Generic collections are used by entity framework to contain child objects.
UPDATE: RadPropertygrid uses the standard System.ComponentMode.Design.CollectionEditor when editing collections. The editor requirements are that the collection being edited implements the IList interface and that it has an indexer (Item in VB.NET) property. Entity Framework generates an ICollection<T> property which holds a HasSet<T> object. This does not fit both requirements of the editor and prevents it from working correctly.