textbox获取焦点选中内容

前台：

    <TextBox VerticalAlignment="Center" HorizontalAlignment="Center" Text="Hover and focus here" Width="150">
            <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();

        }