title: The type or namespace name UpdatePanel does not exist in the namespace System.Web.UI
body: <p>I'm working on a website built by another developer that targets ASP.NET 3.5. I'm trying to add an update panel around a bit of code, but whenever I attempt to do so I get the error posted in the title. I've tried including the ajaxcontroltoolkit dll with no luck.</p>
  
  <p>The following are the referrences from the web config file:</p>
  
  <pre><code>&lt;compilation debug="true"&gt;
        &lt;assemblies&gt;
          &lt;add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/&gt;
          &lt;add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/&gt;
          &lt;add assembly="System.Web.Abstractions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/&gt;
          &lt;add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/&gt;
          &lt;add assembly="System.Data.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/&gt;
          &lt;add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/&gt;
          &lt;add assembly="System.ServiceModel.Web, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/&gt;
          &lt;add assembly="System.Data.Services.Client, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/&gt;
          &lt;add assembly="System.Data.Services.Design, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/&gt;
          &lt;add assembly="System.Data.Entity, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/&gt;
        &lt;/assemblies&gt;
      &lt;/compilation&gt;
  </code></pre>
  
  <p>And when I dragged the updatepanel onto the page from the toolbox in Visual Studio 2010, this is the register statement that was added:</p>
  
  <pre><code>&lt;%@ Register assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" namespace="System.Web.UI" tagprefix="asp" %&gt;
  </code></pre>
  
  <p>What am I missing?</p>