I turned my C++ Dynamic link library into Static library just to acquire more knowledge. My question is how can I use the .obj file to compile both projects with C# express/
No, you can't access static libraries directly from C#. You have to use a DLL.
The way to "use" a static library in C# is to first create a Managed C++ wrapper that will provide a facade for the rest of the managed world. As everyone else has already commented, C# only supports DLLs for P/Invoke.
No way to do that.
Only call DLL functions in runtime or create a COM object from your library
"Static library" means that the library is going to be merged with your final application. This concept doesn't exist in .net. .net supports DLLs only.