title: The type or namespace name 'Linq' does not exist in the namespace 'System'
body: <p>When I want to use button to write code in C# it doesn't go to the &quot;.cs&quot; file to write C# code. When I check the project source, I found this error:</p>
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Web;
  using System.Web.UI;
  using System.Web.UI.WebControls;
  </code></pre>
  <blockquote>
  <p>Severity  Code    Description Project File    Line Error  CS0234  The type or
  namespace name 'Linq' does not exist in the namespace 'System' (are
  you missing an assembly reference?)   Golestani C:\Users\javad\Documents\Golestani\Login.aspx.cs 3</p>
  </blockquote>
  <p><a href="https://i.stack.imgur.com/okFlm.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/okFlm.jpg" alt="Image" /></a></p>