Hi In the definition of IOCS_Attach in atl_ax.c, a "this" handle is stored in the window user data with SetWindowLongPtr and the GWLP_USERDATA parameter. This is used when an ActiveX control is attached to a window. The user data is for the user, not for the OS, and a weird interaction happens when a window that already uses the user data is supposed to be hosting an ActiveX object. Yves
