WPF: Auto truncating text in TextBlock

Is it a TextBox or TextBlock?

If it is TextBlock, then you can use:

In C#:

myTextBlock.TextTrimming = TextTrimming.CharacterEllipsis;

OR

In XAML:

<TextBlock Name="myTextBlock" Margin="20" Background="LightGoldenrodYellow" 
  TextTrimming="WordEllipsis" 
  FontSize="14"
>

But if it were a TextBox, then as per my knowledge, you can bind a ValueConverter to the textbox and return the trancated text(with dots : …) into the text box but save the full text into the Tag property of the textbox. So that your original text is not lost.

But, as per my knowledge, it is not a good practice to apply text trimming on text boxes until there is a specific requirement.