Must be used like this:
Autodesk.AutoCAD.Interop.AcadDocument acaddoc = (Autodesk.AutoCAD.Interop.AcadDocument)doc.AcadDocument;
acaddoc.BeginDoubleClick += new _DAcadDocumentEvents_BeginDoubleClickEventHandler(acaddoc_BeginDoubleClick);
NOT like below:
AcadDocument acaddoc =
(AcadDocument)doc.AcadDocument;
acaddoc.BeginDoubleClick += new
_DAcadDocumentEvents_BeginDoubleClickEventHandler(acaddoc_BeginDoubleClick);
The below one can only work well about 3 times in a new/open drawing. It wastes me a whole day to find out! From 8:00PM to 2:00AM.
I think this should be a .NET API defect!