会失败.因此,在调度程序上排队而不是内联执行的操作.SynchronizationContext.CurrentNoFlow

这也会影响SynchronizationContextTaskScheduler,也会进行参考平等检查.

由于WPF和TPL是由不同的团队开发的,因此这两者都可能是一种疏忽.好像它是故意的.不过,在某些情况下,他们主动选择让异步延续更慢,这有点令人费解.他们不能改变行为以允许比较同步上下文的相等性(例如,通过重写Equals和检查它属于同一个Dispatcher)？也许值得打开Connect问题.