Since Mango SDK, there is a property call TextTrimming.
So this xaml
<TextBlock Text="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" TextTrimming="WordEllipsis" Width="200" />
will produce somehting like "aaaaaaa....."
So for your code you just do it like:
Tb1.TextWrapping=TextWrapping.WordEllipsis
EDIT
There is a property called Wrap which u can set both in xaml and c#.
<TextBlock text={Binding Text} TextWarapping="Wrap" />
or in c# as
MyTextBlock.TextWrapping=TextWrapping.wrap;