The items in the SelectedItems collection should be synchronized with the Items being selected in RadGridView. Consequently the GridViewSelectColumn should show those items as checked.
If you modify the SelectedItems collection from your ViewModel in code, then this will be affected in RadGridView i.e. once you add or remove an item from the SelectedItems collection this should properly affect the UI. The added item will become Selected and the removed item will become unselected.
May I ask you to share some more details on your exact case? You say the SelectedItems is synchronized with the collection of objects. Are the items in RadGridView properly selected except from the fact that the GridViewSelectColumn does not mark those rows with a check mark?
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