我不敢相信居然没有人发布Xceed的WPF扩展工具包——WatermarkTextBox。它工作得很好，并且是开源的，如果你想定制的话。

编辑:虽然仍然是开源的，但这个库现在只对非商业用途免费，请参阅价格和许可证。

这里有一个最简单的解决方案:

            <Grid>
                <Label Content="Placeholder text" VerticalAlignment="Center" Margin="10">
                    <Label.Style>
                        <Style TargetType="Label">
                            <Setter Property="Foreground" Value="Transparent"/>
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding Expression}" Value="">
                                    <Setter Property="Foreground" Value="Gray"/>
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </Label.Style>
                </Label>
                <TextBox HorizontalAlignment="Stretch" Margin="5" Background="Transparent"
                 Text="{Binding Expression, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Center" Padding="5">
                </TextBox>
        </Grid>

这是一个透明背景覆盖标签的文本框。标签的灰色文本通过数据触发器变成透明的，只要绑定的文本不是空字符串就会触发该触发器。

这是一个演示如何在WPF中创建水印文本框的示例:

<Window x:Class="WaterMarkTextBoxDemo.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WaterMarkTextBoxDemo"
    Height="200" Width="400">

    <Window.Resources>

        <SolidColorBrush x:Key="brushWatermarkBackground" Color="White" />
        <SolidColorBrush x:Key="brushWatermarkForeground" Color="LightSteelBlue" />
        <SolidColorBrush x:Key="brushWatermarkBorder" Color="Indigo" />

        <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
        <local:TextInputToVisibilityConverter x:Key="TextInputToVisibilityConverter" />

        <Style x:Key="EntryFieldStyle" TargetType="Grid" >
            <Setter Property="HorizontalAlignment" Value="Stretch" />
            <Setter Property="VerticalAlignment" Value="Center" />
            <Setter Property="Margin" Value="20,0" />
        </Style>

    </Window.Resources>


    <Grid Background="LightBlue">

        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition />
            <RowDefinition />
        </Grid.RowDefinitions>

        <Grid Grid.Row="0" Background="{StaticResource brushWatermarkBackground}" Style="{StaticResource EntryFieldStyle}" >
            <TextBlock Margin="5,2" Text="This prompt dissappears as you type..." Foreground="{StaticResource brushWatermarkForeground}"
                       Visibility="{Binding ElementName=txtUserEntry, Path=Text.IsEmpty, Converter={StaticResource BooleanToVisibilityConverter}}" />
            <TextBox Name="txtUserEntry" Background="Transparent" BorderBrush="{StaticResource brushWatermarkBorder}" />
        </Grid>

        <Grid Grid.Row="1" Background="{StaticResource brushWatermarkBackground}" Style="{StaticResource EntryFieldStyle}" >
            <TextBlock Margin="5,2" Text="This dissappears as the control gets focus..." Foreground="{StaticResource brushWatermarkForeground}" >
                <TextBlock.Visibility>
                    <MultiBinding Converter="{StaticResource TextInputToVisibilityConverter}">
                        <Binding ElementName="txtUserEntry2" Path="Text.IsEmpty" />
                        <Binding ElementName="txtUserEntry2" Path="IsFocused" />
                    </MultiBinding>
                </TextBlock.Visibility>
            </TextBlock>
            <TextBox Name="txtUserEntry2" Background="Transparent" BorderBrush="{StaticResource brushWatermarkBorder}" />
        </Grid>

    </Grid>

</Window>

TextInputToVisibilityConverter定义为:

using System;
using System.Windows.Data;
using System.Windows;

namespace WaterMarkTextBoxDemo
{
    public class TextInputToVisibilityConverter : IMultiValueConverter
    {
        public object Convert( object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture )
        {
            // Always test MultiValueConverter inputs for non-null
            // (to avoid crash bugs for views in the designer)
            if (values[0] is bool && values[1] is bool)
            {
                bool hasText = !(bool)values[0];
                bool hasFocus = (bool)values[1];

                if (hasFocus || hasText)
                    return Visibility.Collapsed;
            }

            return Visibility.Visible;
        }


        public object[] ConvertBack( object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture )
        {
            throw new NotImplementedException();
        }
    }
}

注意:这不是我的代码。我是在这里找到的，但我认为这是最好的方法。

<Window.Resources>

    <Style x:Key="TextBoxUserStyle" BasedOn="{x:Null}" TargetType="{x:Type TextBox}">
      <Setter Property="Foreground" Value="Black"/>
      <Setter Property="HorizontalAlignment" Value="Center"/>
      <Setter Property="VerticalContentAlignment" Value="Center"/>
      <Setter Property="Width" Value="225"/>
      <Setter Property="Height" Value="25"/>
      <Setter Property="FontSize" Value="12"/>
      <Setter Property="Padding" Value="1"/>
      <Setter Property="Margin" Value="5"/>
      <Setter Property="AllowDrop" Value="true"/>
      <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="{x:Type TextBox}">
            <Border x:Name="OuterBorder" BorderBrush="#5AFFFFFF" BorderThickness="1,1,1,1" CornerRadius="4,4,4,4">
              <Border x:Name="InnerBorder" Background="#FFFFFFFF" BorderBrush="#33000000" BorderThickness="1,1,1,1" CornerRadius="3,3,3,3">
                <ScrollViewer SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" x:Name="PART_ContentHost"/>
              </Border>
            </Border>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>

    <Style x:Key="PasswordBoxVistaStyle" BasedOn="{x:Null}" TargetType="{x:Type PasswordBox}">
      <Setter Property="Foreground" Value="Black"/>
      <Setter Property="HorizontalAlignment" Value="Center"/>
      <Setter Property="VerticalContentAlignment" Value="Center"/>
      <Setter Property="Width" Value="225"/>
      <Setter Property="Height" Value="25"/>
      <Setter Property="FontSize" Value="12"/>
      <Setter Property="Padding" Value="1"/>
      <Setter Property="Margin" Value="5"/>
      <Setter Property="AllowDrop" Value="true"/>
      <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="{x:Type PasswordBox}">
            <Border x:Name="OuterBorder" BorderBrush="#5AFFFFFF" BorderThickness="1,1,1,1" CornerRadius="4,4,4,4">
              <Border x:Name="InnerBorder" Background="#FFFFFFFF" BorderBrush="#33000000" BorderThickness="1,1,1,1" CornerRadius="3,3,3,3">
                <Grid>
                  <Label x:Name="lblPwd" Content="Password" FontSize="11" VerticalAlignment="Center" Margin="2,0,0,0" FontFamily="Verdana" Foreground="#828385" Padding="0"/>
                  <ScrollViewer SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" x:Name="PART_ContentHost"/>
                </Grid>
              </Border>
            </Border>
            <ControlTemplate.Triggers>
              <Trigger Property="IsFocused" Value="True">
                <Setter Property="Visibility" TargetName="lblPwd" Value="Hidden"/>
              </Trigger>
            </ControlTemplate.Triggers>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>
  </Window.Resources>


        <PasswordBox Style="{StaticResource PasswordBoxVistaStyle}" Margin="169,143,22,0" Name="txtPassword" FontSize="14" TabIndex="2" Height="31" VerticalAlignment="Top" />

这可以帮助检查您的代码。当应用到密码框时，它将显示密码，当用户输入时，它将消失。

我发现这个方法非常快速和简单

<ComboBox x:Name="comboBox1" SelectedIndex="0" HorizontalAlignment="Left" Margin="202,43,0,0" VerticalAlignment="Top" Width="149">
  <ComboBoxItem Visibility="Collapsed">
    <TextBlock Foreground="Gray" FontStyle="Italic">Please select ...</TextBlock>
  </ComboBoxItem>
  <ComboBoxItem Name="cbiFirst1">First Item</ComboBoxItem>
  <ComboBoxItem Name="cbiSecond1">Second Item</ComboBoxItem>
  <ComboBoxItem Name="cbiThird1">third Item</ComboBoxItem>
</ComboBox>

也许它能帮助到任何想这么做的人

来源:http://www.admindiaries.com/displaying-a-please-select-watermark-type-text-in-a-wpf-combobox/

下面是我的方法是伟大的MVVM，在那里我也检查文本框是否有焦点，你也可以使用一个常规的触发器只是文本值以及重点是，当值改变时，我只是改变背景图像:

                    <TextBox.Style>
                        <Style TargetType="TextBox">

                            <Style.Triggers>
                                <MultiTrigger>
                                    <MultiTrigger.Conditions>
                                        <Condition Property="IsFocused" Value="True"/>
                                        <Condition Property="Text" Value=""/>
                                    </MultiTrigger.Conditions>
                                    <MultiTrigger.Setters>
                                        <Setter Property="Background">
                                            <Setter.Value>
                                                <ImageBrush ImageSource="/Images/Scan.PNG" Stretch="Uniform" AlignmentX="Left"/>
                                            </Setter.Value>
                                        </Setter>
                                    </MultiTrigger.Setters>
                                </MultiTrigger>

                            </Style.Triggers>
                        </Style>
                    </TextBox.Style>
                </TextBox>