When using Parallel BeginInvoke is working while Invoke is not - c# 4.0

Question

When i use invoke inside AddListBoxItem function as seen below software become unreponsive and frozen but if i use BeginInvoke it works. Why is that happening ?

visual studio 2010 , C# 4.0

 private void button2_Click(object sender, EventArgs e)
{
    var watch = Stopwatch.StartNew();
    Parallel.For(2, 20, (i) =>
    {
        var result = SumRootN(i);
        AddListBoxItem("root " + i + " : " + result);
    });
    AddListBoxItem(watch.ElapsedMilliseconds.ToString());            
}

private delegate void AddListBoxItemDelegate(object item);

private void AddListBoxItem(object item)
{
    if (this.listBox1.InvokeRequired)
    {
        this.listBox1.Invoke(new AddListBoxItemDelegate(this.AddListBoxItem), item);
    }
    else
    {
        this.listBox1.Items.Add(item);
    }
}

Jon Skeet · Accepted Answer · 2011-10-02T20:10:38.553

3

Your UI thread will wait for Parallel.For to complete before it continues. That means it can't process any further UI messages until it's completed.

Now when the worker threads call Invoke, they wait until the UI thread processes the delegate before they continue. So they're waiting for the UI thread to get free, basically.

Hence, you have a deadlock - the UI thread is waiting for the tasks, which are waiting for the UI thread... BeginInvoke works because then the task threads don't wait for the delegates to be processed in the UI thread.

I would suggest that you don't call Parallel.For in the UI thread to start with. You'll block the UI until it completes anyway, which isn't a good idea. Do the whole thing in a background thread - then you can still use Invoke if you want, and while it's doing the computation the UI will still be responsive.

edited Oct 02 '11 at 20:10

answered Oct 02 '11 at 19:58

Jon Skeet

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how can i call it in different thread ? i mean call parallel.for ? – Furkan Gözükara Oct 02 '11 at 20:05
@MonsterMMORPG: Do the *whole thing* from a different thread in the way you would anything else - from BackgroundWorker, or using ThreadPool.QueueUserWorkItem - or even Task.Factory.StartNew. – Jon Skeet Oct 02 '11 at 20:09
i think tasks are better thanks for answer. now i understood why becoming unresponsive :) – Furkan Gözükara Oct 02 '11 at 20:10
I think dropping it in a Task has the best chance of not needing an extra Thread. – H H Oct 02 '11 at 20:19
@HenkHolterman: Probably about the same as using ThreadPool.QueueUserWorkItem, but definitely cleaner. – Jon Skeet Oct 02 '11 at 20:19

score 3 · Answer 2 · answered Oct 02 '11 at 20:01

It sounds like you are deadlocking the UI thread. This makes perfect sense, as your button2_Click doesn't exit until For completes, and in particular, no message-loop events can be processed until button2_Click has completed. If you are on a different thread, Invoke uses a message-loop event, and does not return until that item is processed - so nothing will ever be done - and For / button2_Click will never complete.

By using BeginInvoke you simply queue this work - BeginInvoke returns immediately. This means that the For can complete, which lets button2_Click complete, which then allows the message-loop events to be processed (updating the UI).

score 1 · Answer 3 · answered Oct 02 '11 at 20:01

I think it's because auf the Mainthread beeing blocked in the above Click event, waiting to finish AddListBoxItem, which is waiting the buttion2_click event to return.
You shouldn't have Controller logic in your UI, so the main problem that the Click isn't calling back a logic in a different thread.

After Implementing an thread for your logic, your GUI wouldn't block and would refresh easy in any situation.

When using Parallel BeginInvoke is working while Invoke is not - c# 4.0

3 Answers3