This solution is just a workaround and is not fully tested but it seems that it will work correctly. RadDataFilter is not designed to work in the presented scenario. For that reason, I try to find for you some workaround.
The control is created to work in two modes. One of them is to communicate(bound) with any kind of collections (even a plain IEnumerable). The control can then be bound to any ItemsControls (GridView, TreeListView, ComboBox, TreeView etc.) to the filtered endpoint called FilteredSource
The second one is the scenarios in which you might want to use RadDataFilter only for its UI without passing any data to it. These scenarios will require you to use the RadDataFilter's Unbound Mode feature. The usage of this feature consists of manually setting some of the RadDataFilter's properties and handling events.
Unfortunately, none of the two approaches will do your job, so I tried to find for you some workaround which seems to work.
Please give it a try and update me if you notice any issues.
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