In visual studio we can create c# class library and reference it in an external project very easy. It can be exported as a Shared Project which I already described here or as a dll file.

So to build the class library project into dll, first of all we have to change the output type to Class Library.
Right Click on the project directory and choose Properties

From the Output Type selection, choose Class Library

We can choose to build either for x64 or x86. It's depend on the target we want to build for. So now I'm building for x64 target

Seem like the configuration is set up, now we can proceed with the build.
Right Click on the project directory and choose Build

Wait until the build process is completed and then we can get the dll class library from this following directory:
/ProjectFolder/bin/x64/Debug/