<TextBox FontSize="" Height="" Margin="230,150,189,0" Name="txt_Account" VerticalAlignment="Top" Foreground="Indigo" TabIndex="" BorderThickness="">

            <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>