"UpdateSourceTrigger=PropertyChanged" equivalent for a Windows Phone 7 TextBox

UpdateSourceTrigger=Explicit doesnt work for me, hence Im using custom class derivated from TextBox

public class TextBoxEx : TextBox
{
    public TextBoxEx()
    {
        TextChanged += (sender, args) =>
                           {
                               var bindingExpression = GetBindingExpression(TextProperty);
                               if (bindingExpression != null)
                               {
                                   bindingExpression.UpdateSource();
                               }
                           };
    }

}