'Declaration
 
Function ReceiveWeakEvent( _
   ByVal managerType As System.Type, _
   ByVal sender As System.Object, _
   ByVal e As System.EventArgs _
) As System.Boolean

System.bool ReceiveWeakEvent( 
   System.Type managerType,
   System.object sender,
   System.EventArgs e
)

true if the listener handled the event. It is considered an error by the WeakEventManager handling in WPF to register a listener for an event that the listener does not handle. Regardless, the method should return false if it receives an event that it does not recognize or handle.