@Veton -我真的很喜欢你的解决方案的简单性，但我的声誉还没有高到足以撞到你。

@Tim Murphy -“双向绑定需要Path或XPath”的错误很容易修复…更新的代码，包括一些其他的小调整(仅WPF测试):

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;

public class TextBoxWatermarked : TextBox
{
  public string Watermark
  {
    get { return (string)GetValue(WaterMarkProperty); }
    set { SetValue(WaterMarkProperty, value); }
  }
  public static readonly DependencyProperty WaterMarkProperty =
      DependencyProperty.Register("Watermark", typeof(string), typeof(TextBoxWatermarked), new PropertyMetadata(new PropertyChangedCallback(OnWatermarkChanged)));

  private bool _isWatermarked = false;
  private Binding _textBinding = null;

  public TextBoxWatermarked()
  {
    Loaded += (s, ea) => ShowWatermark();
  }

  protected override void OnGotFocus(RoutedEventArgs e)
  {
    base.OnGotFocus(e);
    HideWatermark();
  }

  protected override void OnLostFocus(RoutedEventArgs e)
  {
    base.OnLostFocus(e);
    ShowWatermark();
  }

  private static void OnWatermarkChanged(DependencyObject sender, DependencyPropertyChangedEventArgs ea)
  {
    var tbw = sender as TextBoxWatermarked;
    if (tbw == null || !tbw.IsLoaded) return; //needed to check IsLoaded so that we didn't dive into the ShowWatermark() routine before initial Bindings had been made
    tbw.ShowWatermark();
  }

  private void ShowWatermark()
  {
    if (String.IsNullOrEmpty(Text) && !String.IsNullOrEmpty(Watermark))
    {
      _isWatermarked = true;

      //save the existing binding so it can be restored
      _textBinding = BindingOperations.GetBinding(this, TextProperty);

      //blank out the existing binding so we can throw in our Watermark
      BindingOperations.ClearBinding(this, TextProperty);

      //set the signature watermark gray
      Foreground = new SolidColorBrush(Colors.Gray);

      //display our watermark text
      Text = Watermark;
    }
  }

  private void HideWatermark()
  {
    if (_isWatermarked)
    {
      _isWatermarked = false;
      ClearValue(ForegroundProperty);
      Text = "";
      if (_textBinding != null) SetBinding(TextProperty, _textBinding);
    }
  }

}

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。

下面是XAML中另一个简单的解决方案:

XAML:

       <TextBox>
            <TextBox.Resources>
                <Style TargetType="TextBox">
                    <Style.Triggers>
                        <Trigger Property="IsFocused" Value="True">
                            <!--text color-->
                            <Setter Property="Foreground" Value="Black"/>
                            <Setter Property="Text" Value=""/>
                        </Trigger>
                        <Trigger Property="IsFocused" Value="False">
                            <!--placeholder color-->
                            <Setter Property="Foreground" Value="Gray"/>
                            <!--placeholder here-->
                            <Setter Property="Text" Value="Placeholder"/>
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </TextBox.Resources>
        </TextBox>

好吧，这是我的:不一定是最好的，但因为它很简单，很容易编辑到你的口味。

<UserControl x:Class="WPFControls.ShadowedTextBox"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WPFControls"
    Name="Root">
<UserControl.Resources>
    <local:ShadowConverter x:Key="ShadowConvert"/>
</UserControl.Resources>
<Grid>
    <TextBox Name="textBox" 
             Foreground="{Binding ElementName=Root, Path=Foreground}"
             Text="{Binding ElementName=Root, Path=Text, UpdateSourceTrigger=PropertyChanged}"
             TextChanged="textBox_TextChanged"
             TextWrapping="Wrap"
             VerticalContentAlignment="Center"/>
    <TextBlock Name="WaterMarkLabel"
           IsHitTestVisible="False"
           Foreground="{Binding ElementName=Root,Path=Foreground}"
           FontWeight="Thin"
           Opacity=".345"
           FontStyle="Italic"
           Text="{Binding ElementName=Root, Path=Watermark}"
           VerticalAlignment="Center"
           TextWrapping="Wrap"
           TextAlignment="Center">
        <TextBlock.Visibility>
            <MultiBinding Converter="{StaticResource ShadowConvert}">
                <Binding ElementName="textBox" Path="Text"/>
            </MultiBinding>
        </TextBlock.Visibility> 
    </TextBlock>
</Grid>

这个转换器，就像它现在写的那样，它不一定是一个MultiConverter，但这样它可以很容易地扩展

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

namespace WPFControls
{
    class ShadowConverter:IMultiValueConverter
    {
        #region Implementation of IMultiValueConverter

        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            var text = (string) values[0];
            return text == string.Empty
                       ? Visibility.Visible
                       : Visibility.Collapsed;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            return new object[0];
        }

        #endregion
    }
}

最后是背后的代码:

using System.Windows;
using System.Windows.Controls;

namespace WPFControls
{
    /// <summary>
    /// Interaction logic for ShadowedTextBox.xaml
    /// </summary>
    public partial class ShadowedTextBox : UserControl
    {
        public event TextChangedEventHandler TextChanged;

        public ShadowedTextBox()
        {
            InitializeComponent();
        }

        public static readonly DependencyProperty WatermarkProperty =
            DependencyProperty.Register("Watermark",
                                        typeof (string),
                                        typeof (ShadowedTextBox),
                                        new UIPropertyMetadata(string.Empty));

        public static readonly DependencyProperty TextProperty =
            DependencyProperty.Register("Text",
                                        typeof (string),
                                        typeof (ShadowedTextBox),
                                        new UIPropertyMetadata(string.Empty));

        public static readonly DependencyProperty TextChangedProperty =
            DependencyProperty.Register("TextChanged",
                                        typeof (TextChangedEventHandler),
                                        typeof (ShadowedTextBox),
                                        new UIPropertyMetadata(null));

        public string Watermark
        {
            get { return (string)GetValue(WatermarkProperty); }
            set
            {
                SetValue(WatermarkProperty, value);
            }
        }

        public string Text
        {
            get { return (string) GetValue(TextProperty); }
            set{SetValue(TextProperty,value);}
        }

        private void textBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            if (TextChanged != null) TextChanged(this, e);
        }

        public void Clear()
        {
            textBox.Clear();
        }

    }
}

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/

最简单的方法水印的文本框

 <Window.Resources>
    <Style x:Key="MyWaterMarkStyle" TargetType="{x:Type TextBox}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type TextBox}">
                    <Grid>
                        <Border Background="White" BorderBrush="#FF7D8683" BorderThickness="1"/>
                        <ScrollViewer x:Name="PART_ContentHost" Margin="5,0,0,0" VerticalAlignment="Center" />
                        <Label Margin="5,0,0,0" x:Name="WaterMarkLabel" Content="{TemplateBinding Tag}" VerticalAlignment="Center"
                           Visibility="Collapsed" Foreground="Gray" FontFamily="Arial"/>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <MultiTrigger>
                            <MultiTrigger.Conditions>
                                <Condition Property="Text" Value=""/>
                            </MultiTrigger.Conditions>
                            <Setter Property="Visibility" TargetName="WaterMarkLabel" Value="Visible"/>
                        </MultiTrigger>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter Property="Foreground" Value="DimGray"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>

并添加文本框StaticResource样式

  <TextBox
                Style="{StaticResource MyWaterMarkStyle}"
                Tag="Search Category"
                Grid.Row="0"
                Text="{Binding CategorySearch,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                TextSearch.Text="Search Category"
                >