Hi Li Yang,
[quote user="李杨 Li Yang"] I am not so clear about the difference between COM Application object or standard MicroStation NET addin. [/quote]
MicroStation NET addin code is derived from Addin class and NET assembly is loaded by MicroStation (using standard MDL LOAD key-in) and it runs in the same space, so it can be treated as "a part of MicroStation".
Using COM Application (also named COM Automation or just Automation) is based on Application Object Connector object, which allows to connect to MicroStation through COM API. In this scenario, there are two separate processes - MicroStation and the application. Because the application is running separately, in some situations special care is required and some extra code is required. It's described in MicroStation VBA help file.
With regards,
Jan