title: Where am i supposed to use `RegisterViewWithRegion` in Prism?
body: <p>I'm creating "layout" <code>View</code> which includes few <code>ContentControl</code> elements, which results in need for me to register a "default" control to be desplayed at start. As I realize <code>RegisterViewWithRegion</code> solves this problem, but I am not sure where I'm supposed to use it?</p>
  
  <p>So far I have been using <code>RegisterViewWithRegion</code> inside constructor of layout's <code>ViewModel</code>, but this results in</p>
  
  <blockquote>
    <p>ArgumentException: Region with the given name is already registered: AuthContentRegion</p>
  </blockquote>
  
  <p>error once <code>ViewModel</code> is constructed again (second time).</p>
  
  <p>View:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;DockPanel LastChildFill="True"&gt;
      &lt;ContentControl prism:RegionManager.RegionName="{x:Static core:RegionNames.NavRegion}" DockPanel.Dock="Top" Margin="5" /&gt;
      &lt;ContentControl prism:RegionManager.RegionName="{x:Static core:RegionNames.AuthContentRegion}"  Margin="5" /&gt;
  &lt;/DockPanel&gt;
  </code></pre>
  
  <p>View Model:</p>
  
  <pre><code>public AdminViewModel(IRegionManager regionManager)
  {
      regionManager.RegisterViewWithRegion(RegionNames.NavRegion, typeof(Views.AdminNavView));
      regionManager.RegisterViewWithRegion(RegionNames.AuthContentRegion, typeof(Views.RailwayListView));
  }
  </code></pre>
  
  <p>Essentially what I want is to have default <code>UserControl</code> registered as <code>ContentControl</code>/<code>Region</code> without getting any exceptions once <code>View</code>/<code>ViewModel</code> is constructed again.</p>