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.NET Forum / Languages / C# / October 2007Fire event in the same thread as the UI like BackgroundWorker
This is what I've always been wondered. Suppose I've created a class named Agent, and the Agent does some lengthy job. Of course I don't want to block the main window, so the Agent does the job in a separate thread. If the job is progressed it fires an event, and the main window handled the event by changing the value of a progress bar. The problem is that this event is fired in another thread so when the handler in the main window tries to access the progress bar, an exception occurs. Until now, I've been solving this problem by checking InvokeRequired and creating a delegate. But the .NET 2.0's built-in BackgroundWorker seems to have solved this with a very elegant way. When a BackgroundWorker fires the progress event, it looks like the event is fired in the same thread as the main window, because the InvalidOperationException does not occur when I try to access UI controls in the handler. How could this be possible? I want my Agent class to be able to the same thing. Making a lot of delegations in the main window code makes the code dirty. Please give me an idea. Thank you. > [...] > But the .NET 2.0's built-in BackgroundWorker seems to have solved this [quoted text clipped - 6 lines] > > Please give me an idea. Thank you. I don't know the specifics of how BackgroundWorker does it. Since it doesn't require a specific Control instance given to it, I suspect it's using AsyncOperation or something similar. I'm pretty sure it raises events on the same thread that created the BackgroundWorker when possible (though, if I recall correctly that's not actually what it does in a console application...it does depend on having a message pump), not specifically the main UI thread. You could either use AsyncOperation yourself (you'd initialize it when your object is created, and then use it to raise events on the same thread used to create the object). Alternatively, if your object depends on a specific Control instance, you may find it easier to just use the Control instance to invoke a method to raise the event. Pete Yes, BackgroundWorker uses AsyncOperation to marshal the call back to the correct thread. It doesn't use a particular control thread, it simply uses the thread that was used to create the BackgroundWorker object.  SignatureBrowse http://connect.microsoft.com/VisualStudio/feedback/ and vote. http://www.peterRitchie.com/blog/ Microsoft MVP, Visual Developer - Visual C# > > [...] > > But the .NET 2.0's built-in BackgroundWorker seems to have solved this [quoted text clipped - 23 lines] > > Pete Interesting. I hadn't seen the AsyncOperation class before. It's worth noting that it is just a wrapper for the SynchronizationContext class though. It is the particular current SynchronizationContext (available via SynchronizationContext.Current) that does the actual work of marshalling to the correct thread. If you're running Winforms, it'll be a WindowsFormsSynchronizationContext. For WPF, it'll be a DispatcherSynchronizationContext. So AsyncOperation makes it easier to post progress and completion of an operation to the UI (or "contextual") thread. Cool stuff. Kent On Oct 23, 1:00 am, "ken...@internode.on.net" <ken...@internode.on.net> wrote: > Interesting. I hadn't seen the AsyncOperation class before. It's worth > noting that it is just a wrapper for the SynchronizationContext class [quoted text clipped - 8 lines] > > Kent Right after I posted the original post, I thought maybe disassembling the BackgroundWorker would give me a hint. So I did it. It used several classes I'd never seen before, and the logic was quite unfamilar to me. But after all, I modified my class according to the way BackgroundWoker works, and now it works just like the BackgroundWorker. I don't need to use InvokeRequired every time. I think it would be worth for other people to try this. Thank you for your replies, all. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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