Plug in concept

Question

I want to develop plug in concept in my work. First time I want to build a program and after it I only add few plug in dll to make better my work without touching my main program. But i am unable to implement it.

Please help me.

I give you my little idea. a small *.exe has three textfield , comboBox & one button. One plug in has add function, another has subtract method.

So, first that *.exe will consider them that those(*.dll) are extend part of them, and load them. then choosing the method in comboBox the operation will executed, (add,subtract etc).

That's the idea but I can't do anything.

Please share me your Idea.

Plug in concept will help every one for better project implementation.

Thank you in advance.