C# Directives

The C# File have directives such like C and C++.
Each directives have different purpose..

using system;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using system.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Xml.Linq;

public partial class : System.Web.UI.Page
{
protected void page_load(object sender, EventArgs e)
{
Label1.Text = "Asp.net";
}
}

There are many more directives which are used as per requirement...
if we have to use database connection then OleDb, SQLClient, Data etc. will be included.