CollectionChanged notification across threads?

Question

I'm writing a download manager using C#/WPF, and I just encountered this error:

This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread.

The basic flow of my program is that a few web pages/downloads are enqueued at the start, and then they're downloaded asynchronously. When an HTML page has completed downloading, I parse it and look for more stuff to download, then enqueue it directly from within the worker thread.

I get that error when trying to send out the CollectionChanged event on my customized queue class. However, I need to fire that event so that the GUI can get updated.

What are my options?

Answer 1

You just need to make sure that you use the Dispatcher for the relevant thread to invoke any code that updates the collection, or fires the event.

http://msdn.microsoft.com/en-us/magazine/cc163328.aspx

Update

The Dispatcher is a property on all of the classes that derive from DispatcherObject, which includes all DependencyObjects, Visuals, etc.

So, your GUI objects will all have a Dispatcher property.

Answer 2

You might want to look into the Reactive Extensions from Microsoft, they're designed for just this sort of thing. They create "observable collections" and are useful in paralell and multi-core scenarios.

UPDATE:

You may also find the SyncronizationContext useful as well. That's a Windows Forms sample, but there are variations for WPF as well.

Answer 3

Whenever I had to deal with collection change notifications across threads, I always turned to How can I propagate changes across threads blog article for help.

I am not sure from the context of your question if this can indeed help you, but if your collection is something you directly control, you can benefit from this.