If RadGridView is bound to custom objects that implement IComparable<T>, Excel-like filtering does not work. Currently, the issue can be avoided through implementing IComparable instead.
IComparable<T> is used for comparing elements in generic collections. RadGridView is not a generic control and therefore cannot use the IComparable<T> interface. In order to control the grid sorting/filtering one must implement the non-generic IComparable interface.