UI update after background operation blocks binding on ObservableCollection

Question

I have this:

1. Shows a waiting animation to 'block' the UI while performs a loading operation in the background.
2. At the end of the loading I call a method that instances a User Control and displays some data by using Bindings (and ObservableCollection among others)
3. This User Control gets displayed and user can interact with it, however the ObservableCollection seems to be stuck in another thread as it doesn't allow to add new items to it.

I've tried to update the UI at the Completed event of a BackgroundWorker, using Dispatcher, using DispatchTimer... all of this displays the User Control, but the ObservableCollection stays of out reach for adding.

The code that tries to add items to the collection is inside the UserControl.

The exact error is: "This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread"

This does not happen if I don't do the loading in the background.

Thank you for any workaround for this.

By the way, trying to add the items using Dispatcher doesn't work either.

In other words, what I would like to do is to create an object in the UI Thread while being in the background... I know this may sounds silly.

Answer 1

You may have to check which Dispatcher you are using? In your case you could be referring to two different dispatchers.

Also why not use thread safe observable collection?

Answer 2

Usually I will create the objects on my UI thread, then populate them with data obtained from a background thread.

For example,

void async LoadButton_Clicked(object sender, EventArgs e)
{
    MyCollection = new ObservableCollection<SomeItem>();

    // Can also use a BackgroundWorker
    var collectionData = await GetCollectionData();

    foreach(var item in collectionData)
    {
        MyCollection.Add(item);
    }
}

I'm using C# 5.0 async and await keywords for asynchronous operations, but you can also use a BackgroundWorker that does your background work.

You can also use Dispatcher.BeginInvoke() for some lighter background work (such as copying data into MyCollection), although for heavy work I find it still locks up the UI so I prefer to use background threads.

Answer 3

It is not possible to modify the contents of an ObservableCollection on a separate thread if a view is bound to this collection, instead you can override ObservableCollection and provide support for it and use it across your application.

This sample contains exactly what you want - http://tomlev2.wordpress.com/2009/04/17/wpf-binding-to-an-asynchronous-collection/

Answer 4

When it comes to threads and ui-elements one of the most important rules to follow which may safe you a lot of trouble in the long run is to keep ui-element instantiation on the ui-thread. Surely you can manage that. And if you need to change those objects from another thread you can use the Dispatcher.

(The threading model reference may also be of interest)

Answer 5

Thank you everyone for your help... a guy from MS visited the company (sorry for the commercial annotation) to do other things, I stoled him and show this behavior. In a matter of 2 minutes founds the source of the problem... which I'm not sure to really understand.

It happens that I'm using an ICollectionView to display a sorted/filtered version of my problematic ObservableCollection. I was creating this ICollectionView in the constructor of my class, so at the moment of deserialization it was created in another thread. He suggested to move this creation to a further time in code (when the related property gets read). This solved the problem.

However the ObservableCollection, created in that other thread, now lets me add new item. Not sure why, but now it works.

Sorry for being this late and thank you again.