title: Create a simple wpf trigger on one object that affects another
body: <p>This is the closest that I have come to creating a simple trigger on this.  I just want the datagrid's IsMouseOver == true to show the button.
  The problem is that the Setter's TargetName says: The property 'TargetName' does not represent a valid target for the 'Setter' because an element named 'ButtonExpand' was not found. Make sure that the target is declared before any Setters, Triggers or Conditions that use it.
  What am I doing wrong?</p>
  
  <p></p>
  
  <pre><code>&lt;UserControl.Resources&gt;
      &lt;Style TargetType="DataGrid"&gt;
          &lt;Style.Triggers&gt;
              &lt;Trigger Property="IsMouseOver" Value="True"&gt;
                  &lt;Setter TargetName="ButtonExpand" Property="Visibility" Value="Visible" /&gt;
              &lt;/Trigger&gt;
          &lt;/Style.Triggers&gt;
      &lt;/Style&gt;
  &lt;/UserControl.Resources&gt;
  &lt;Grid&gt;
  
  
  
      &lt;DataGrid Name="MainDataGrid" ItemsSource="{Binding Programs}" IsReadOnly="True" AutoGenerateColumns="false" &gt;
          &lt;DataGrid.Columns&gt;
              &lt;DataGridTextColumn Header="Name" Binding="{Binding Name}"/&gt;
              &lt;DataGridTextColumn Header="Version" Binding="{Binding Version}"/&gt;
              &lt;DataGridTextColumn Header="Publisher" Binding="{Binding Publisher}"/&gt;
          &lt;/DataGrid.Columns&gt;
      &lt;/DataGrid&gt;
  
      &lt;Button Name="ButtonExpand" Height="25" Width="25" HorizontalAlignment="Right" VerticalAlignment="Bottom" Visibility="Hidden"&gt;+&lt;/Button&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p></p>