ASP.NET C#: using System
At the top of the .cs file (code behind) you will see a list of using statements, which act almost like includes, and provide functions from the specified file being used.
For example, the oft used ArrayList collection object is part of the System.Collections namespace. Without the directive used here below, you won’t be able to use an ArrayList in your application.
Most of these are included by default.
using System; using System.IO; using System.Data; using System.Collections; //must be in the .cs file for an ArrayList to be used. using System.Collections.Generic; using System.Linq; using System.Web; using System.Web.UI; using System.Web.UI.WebControls; using System.Web.Services;