为什么SynchronizationContext无法正常工作？

Question

我有以下代码:

[TestMethod]
public void StartWorkInFirstThread()
{
    if (SynchronizationContext.Current == null)
        SynchronizationContext.SetSynchronizationContext(
            new SynchronizationContext());

    var syncContext = SynchronizationContext.Current;

    Console.WriteLine("Start work in the first thread ({0})", 
        Thread.CurrentThread.ManagedThreadId);

    var action = ((Action) DoSomethingInSecondThread);
    action.BeginInvoke(CallbackInSecondThread, syncContext);

    // Continue its own work
}

private static void DoSomethingInSecondThread()
{
    Console.WriteLine("Do something in the second thread ({0})", 
        Thread.CurrentThread.ManagedThreadId);   
}

private void CallbackInSecondThread(IAsyncResult ar)
{
    Console.WriteLine("Callback in the second thread ({0})", 
        Thread.CurrentThread.ManagedThreadId);
    var syncContext = (SynchronizationContext) ar.AsyncState;
    syncContext.Post(CallbackInFirstThread, null);
}

private void CallbackInFirstThread(object obj)
{
    Console.WriteLine("Callback in the first thread ({0})",
        Thread.CurrentThread.ManagedThreadId);
}

我希望最后一个方法在第一个线程中执行,即从中获取SynchronizationContext的初始线程,因为我调用Post()了这个上下文的方法.就是这样的:

Start work in the first thread (28)
Do something in the second thread (17)
Callback in the second thread (17)
Callback in the first thread (28)

是不是SynchronizationContext的含义？但实际上我有以下输出:

Start work in the first thread (28)
Do something in the second thread (17)
Callback in the second thread (17)
Callback in the first thread (7)

问题是什么？SynchronizationContext出了什么问题,或者我有一些误解？

更新:我使用Resharper测试运行器将此方法称为单元测试.

Answer 1

请参见http://www.codeproject.com/KB/threads/SynchronizationContext.aspx

有你需要的答案.您必须覆盖SynchronizationContext以使其正确处理您的操作.

阅读从:

请注意,DoWork在线程11上执行,与Run1相同.进入主线程的SynchronizationContext并不多.为什么？这是怎么回事？嗯......当你意识到生活中没有任何东西是免费的时候,这就是你的一部分.线程不能只是在它们之间切换上下文,它们必须有一个内置的基础结构才能这样做.例如,UI线程使用消息泵,在其SynchronizationContext中,它利用消息泵同步到UI线程.

Answer 2

SynchronizationContext的默认实现只是在调用线程中执行传递的委托(在调用Send/Post方法的线程中,而不是捕获上下文的线程).如果您需要某些特定行为,例如某些操作的线程关联,则应手动实现.BCL包含一些简化UI互操作性的现成实现,如WindowsFormsSynchronizationContext或DispatcherSynchronizationContext.