WPF 设置TextBox为空时，背景为文字提示。

<TextBox FontSize="17" Height="26" Margin="230,150,189,0" Name="txt_Account" VerticalAlignment="Top" Foreground="Indigo" TabIndex="0" BorderThickness="1">
            <TextBox.Resources>
                <VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.3" Stretch="None" AlignmentX="Left">
                    <VisualBrush.Visual>
                        <TextBlock FontStyle="Italic" Text="请输入用户名"/>
                    </VisualBrush.Visual>
                </VisualBrush>
            </TextBox.Resources>
            <TextBox.Style>
                <Style TargetType="TextBox">
                    <Style.Triggers>
                        <Trigger Property="Text" Value="{x:Null}">
                            <Setter Property="Background" Value="{StaticResource HelpBrush}"/>
                        </Trigger>
                        <Trigger Property="Text" Value="">
                            <Setter Property="Background" Value="{StaticResource HelpBrush}"/>
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </TextBox.Style>
        </TextBox>