I have a VB6 project(windows application) and I have to redevelop a module in the existing VB6 project in C#.net.
The module that I develop in C#.net should be a dll an
The only way to do it is to expose your C# class as a COM object (also called a CCW - COM Callable Wrapper), and create an instance of that COM object in your VB6 code.
This should help you get started:
http://www.bing.com/search?q=C%23+CCW&go=&form=QBRE&qs=n