title: Getting an error trying to use System namespace in xaml
body: <p>I get the following error trying to use the String type in xaml:</p>
  
  <p>XLS0419 Undefined CLR namespace. The 'clr-namespace' URI refers to a namespace 'System' that could not be found.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>&lt;ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                      xmlns:markup="clr-namespace:MahApps.Metro.Markup;assembly=MahApps.Metro"
                      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                      xmlns:options="http://schemas.microsoft.com/winfx/2006/xaml/presentation/options"
                      xmlns:system="clr-namespace:System;assembly=mscorlib"
                      mc:Ignorable="options"&gt;
  
      &lt;!--  Matadata  --&gt;
      &lt;system:String x:Key="Theme.Name"&gt;Dark.Blue&lt;/system:String&gt;
  </code></pre>
  
  <p>The line 'xmlns:system="clr-namespace:System;assembly=mscorlib"' is the location of the error. I have simply copied this code from another file in another project in the solution, and the code works fine there. I have no idea how to resolve this.</p>
  
  <p>I have tried shutting down Visual Studio and restarting the computer.</p>