WPF TextBox文本为空时，显示水印

 public class TextBoxExtHelper
    {
 
        public static string GetIsNullString(DependencyObject obj)
        {
            return (string)obj.GetValue(IsNullStringProperty);
        }

        public static void SetIsNullString(DependencyObject obj, string value)
        {
            obj.SetValue(IsNullStringProperty, value);
        }

        // Using a DependencyProperty as the backing store for IsNullString.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty IsNullStringProperty =
            DependencyProperty.RegisterAttached("IsNullString", typeof(string), typeof(TextBoxExtHelper), new PropertyMetadata(""));
 
    }

添加个附加属性，文本为空时，显示的水印名称

    <Style x:Key="TextBoxStyleWithWatermark" TargetType="{x:Type TextBox}">
           
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type TextBox}">
                        <Border x:Name="Bg"
                            SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}">
                            <Grid x:Name="PART_InnerGrid">
                                
                                <!--文本和水印-->
                                <ScrollViewer x:Name="PART_ContentHost"
                                          BorderThickness="0"
                                          Grid.Column="1" 
                                          IsTabStop="False"
                                          Margin="2"
                                          VerticalAlignment="Stretch"
                                          Background="{x:Null}" />
                                <TextBlock x:Name="WaterMark"
                                       Grid.Column="1"
                                       VerticalAlignment="Center"
                                       Foreground="Silver"
                                       FontSize="18"
                                       Text="{TemplateBinding txtExt:TextBoxExtHelper.IsNullString}"
                                       Visibility="Collapsed"
                                       Padding="5,0,0,0" />
 
                            </Grid>
                        </Border>

                        <ControlTemplate.Triggers>
                            <!--当Text为空时，隐藏删除按钮-->
                            <DataTrigger Binding="{Binding RelativeSource={RelativeSource Self},Path=Text}" Value="">
                                <Setter Property="Visibility" TargetName="WaterMark" Value="Visible" />
                            </DataTrigger>

                            
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

添加textbox的style

用法如下：

<TextBox x:Name="txt" Style="{StaticResource TextBoxStyleWithWatermark}" Width="120" Height="23" txtExt:TextBoxExtHelper.IsNullString="请输入文本"/>