You can't two-way bind a property which is read-only.
What you can do is bind your grid to an instance of QueryableCollectionView residing in your view model. This instance will at all times represent the current view of data, i.e. what the user sees in RadGridView.
So, instead of binding the grid to your sourceCollection, you will create a QueryanleCollectionView(sourceCollection), expose it as a property on your view model and then bind the grid to this new property. Enumerating this QueryanleCollectionView instance will provide you with all items that are currently in view.
I hope this helps.
the Telerik team
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