public class TextBlockUtils
{
  /// <summary>
  /// Gets the value of the AutoTooltipProperty dependency property
  /// </summary>
  public static bool GetAutoTooltip(DependencyObject obj)
  {
    return (bool)obj.GetValue(AutoTooltipProperty);
  }
 
  /// <summary>
  /// Sets the value of the AutoTooltipProperty dependency property
  /// </summary>
  public static void SetAutoTooltip(DependencyObject obj, bool value)
  {
    obj.SetValue(AutoTooltipProperty, value);
  }
 
  /// <summary>
  /// Identified the attached AutoTooltip property. When true, this will set the
  /// TextBlock TextTrimming property to WordEllipsis, and display a tooltip with
  /// the full text whenever the text is trimmed.
  /// </summary>
  public static readonly DependencyProperty AutoTooltipProperty =  
           DependencyProperty.RegisterAttached("AutoTooltip", typeof(bool), typeof(TextBlockUtils),
            new PropertyMetadata(false, OnAutoTooltipPropertyChanged));
 
  private static void OnAutoTooltipPropertyChanged(DependencyObject d,
                                     DependencyPropertyChangedEventArgs e)
  {
    TextBlock textBlock = d as TextBlock;
    if (textBlock == null)
      return;
 
    if (e.NewValue.Equals(true))
    {
      textBlock.TextTrimming = TextTrimming.WordEllipsis;
      ComputeAutoTooltip(textBlock);
      textBlock.SizeChanged += TextBlock_SizeChanged;
    }
    else
    {
      textBlock.SizeChanged -= TextBlock_SizeChanged;
    }
  }
 
  private static void TextBlock_SizeChanged(object sender, SizeChangedEventArgs e)
  {
    TextBlock textBlock = sender as TextBlock;
    ComputeAutoTooltip(textBlock);
  }
 
    /// <summary>
  /// Assigns the ToolTip for the given TextBlock based on whether the text is trimmed
  /// </summary>
  private static void ComputeAutoTooltip(TextBlock textBlock)
  {
    FrameworkElement parentElement = VisualTreeHelper.GetParent(textBlock) as FrameworkElement;
    if (parentElement != null)
    {
      if (textBlock.ActualWidth > parentElement.ActualWidth)
      {
        ToolTipService.SetToolTip(textBlock, textBlock.Text);
      }
      else
      {
        ToolTipService.SetToolTip(textBlock, null);
      }
    }
  }
}

<Grid util:GridUtils.ColumnDefinitions="2*,"
      util:GridUtils.RowDefinitions=",">
 
  <sdk:GridSplitter Grid.Column="0" Grid.Row="0" Grid.RowSpan="2"
                    ShowsPreview="True"
                    Background="LightGray"
                    Width="10"
                    HorizontalAlignment="Right"/>
 
  <Grid Grid.Column="0"
        Margin="0,0,10,0">
    <TextBlock Text="The rain in Spain stays mainly in the plain"   
                util:TextBlockUtils.AutoTooltip="True"/>
  </Grid>
 
  <Grid Grid.Column="2">
    <TextBlock Text="In Hertford, Hereford, and Hampshire, hurricanes hardly ever happen"
                util:TextBlockUtils.AutoTooltip="True"/>
  </Grid>
 
  <Grid Grid.Column="0" Grid.Row="1"
        Margin="0,0,10,0">
    <TextBlock Text="Peter Piper picked a peck of pickled peppers"
                util:TextBlockUtils.AutoTooltip="True"/>
  </Grid>
 
  <Grid Grid.Column="2" Grid.Row="1">
    <TextBlock Text="She sells sea-shells on the sea-shore"
                util:TextBlockUtils.AutoTooltip="True"/>
  </Grid>
</Grid>