How to create a dll or exe for an Winform application ?

An application is an exe, not a dll. You create it by writing it and compiling it. A dll for an app to use, you create the right type of project, and compile that, then add a reference in your exe project. Is that really what you meant to ask, b.c if it was, you should take a class, rather than expecting to understand the answers we can give. Our answers do assume a basic level of knowledge, which is best got from books or classes.