Using 2 DLL's in one solution

Question

Hello there,

I'm facing a problem for which I hope to get some help on this forum.
I simplified the idea with the following case:

We have an executable (general.exe), and 2 DLL's (Calculation1.dll and Calculation2.dll). All made with C#. Both DLL's got a function called Calc(int x, int y) and return an integer.
The first dll returns value x*y and the second returns the sum of x and y.

The executable imports only Calculation.dll. Depending on which of the two DLL's I call Calculation.dll I want the general.exe to print the x*y or x+y. I'd like to be able to make another DLL with devides x by y as well for the future without changing the general.exe.

I hope I made the case clear. If you have any questions feel free to contact me.

With kind regards,
Niels