Question

我刚刚注意到，使用.NET 4.5，每个Dispatcher.BeginInvoke / InvokeAsync回调都在其自己非常独特的同步上下文（DispatcherSynchronizationContext的实例）上执行。 这种变化背后的原因是什么？

以下简单的WPF应用说明了这一点：

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
...

将BaseCompatibilityPreferences.ReuseDispatcherSynchronizationContextInstance设置为true可恢复.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
...

为DispatcherOperation学习the .NET sources表明了这一点：

[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 / InvokeAsync排队的回调无论如何都要在已经安装了DispatcherSynchronizationContext实例的正确线程上执行

此更改的一个有趣的副作用是await TaskCompletionSource.Task continuation（由TaskCompletionSource.SetResult触发）在.NET 4.5 WPF中几乎总是异步的，与WinForms或v4.0 WPF不同（{{3} }）。

Answer 1

源代码中的解释很长。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也捕获了操作DispatcherPriority，因此无法重复使用（此行为也可以通过BaseCompatibilityPreferences.FlowDispatcherSynchronizationContextPriority配置）。

关于await - 在SynchronizationContextAwaitTaskContinuation中，异步方法捕获的同步上下文与当前之间存在 referencial 相等（返回者为SynchronizationContext.CurrentNoFlow），如果不重用上下文，当然会失败。因此，在调度程序上排队的操作不是内联执行。

这也会影响SynchronizationContextTaskScheduler，同时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，因此这些都可能是一种疏忽。~~似乎是故意这样做的。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，通过覆盖Equals并检查它是否属于同一个Dispatcher）？也许值得打开Connect问题。

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

2 个答案: