Incorporate C# Code

And how difficult or easy is it to incorporate a C# source file into a Window Application...Say you have an application that you can enter 20 grades and get the sum of the grades then the average of the grades then print out what grade the student earned ....Can you implement this in a window application....I know you would take some steps in setting the behavior of the textboxes, buttons, ect....but is it possible....Should you start off with the acutual Window Application or the C# source file?