How to prevent a dll from being loaded in other apps

currently I develop a C#.Net application in which I'm using a custom control I developed some time ago. I need the dll to be shipped within the new application - but understandably I do not want the dll file to be used for foreign apps.

That's why I need the custom dll to be somehow compiled within the new application. Currently the dll is copied into the application directory.

Any ideas? Should be trivial imho.