Please Help! How do I add a class to another class in another file

Question

This seems like it should be incredibly obvious. That said, I've looked through all of my books and have been searching the web forever for this very basic piece of info. QUESTION: How do I add a custom class from another .cs file into my current class. If the class exists in .net, I use: using System.Web; using System.Text.RegularExpressions; How do I do something like using MyClass.cs; In the ASP.Net page side I can use: <%@ assembly src="SHXFilters.cs" %> Then I can reference the other class. Is there an equivalent inside of a .cs file? Do I need to precompile to DLL? Thank for your help. Sunny Hirai