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>

我决定通过一个行为来解决这个问题。它使用Hint属性定义要显示的文本(如果您愿意，也可以是一个对象)，并使用Value属性计算提示是否应该可见。

行为声明如下:

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

    public class HintBehavior : Behavior<ContentControl>
    {
        public static readonly DependencyProperty HintProperty = DependencyProperty
            .Register("Hint", typeof (string), typeof (HintBehavior)
            //, new FrameworkPropertyMetadata(null, OnHintChanged)
            );

        public string Hint
        {
            get { return (string) GetValue(HintProperty); }
            set { SetValue(HintProperty, value); }
        }

        public static readonly DependencyProperty ValueProperty = DependencyProperty
            .Register("Value", typeof (object), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(null, OnValueChanged));

        private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var visible = e.NewValue == null;
            d.SetValue(VisibilityProperty, visible ? Visibility.Visible : Visibility.Collapsed);
        }

        public object Value
        {
            get { return GetValue(ValueProperty); }
            set { SetValue(ValueProperty, value); }
        }

        public static readonly DependencyProperty VisibilityProperty = DependencyProperty
            .Register("Visibility", typeof (Visibility), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(Visibility.Visible
                    //, new PropertyChangedCallback(OnVisibilityChanged)
                    ));

        public Visibility Visibility
        {
            get { return (Visibility) GetValue(VisibilityProperty); }
            set { SetValue(VisibilityProperty, value); }
        }

        public static readonly DependencyProperty ForegroundProperty = DependencyProperty
            .Register("Foreground", typeof (Brush), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(new SolidColorBrush(Colors.DarkGray)
                    //, new PropertyChangedCallback(OnForegroundChanged)
                    ));

        public Brush Foreground
        {
            get { return (Brush) GetValue(ForegroundProperty); }
            set { SetValue(ForegroundProperty, value); }
        }

        public static readonly DependencyProperty MarginProperty = DependencyProperty
            .Register("Margin", typeof (Thickness), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(new Thickness(4, 5, 0, 0)
                    //, new PropertyChangedCallback(OnMarginChanged)
                    ));

        public Thickness Margin
        {
            get { return (Thickness) GetValue(MarginProperty); }
            set { SetValue(MarginProperty, value); }
        }


        private static ResourceDictionary _hintBehaviorResources;

        public static ResourceDictionary HintBehaviorResources
        {
            get
            {
                if (_hintBehaviorResources == null)
                {
                    var res = new ResourceDictionary
                    {
                        Source = new Uri("/Mayflower.Client.Core;component/Behaviors/HintBehaviorResources.xaml",
                            UriKind.RelativeOrAbsolute)
                    };
                    _hintBehaviorResources = res;
                }
                return _hintBehaviorResources;
            }
        }


        protected override void OnAttached()
        {
            base.OnAttached();
            var t = (ControlTemplate) HintBehaviorResources["HintBehaviorWrapper"];
            AssociatedObject.Template = t;
            AssociatedObject.Loaded += OnLoaded;
        }

        private void OnLoaded(object sender, RoutedEventArgs e)
        {
            AssociatedObject.Loaded -= OnLoaded;
            var label = (Label) AssociatedObject.Template.FindName("PART_HintLabel", AssociatedObject);
            label.DataContext = this;
            //label.Content = "Hello...";
            label.SetBinding(UIElement.VisibilityProperty, new Binding("Visibility") {Source = this, Mode = BindingMode.OneWay});
            label.SetBinding(ContentControl.ContentProperty, new Binding("Hint") {Source = this, Mode = BindingMode.OneWay});
            label.SetBinding(Control.ForegroundProperty, new Binding("Foreground") {Source = this, Mode = BindingMode.OneWay});
            label.SetBinding(FrameworkElement.MarginProperty, new Binding("Margin") {Source = this, Mode = BindingMode.OneWay});
        }
    }

它用自己的模板包装目标，并向其添加一个标签:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ControlTemplate x:Key="HintBehaviorWrapper" TargetType="{x:Type ContentControl}">
        <Grid>
            <ContentPresenter Content="{TemplateBinding Content}" />
            <Label x:Name="PART_HintLabel" IsHitTestVisible="False" Padding="0" />
        </Grid>
    </ControlTemplate>
</ResourceDictionary>

要使用它，只需将其添加为一个行为并绑定你的值(在我的情况下，我将它添加在一个ControlTemplate中，因此绑定):

<ContentControl>
    <i:Interaction.Behaviors>
        <behaviors:HintBehavior Value="{Binding Property, RelativeSource={RelativeSource TemplatedParent}}"
                                                        Hint="{Binding Hint, RelativeSource={RelativeSource TemplatedParent}}" />
    </i:Interaction.Behaviors>
    <TextBox ... />
</ContentControl>

如果这是一个干净的解决方案，我希望得到反馈。它不需要静态字典，因此没有内存泄漏。

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;
}

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);
    }
}

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/