HttpContext namespace

I need to write a NUnit script to test the cache data. I added a project(Class libray) in my project and add reference to NUnit.Framework. But i am not getting the "HttpContext" in my code. Please tell me which namespace i should include to get this. I am doing with the help of the post

Unit test HttpContext.Curre开发者_如何学JAVAnt.Cache or other server-side methods in C#?

Please help

Thanks San

HttpContext is a class in NameSpace System.Web