The type or namespace name 'HtmlEditorExtender' does not exist in the namespace 'AjaxControlToolkit'
<pre><code> &lt;form id="form1" runat="server"&gt;   
     &lt;div&gt;
    &lt;asp:ScriptManager ID="ScriptManager1" runat="server"&gt;&lt;/asp:ScriptManager&gt;&lt;ajaxToolkit:ToolkitScriptManagerID="ToolkitScriptManager1"runat="server"&gt;
&lt;/ajaxToolkit:ToolkitScriptManager&gt;
    &lt;asp:TextBox ID="TxtEditor" runat="server"/&gt;
        &lt;ajaxToolkit:HtmlEditorExtender ID="HtmlEditorExtender1" runat="server" TargetControlID="TxtEditor"&gt;                
        &lt;/ajaxToolkit:HtmlEditorExtender&gt;
    &lt;br/&gt;
    &lt;asp:Button runat="server" Text="Submit" OnClick="Submit"/&gt;

&lt;/form&gt;
</code></pre>

<blockquote>
  <p>This is the design page and I add "&lt;%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>" at the top of the page</p>
</blockquote>

<pre><code>protected global::AjaxControlToolkit.HtmlEditorExtender HtmlEditorExtender1;
</code></pre>

<blockquote>
  <p>I am getting error on this line.</p>
</blockquote>

<pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
    &lt;configuration&gt;
    &lt;configSections&gt;
   &lt;sectionGroup name="system.web"&gt;
    &lt;section name="sanitizer" requirePermission="false" type="AjaxControlToolkit.Sanitizer.ProviderSanitizerSection, AjaxControlToolkit" /&gt;
&lt;/sectionGroup&gt;
 &lt;section name="ajaxControlToolkit" type="AjaxControlToolkit.AjaxControlToolkitConfigSection, AjaxControlToolkit" /&gt;&lt;/configSections&gt;
 &lt;system.web&gt;

&lt;compilation debug="true" targetFramework="4.5" /&gt;
&lt;httpRuntime targetFramework="4.5" /&gt;
&lt;compilation debug="true" targetFramework="4.0"&gt;
  &lt;assemblies&gt;
    &lt;add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" /&gt;
    &lt;add assembly="System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" /&gt;
    &lt;add assembly="System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" /&gt;
  &lt;/assemblies&gt;
&lt;/compilation&gt;
&lt;sanitizer defaultProvider="HtmlAgilityPackSanitizerProvider"&gt;
  &lt;providers&gt;
    &lt;add name="HtmlAgilityPackSanitizerProvider" type="AjaxControlToolkit.Sanitizer.HtmlAgilityPackSanitizerProvider"&gt;&lt;/add&gt;
  &lt;/providers&gt;
&lt;/sanitizer&gt;

  &lt;pages&gt;
  &lt;controls&gt;
    &lt;add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" /&gt;
  &lt;/controls&gt;
&lt;/pages&gt;&lt;/system.web&gt;
&lt;ajaxControlToolkit htmlSanitizer="AjaxControlToolkit.HtmlEditor.Sanitizer.DefaultHtmlSanitizer, AjaxControlToolkit.HtmlEditor.Sanitizer" /&gt;
&lt;runtime&gt;
&lt;assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1"&gt;
  &lt;dependentAssembly&gt;
    &lt;assemblyIdentity name="AjaxControlToolkit" publicKeyToken="28f01b0e84b6d53e" culture="neutral" /&gt;
    &lt;bindingRedirect oldVersion="0.0.0.0-1.0.20229.24049" newVersion="1.0.20229.24049" /&gt;
  &lt;/dependentAssembly&gt;
&lt;/assemblyBinding&gt;
&lt;/runtime&gt;
&lt;/configuration&gt;
</code></pre>

<blockquote>
  <p>This is my web.config page</p>
</blockquote>