Active Technology and DLLs
Active technology allows object servers to be completely implemented inside a DLL. This type of server is called an "in-process server." MFC does not completely support in-process servers for all of the features of visual editing, mainly because Active technology does not provide a way for a server to hook into the container's main message loop. MFC requires access to the container application's message loop to handle accelerator keys and idle-time processing.
If you are writing an Automation server and your server has no user interface, you can make your server an in-process server and put it completely into a DLL.