It could be interesting to attach a command to a TextBox if the enter key is pressed while the TextBox is focused so to provide a better UX to the end user, the TextBoxManager Command attached property (or behavior) comes to the rescue in this scenario:

<TextBox Text="bla bla...">
<behaviors:TextBoxCommandBehavior Command="{Binding Path=MyCommand}" />
<TextBox Text="bla bla..." behaviors:TextBoxManager.Command="{Binding Path=MyCommand}" />

the attached property, and the behavior too, is defined in the xml namespace.