title: set ellipse stroke(border color) when mouse enters it in wpf
body: <p>Ellipse has a property called IsMouseOver, we can use it to set ellipse color, just like <a href="https://stackoverflow.com/questions/8531516/change-the-color-of-ellipse-when-mouse-over">this post</a> does.
  But in practice, when the mouse is <em>over</em> ellipse, the stroke will be changed(image we draw ellipse as a circle), when the mouse is <em>inside</em> the ellipse(circle), the color backs to original value.
  I know ellipse has an event called <em>MouseEnter</em>, we can use EventTrigger, but only StoryBoard can be set in EventTrigger.</p>
  
  <pre><code>&lt;Grid&gt;
      &lt;Grid.ColumnDefinitions&gt;
          &lt;ColumnDefinition Width="10*" /&gt;
          &lt;ColumnDefinition Width="90*" /&gt;
      &lt;/Grid.ColumnDefinitions&gt;
      &lt;Ellipse x:Name="checkButton" Grid.Column="0" Stroke="Black"&gt;&lt;/Ellipse&gt;
      &lt;TextBlock x:Name="txtContent" Grid.Column="1" FontWeight="Bold" VerticalAlignment="Center" FontSize="14" HorizontalAlignment="Center" TextAlignment="Center"&gt;
          &lt;ContentPresenter /&gt;
      &lt;/TextBlock&gt;
  &lt;/Grid&gt;
  &lt;ControlTemplate.Triggers&gt;
      &lt;EventTrigger RoutedEvent="MouseEnter"&gt;
          &lt;BeginStoryboard&gt;&lt;/BeginStoryboard&gt;
          // something like &lt;Setter Property="Stroke" Value="Red" /&gt; here
      &lt;/EventTrigger&gt;
  &lt;/ControlTemplate.Triggers&gt;
  </code></pre>
  
  <p>All I want is when MouseEnter happens, set ellipse stroke; when MouseLeave happens, set it back.
  Does anyone have any idea? </p>
  
  <p>Thanks in advance!</p>