You don't use them in C#. C# is more "programmer friendly". When dealing with files of the same project, instead of manually specifying "header file" every time, it will go and look in all the project files for a match according to the namespace.
We benefit hugely from resources on the web so we decided we should try and give back some of our knowledge and resources to the community by opening up many of our company’s internal notes and libraries through mini sites like this. We hope you find the site helpful.