title: Extend DataGrid ColumnHeaderStyle programmatically
body: <p>I'm defining the style of DataGridColumnHeader by ResourceDictionary with a Setter there:</p>
  
  <pre><code>&lt;Style TargetType="{x:Type DataGridColumnHeader}"&gt;
    &lt;Setter Property="Background"&gt;
      &lt;Setter.Value&gt;
        &lt;LinearGradientBrush EndPoint="0,0" StartPoint="0,1"&gt;
  </code></pre>
  
  <p>and so on (not really important.</p>
  
  <p>Now I want to extend the style by a tooltip for the ColumnHeader. I have to set this tooltip in code because it is different for some situations.</p>
  
  <p>I could do it that way:</p>
  
  <pre><code>var style = new Style(typeof(System.Windows.Controls.Primitives.DataGridColumnHeader));
  style.Setters.Add(new Setter(ToolTipService.ToolTipProperty,"my tooltop"));
  dgcol1.HeaderStyle = style;
  </code></pre>
  
  <p>But obviously all other style setters from the recource dictionary are overwritten then.
  How can I add my tooltip to the ColumnHeader by code?
  Does anyone have any idea? Thank you!</p>