Showing single dialog from multiple threads

Question

My scenario at a high level is the following:

Application loads and caching of data that never is going to change starts, for this I do batches of 5 threads that do a call to a WCF service.

If the service is down I'll get a popup informing the user that there was a communication error and ask whether he wants to retry or cancel the operation (this logic happens inside the thread that is making the service call, which doesn't care/know of any other running threads).

As I'm doing 5 calls at once and the service is down, I'll get asked 5 consecutive times about if I want to retry the operation or cancel it.

What I'd like to do is: as I'm showing the same question only ask the user once and return the same result to all waiting threads. Any ideas on how to achieve this? It sounds as a bad idea so I'm also open to hear other ways to achieve the same behaviour.

public string Show(string key, Action<DialogBuilder> action)
{
    lock (this) //now 4 threads are waiting here
    {       
         //marshal the dialog to the UI thread  
         return _dispatcher.Send(() => new MyDialogForm().ShowDialog()); 
    }
}

Thanks!

Teoman Soygul · Accepted Answer · 2011-05-26 09:15:19Z

1

Just cache the dialog result and provide it to all other dialogs like this (below code may need some tweaking as I wrote it in notepad):

private DialogResult threadedDialogResult;
private bool dialogShown;

public string Show(string key, Action<DialogBuilder> action)
{
    lock (this) //now 4 threads are waiting here
    {
        if (!dialogShown)
        {
            //marshal the dialog to the UI thread  
            return _dispatcher.Send(() => { dialogShown = true; this.threadedDialogResult = new MyDialogForm().ShowDialog(); });
        }
        else
        {
            // Use the cached dialog result as we know it was shown
            return threadedDialogResult.ToString();
        }
    }
}

answered May 26, 2011 at 9:15

Teoman Soygul

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4 Comments

Sebastian Piu Over a year ago

Hi Teoman, where would be the best way to reset the dialogShown flag?

Teoman Soygul Over a year ago

It is not thread safe it is best to reset it anywhere in the lock(this) { } block.

Sebastian Piu Over a year ago

but then wouldnt it be always 'false' for the next incoming thread?

Teoman Soygul Over a year ago

Well then use yet anoter private backing field like private int threadUsed = 0; and increment it in the lock() block and when it reaches 4, reset the flag.

vityanya · Accepted Answer · 2011-05-26 09:06:47Z

0

You have to use static class/method with lock in this case. But with additional logic that handle user answer and return it to other threads after unlock without showing the popup again.

answered May 26, 2011 at 9:06

vityanya

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