MahApps。Metro for WPF有一个内置的水印控制，如果你不想自己滚动。使用起来相当简单。

 <AdornerDecorator>
            <TextBox Name="txtSomeText"
                     Width="200"
                     HorizontalAlignment="Right">
                <Controls:TextBoxHelper.Watermark>I'm a watermark!</Controls:TextBoxHelper.Watermark>
            </TextBox>
        </AdornerDecorator>

使用风格的简单解决方案:

<TextBox>
    <TextBox.Style>
        <Style TargetType="TextBox" xmlns:sys="clr-namespace:System;assembly=mscorlib">
            <Style.Resources>
                <VisualBrush x:Key="CueBannerBrush" AlignmentX="Left" AlignmentY="Center" Stretch="None">
                    <VisualBrush.Visual>
                        <Label Content="MM:SS:HH AM/PM" Foreground="LightGray" />
                    </VisualBrush.Visual>
                </VisualBrush>
            </Style.Resources>
            <Style.Triggers>
                <Trigger Property="Text" Value="{x:Static sys:String.Empty}">
                    <Setter Property="Background" Value="{StaticResource CueBannerBrush}" />
                </Trigger>
                <Trigger Property="Text" Value="{x:Null}">
                    <Setter Property="Background" Value="{StaticResource CueBannerBrush}" />
                </Trigger>
                <Trigger Property="IsKeyboardFocused" Value="True">
                    <Setter Property="Background" Value="White" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>

伟大的解决方案:

https://code.msdn.microsoft.com/windowsdesktop/In-place-hit-messages-for-18db3a6c

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/

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);
    }
}

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();
        }

    }
}

看看另一个简单的解决方案:

我专注于“得到专注”和“失去专注”事件。

XAML:

<Grid>
<TextBlock x:Name="DosyaİhtivaEdenDizinYansıması" Text="Hedef Dizin Belirtin" VerticalAlignment="Center" HorizontalAlignment="Center" TextAlignment="Center" Foreground="White" Background="Transparent" Width="500" MinWidth="300" Margin="10,0,0,0" Opacity="0.7"/>
<TextBox x:Name="DosyaİhtivaEdenDizin" CaretBrush="White" Foreground="White" Background="Transparent" VerticalContentAlignment="Center" HorizontalContentAlignment="Center" VerticalAlignment="Center" HorizontalAlignment="Center" MinHeight="40" BorderThickness="1" BorderBrush="White" Width="500" MinWidth="300" Margin="10,0,0,0" GotFocus="DosyaİhtivaEdenDizin_GotFocus" LostFocus="DosyaİhtivaEdenDizin_LostFocus"/>
</Grid>

C#:

    #region DosyaİhtivaEdenDizin
    private void DosyaİhtivaEdenDizin_GotFocus(object sender, RoutedEventArgs e)
    {
        if (DosyaİhtivaEdenDizin.Text.Length == 0)
        {
            DosyaİhtivaEdenDizinYansıması.Text = "";
        }
    }

    private void DosyaİhtivaEdenDizin_LostFocus(object sender, RoutedEventArgs e)
    {
        if (DosyaİhtivaEdenDizin.Text.Length == 0)
        {
            DosyaİhtivaEdenDizinYansıması.Text = "Hedef Dizin Belirtin";
        }
    }
    #endregion