When we create a .cs file means code file class is automatically create. That .cs file means class has default usings for namespace.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
We don't use all namespaces in that class so we have to remove unused namespaces from file. We can remove namespace manually but it takes time and need full knowledge of class library so we can use Visual Studio .
Step 1: Right Click in .cs File.
Step 2: Move on Organize usings.
Step 3: Click on Remove Unused Usings.
After that we have only those namespaces in the file these are using.