You have an instance of the Microsoft HTML parsing and rendering engine (MSHTML) that is hosted by Microsoft Internet Explorer or by a WebBrowser control. When you use the ICustomDoc::SetUIHandler method to set the IDocHostUIHandler interface for the MSHTML instance, a memory leak occurs.
The ICustomDoc interface is designed for applications that host MSHTML directly. It is not intended to replace an existing IDocHostUIHandler object that is provided by the true MSHTML host. Either Internet Explorer or a WebBrowser control will host MSHTML and will provide their own implementation of the IDocHostUIHandler interface. If you try to replace that interface by using the ICustomDoc interface, you may experience unexpected behavior.
Applications that directly host the WebBrowser control or MSHTML can provide an IDocHostUIHandler interface by using the method that is described on the following Microsoft Developer Network (MSDN) Web site: