Interaction with C# DLLs should be done with COM, not with Interop.Call. You need to make your classes and methods "ComVisible" so that they are exposed for COM in the DLL. Have a look at
trying to create a c# COM server.
If you want your C# project to interact with the Plus! objects (Messenger, MsgPlus,...), see if you can find something in this
Tutorial on How to Let a Script-Callable DLL Handle Scripting Objects. It's written for C++, but you should be able to do the same in C# as well.
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