title: VS Designer not working when adding a user control from another project
body: <p>I am creating a universal app in Visual Studio 2015. My universal app has a reference to a universal library called <code>UIComponents</code>.</p>
  
  <p>In <code>UIComponents</code> I created a user control:</p>
  
  <pre><code>namespace MyProj.UIComponents {
    public sealed partial class MyControl : UserControl
    {
      public MyControl()
      {
        this.InitializeComponent();
      }
    }
  }
  </code></pre>
  
  <p>With the following xaml:</p>
  
  <pre><code>&lt;UserControl
      x:Class="MyProj.UIComponents.MyControl"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="using:MyProj.UIComponents"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      mc:Ignorable="d"
      d:DesignHeight="300"
      d:DesignWidth="400"&gt;
  
      &lt;Grid&gt;
          &lt;Rectangle Fill="White" HorizontalAlignment="Left" Height="280" Stroke="Black" VerticalAlignment="Top" Width="380" Margin="10,10,0,0"/&gt;
          &lt;TextBox x:Name="textBox" Margin="20,20,20,20" TextWrapping="Wrap" Text="TextBox"/&gt;
  
      &lt;/Grid&gt;
  &lt;/UserControl&gt;
  </code></pre>
  
  <p>Inside my app project, which references <code>UIComponents</code>, I do this:</p>
  
  <pre><code>&lt;Page
      x:Class="MyProj.App.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="using:MyProj.App"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      xmlns:ui="using:MyProj.UIComponents"
      mc:Ignorable="d"&gt;
  
      &lt;Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"&gt;
          &lt;ui:MyControl&gt;&lt;/ui:MyControl&gt;
      &lt;/Grid&gt;
  &lt;/Page&gt;
  </code></pre>
  
  <p>But when I try to get the designer display the page I get:</p>
  
  <p><a href="https://i.stack.imgur.com/Q3WCv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Q3WCv.png" alt="enter image description here"></a></p>
  
  <p>The error list shows this:</p>
  
  <blockquote>
    <p>The name "MyControl" does not exist in the namespace
    "using:MyProj.UIComponents".</p>
  </blockquote>
  
  <p>Funny thing is that the whole solution builds just fine, but the designer is not collaborating.</p>
  
  <h3>Attempt using <code>clr-namespace</code></h3>
  
  <p>There are similar questions about this in WPF, so not strictly universal apps, and they are marked as solved on answers where the solution was to use:</p>
  
  <pre><code>xmlns:ui="clr-namespace:MyProj.UIComponents" 
  </code></pre>
  
  <p>Bu that does not work:</p>
  
  <blockquote>
    <p>Undefined CLR namespace. The 'clr-namespace' URI refers to a namespace
    'MyProj.UIComponents' that could not be found.</p>
  </blockquote>