title: ASP.NET validation message "Attribute ... is not a valid attribute of element ..." for user control
body: <p>We have ascx user controls written in VB.NET for an ASP.NET 4.0 project. The built-in VS page validator always displays the message from the question for all custom properties of our user control.</p>
  
  <p>For instance, here is the beginning of the code of one of our controls:</p>
  
  <pre><code>&lt;%@ Control Language="VB" ClassName="PicView" %&gt;
  &lt;%@ Import Namespace="System.Drawing" %&gt;
  &lt;script runat="server"&gt;
      Public ImageUrl As String
  </code></pre>
  
  <p>When we try to use this control using code like this</p>
  
  <pre><code>&lt;%@ Register TagPrefix="foo" TagName="PicView" src="~/ascx/PicView.ascx" %&gt;
  &lt;foo:PicView ImageUrl="screenshots/image.gif" runat="server" /&gt;
  </code></pre>
  
  <p>, the "Error List" pane displays this message:</p>
  
  <p>Attribute 'ImageUrl' is not a valid attribute of element 'PicView'</p>
  
  <p>The property works fine in compiled aspx pages, but how to get rid of this in the VS IDE? And enable IntelliSense for such properties if it's possible?</p>