Question

I have following code:

[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);
}

I expect last method to be executed in the first thread, i.e. initial thread where SynchronizationContext is taken from, because I call Post() method of this context. I.e. something like this:

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)

Isn’t it the meaning of SynchronizationContext? But actually I have following output:

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)

What is the problem? Does something go wrong with SynchronizationContext or I have some misunderstanding?

Update: I call this method as a unit test using Resharper test runner.

Answer 1

See http://www.codeproject.com/KB/threads/SynchronizationContext.aspx

There is the answer you need. You must override SynchronizationContext to make it properly handling your operations.

Read starting from:

Notice that DoWork is executed on
thread 11, the same thread as Run1.
Not much of a SynchronizationContext
into the main thread. Why? What’s
going on? Well… This is the part
when you realize that nothing is for
free in life. Threads can’t just
switch contexts between them, they
must have an infrastructure built-in
into them in order to do so. The UI
thread, for example, uses a message
pump, and within its
SynchronizationContext, it leverages
the message pump to sync into the UI
thread.