title: Compilation Error when using AJAX Control From AJAX ToolKit
body: <p>I tried using AJAX Control for my first time after I installed the AJAX Toolkit for ASP.NET. I created a new ASP.NET website and added the ConfirmButtonExtender. 
  After I run it without changing anything I get this error:</p>
  
  <blockquote>
    <p>Compilation Error<br>
    Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately. </p>
    
    <p>Compiler Error Message: CS0012: The type 'System.Web.UI.ExtenderControl' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.</p>
  </blockquote>
  
  <p>Source Error:</p>
  
  <pre><code>Line 16:         &lt;br /&gt;
  Line 17:         &lt;asp:Label ID="Label1" runat="server" Text="Label" Width="229px"&gt;&lt;/asp:Label&gt;&lt;br /&gt;
  **Line 18:         &lt;asp:ConfirmButtonExtender ID="ConfirmButtonExtender1" runat="server"** ConfirmText="are you sure"
  Line 19:             TargetControlID="Button1"&gt;
  Line 20:         &lt;/asp:ConfirmButtonExtender&gt;
  </code></pre>
  
  <p>I tried connecting this Control with a button as I saw in a tutorial video or adding the ToolkitScriptManager but it gave me the same error.</p>
  
  <p>Does anyone know how can I solve this error?</p>