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");
                }
            }
        }
}

当使用@john-myczek的代码绑定文本框时，我遇到了一些困难。由于TextBox在更新时不会引发焦点事件，水印将在新文本下面保持可见。为了解决这个问题，我简单地添加了另一个事件处理程序:

if (d is ComboBox || d is TextBox)
{
    control.GotKeyboardFocus += Control_GotKeyboardFocus;
    control.LostKeyboardFocus += Control_Loaded;

    if (d is TextBox)
        (d as TextBox).TextChanged += Control_TextChanged;
}


private static void Control_TextChanged(object sender, RoutedEventArgs e)
{
    var tb = (TextBox)sender;
    if (ShouldShowWatermark(tb))
    {
        ShowWatermark(tb);
    }
    else
    {
        RemoveWatermark(tb);
    }
}

你可以使用GetFocus()和LostFocus()事件来做到这一点

下面是例子:

    private void txtData1_GetFocus(object sender, RoutedEventArgs e)
    {
        if (txtData1.Text == "TextBox1abc")
        {
            txtData1.Text = string.Empty;
        }
    }

    private void txtData1_LostFocus(object sender, RoutedEventArgs e)
    {
        if (txtData1.Text == string.Empty)
        {
            txtData1.Text = "TextBox1abc";
        }
    }

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>

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。

您可以为输入的文本保留一个单独的值，并且可以在“GotFocus”和“LostFocus”事件中设置文本框的“text”字段。当您获得焦点时，如果没有值，则需要清除文本框。当你失去焦点时，你会想要设置从文本框中获取“文本”值，然后将文本框的“文本”值重置为占位符(如果它是空的)。

private String username = "";

private void usernameTextBox_GotFocus(object sender, RoutedEventArgs e) {
  if (String.IsNullOrEmpty(username)) {
    usernameTextBox.Text = "";
  }
}

private void usernameTextBox_LostFocus(object sender, RoutedEventArgs e) {
  username = usernameTextBox.Text;
  if (String.IsNullOrEmpty(usernameTextBox.Text)) {
    usernameTextBox.Text = "Username";
  }
}

然后，您只需确保文本框的“Text”值初始化为占位符文本。

<TextBox x:Name="usernameTextBox" Text="Username" GotFocus="usernameTextBox_GotFocus" LostFocus="usernameTextBox_LostFocus" />

您可以进一步将其提取到扩展“TextBox”类的类中，然后在整个项目中重用它。

namespace UI {
  public class PlaceholderTextBox : TextBox {
    public String Value { get; set; }
    public String PlaceholderText { get; set; }
    public Brush PlaceholderBrush { get; set; }
    private Brush ValuedBrush { get; set; }

    public PlaceholderTextBox() : base() {}

    protected override void OnInitialized(EventArgs e) {
      base.OnInitialized(e);

      ValuedBrush = this.Foreground;

      if (String.IsNullOrEmpty(this.Text)) {
        this.Text = PlaceholderText;
        this.Foreground = PlaceholderBrush;
      }
    }

    protected override void OnGotFocus(System.Windows.RoutedEventArgs e) {
      this.Foreground = ValuedBrush;
      if (String.IsNullOrEmpty(Value)) {
        this.Text = "";
      }

      base.OnGotFocus(e);
    }

    protected override void OnLostFocus(System.Windows.RoutedEventArgs e) {
      Value = this.Text;
      if (String.IsNullOrEmpty(this.Text)) {
        this.Text = PlaceholderText;
        this.Foreground = PlaceholderBrush;
      }

      base.OnLostFocus(e);
    }
  }
}

然后这个可以直接添加到xaml中。

<Window x:Class="UI.LoginWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:m="clr-namespace:UI"
        Initialized="Window_Initialized">
    <Grid>
        <m:PlaceholderTextBox x:Name="usernameTextBox" PlaceholderText="Username" PlaceholderBrush="Gray" />
    </Grid>
</Window>