namespace PlaceholderForRichTexxBoxInWPF
{
public MainWindow()
        {
            InitializeComponent();
            Application.Current.MainWindow.WindowState = WindowState.Maximized;// maximize window on load

            richTextBox1.GotKeyboardFocus += new KeyboardFocusChangedEventHandler(rtb_GotKeyboardFocus);
            richTextBox1.LostKeyboardFocus += new KeyboardFocusChangedEventHandler(rtb_LostKeyboardFocus);
            richTextBox1.AppendText("Place Holder");
            richTextBox1.Foreground = Brushes.Gray;
        }
 private void rtb_GotKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
        {
            if (sender is RichTextBox)
            {
                TextRange textRange = new TextRange(richTextBox1.Document.ContentStart, richTextBox1.Document.ContentEnd); 

                if (textRange.Text.Trim().Equals("Place Holder"))
                {
                    ((RichTextBox)sender).Foreground = Brushes.Black;
                    richTextBox1.Document.Blocks.Clear();
                }
            }
        }


        private void rtb_LostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
        {
            //Make sure sender is the correct Control.
            if (sender is RichTextBox)
            {
                //If nothing was entered, reset default text.
                TextRange textRange = new TextRange(richTextBox1.Document.ContentStart, richTextBox1.Document.ContentEnd); 

                if (textRange.Text.Trim().Equals(""))
                {
                    ((RichTextBox)sender).Foreground = Brushes.Gray;
                    ((RichTextBox)sender).AppendText("Place Holder");
                }
            }
        }
}

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>

设置文本框的占位符文本在一个柔和的颜色…

public MainWindow ( )
{
    InitializeComponent ( );
    txtInput.Text = "Type something here...";
    txtInput.Foreground = Brushes.DimGray;
}

当文本框获得焦点时，清除它并更改文本颜色

private void txtInput_GotFocus ( object sender, EventArgs e )
{
    MessageBox.Show ( "got focus" );
    txtInput.Text = "";
    txtInput.Foreground = Brushes.Red;
}

Telerik有一个控件叫做RadWatermarkTextBox专门解决这个问题。如果您正在使用Telerik控件，只需按以下方式使用即可

<telerik:RadWatermarkTextBox
    Text="{Binding Path=MyTextBoxText}"
    WatermarkContent="Please enter some text" />

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。

<TextBox x:Name="OrderTxt" HorizontalAlignment="Left" VerticalAlignment="Top" VerticalContentAlignment="Center" Margin="10,10,0,0" Width="188" Height="32"/>

<Label IsHitTestVisible="False" Content="Order number" DataContext="{Binding ElementName=OrderTxt}" Foreground="DarkGray">
    <Label.Style>
        <Style TargetType="{x:Type Label}">
            <Setter Property="Visibility" Value="Collapsed"/>
            <Setter Property="Width" Value="{Binding Width}"/>
            <Setter Property="Height" Value="{Binding Height}"/>
            <Setter Property="Margin" Value="{Binding Margin}"/>
            <Setter Property="VerticalAlignment" Value="{Binding VerticalAlignment}"/>
            <Setter Property="HorizontalAlignment" Value="{Binding HorizontalAlignment}"/>
            <Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Text}" Value="">
                    <Setter Property="Visibility" Value="Visible"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Label.Style>
</Label>