TextTrimming not working on WPF TextBlock

Hi,

TextTrimming is not working as expected and I am not getting ... following my text. Please note, I don't want to give fix width of TextBLock

<ColumnDefinition x:Uid="ColumnDefinition_2" Width="140"/>
...
...
...
<StackPanel ...><Image ... /><TextBlock ToolTip="{Binding MyTooltip}"><TextBlock Name="MyName" Text="Really a long text to test given scenario"  VerticalAlignment="Top" Height="auto"                               
                              HorizontalAlignment="Left" TextTrimming="CharacterEllipsis" ></TextBlock></TextBlock></StackPanel>

Regards, http://www.shwetalodha.blogspot.in/