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What is the difference between SynchronizationContext.Send and SynchronizationContext.Post?

Thanks to Jeremy Miller's good work in Functional Programming For Everyday .NET Development, I have a working command executor that does everything I want it to (do heavy lifting on the thread pool, send results or errors back to the synchronization context, and even post progress back to the synchronization context), but I can't explain why it uses SynchronizationContext.Send from the thread-pool and Synchronization.Post from the Func passed into the method that does the heavy lifting. I have read the documentation, several times, but I just can't get a gut sense for what the difference is. What am I supposed to get from the fact that one is called Send and one is called Post? I sense the magic is in the fact Send "starts a synchronous request" and Post "starts an asynchronous request", but both requests come from the thread pool and need to be sent/posted back to the UI thread.

Can someone explain the difference, even if it is just a mnemonic device that lets me know when to choose one over the other?

In case it matters, this is my test code where I use Post to send progress back to the UI:

private Action _ExecuteCommand
                (SynchronizationContext context
                 , Action<int, int> progress
                 , Action<int, int> after)
{
    int count = 3;
    int accumulatedValue = 0;
    int threadId = Thread.CurrentThread.ManagedThreadId;
    for (int i = 0; i < count; i++)
    {
        Thread.Sleep(1000);
        context.Post(delegate { progress(i + 1, threadId); });
        accumulatedValue += i;
    }

    return () => after(threadId, accumulatedValue);
}

That _ExecuteCommand method is passed in as the command parameter below, mostly from the original article, that uses Send to send completion and error message back to the UI:

public void Execute(Func<Action> command, Action<Exception> error)
{
    ThreadPool.QueueUserWorkItem(o =>
     {
         try
         {
             Action continuation = command();
             _Context.Send(s => continuation());
         }
         catch (Exception e)
         {
             _Context.Send(s => error(e));
         }
     });
}
like image 662
flipdoubt Avatar asked Mar 18 '10 20:03

flipdoubt


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1 Answers

Send - synchronous: wait for answer (or action completed)

Post - asynchronous: drop off and continue

So your example uses the correct methods at the right moments. There is no need to halt the for-loop until the progress update is complete (on the contrary).
And Execute does want to wait for the Action to complete, otherwise the exception handling has no purpose.

like image 147
Henk Holterman Avatar answered Sep 21 '22 13:09

Henk Holterman