I have a legacy delphi program and want to add some content implemented with WPF. So I encapsulate the WPF control with a .NET/ActiveX interop technology. That means something like: [ComRegisterFunction()] public static void RegisterClass(string key); [ComUnregisterFunction()] public static void UnregisterClass(string key); The activeX component is a WinForms User Control and the WPF materials are attached to an ElemenHost in this User Control. It works fine if the host app of this ActiveX is a MFC program even without /clr switch. But my legacy app is a delphi program, and it always throw a stackoverflow exception within the constructor of my WPF user control as the program be started. I have no clue, Google is no help. and it has puzzled me for days.