textbox获取焦点选中内容

前台：

     <TextBox VerticalAlignment="Center" HorizontalAlignment="Center" Text="Hover and focus here" Width="">
             <TextBox.Style>

                 <Style TargetType="TextBox">
                     <Style.Triggers>
                         <MultiTrigger>
                             <MultiTrigger.Conditions>
                                 <Condition Property="IsKeyboardFocused" Value="True" />
                                 <Condition Property="IsMouseOver" Value="True" />
                             </MultiTrigger.Conditions>
                             <MultiTrigger.Setters>
                                 <Setter Property="Background" Value="LightGreen" />

                             </MultiTrigger.Setters>
                         </MultiTrigger>
                     </Style.Triggers>

                     <EventSetter Event="PreviewMouseUp" Handler="TextBox_MouseDown"/>
                 </Style>

             </TextBox.Style>
         </TextBox>

后台：

    private void TextBox_MouseDown(object sender, MouseButtonEventArgs e)
         {
            // MessageBox.Show("ok");
             TextBox tb = sender as TextBox;
             tb.SelectAll();

         }