还有，看看这个答案。你可以用VisualBrush和Style中的一些触发器更容易地完成这一点:

 <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="Search" 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>

为了增加此样式的可重用性，您还可以创建一组附加属性来控制实际的提示横幅文本、颜色、方向等。

我的解决办法很简单。

在我的登录窗口。xaml是这样的。

 <DockPanel HorizontalAlignment="Center" VerticalAlignment="Center" Height="80" Width="300" LastChildFill="True">
        <Button Margin="5,0,0,0" Click="login_Click" DockPanel.Dock="Right"  VerticalAlignment="Center" ToolTip="Login to system">
            Login
        </Button>
        <StackPanel>
            <TextBox x:Name="userNameWatermarked" Height="25" Foreground="Gray" Text="UserName" GotFocus="userNameWatermarked_GotFocus"></TextBox>
            <TextBox x:Name="userName" Height="25"  TextChanged="loginElement_TextChanged" Visibility="Collapsed" LostFocus="userName_LostFocus" ></TextBox>
            <TextBox x:Name="passwordWatermarked" Height="25" Foreground="Gray" Text="Password"  Margin="0,5,0,5" GotFocus="passwordWatermarked_GotFocus"></TextBox>
            <PasswordBox x:Name="password" Height="25" PasswordChanged="password_PasswordChanged" KeyUp="password_KeyUp" LostFocus="password_LostFocus" Margin="0,5,0,5" Visibility="Collapsed"></PasswordBox>
            <TextBlock x:Name="loginError" Visibility="Hidden" Foreground="Red" FontSize="12"></TextBlock>
        </StackPanel>
    </DockPanel>

代码是这样的。

private void userNameWatermarked_GotFocus(object sender, RoutedEventArgs e)
    {
        userNameWatermarked.Visibility = System.Windows.Visibility.Collapsed;
        userName.Visibility = System.Windows.Visibility.Visible;
        userName.Focus();
    }

    private void userName_LostFocus(object sender, RoutedEventArgs e)
    {
        if (string.IsNullOrEmpty(this.userName.Text))
        {
            userName.Visibility = System.Windows.Visibility.Collapsed;
            userNameWatermarked.Visibility = System.Windows.Visibility.Visible;
        }
    }

    private void passwordWatermarked_GotFocus(object sender, RoutedEventArgs e)
    {
        passwordWatermarked.Visibility = System.Windows.Visibility.Collapsed;
        password.Visibility = System.Windows.Visibility.Visible;
        password.Focus();
    }

    private void password_LostFocus(object sender, RoutedEventArgs e)
    {
        if (string.IsNullOrEmpty(this.password.Password))
        {
            password.Visibility = System.Windows.Visibility.Collapsed;
            passwordWatermarked.Visibility = System.Windows.Visibility.Visible;
        }
    }

仅仅决定隐藏或显示水印文本框就足够了。虽然不漂亮，但工作得很好。

<TextBox Controls:TextBoxHelper.Watermark="Watermark"/>

添加mahapps。地铁到你的项目。 将带有上述代码的文本框添加到窗口中。

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;
}

我看到了John Myczek的解决方案，以及它关于ComboBox和PasswordBox兼容性的评论，所以我改进了John Myczek的解决方案，它是:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Documents;

/// <summary>
/// Class that provides the Watermark attached property
/// </summary>
public static class WatermarkService
{
    /// <summary>
    /// Watermark Attached Dependency Property
    /// </summary>
    public static readonly DependencyProperty WatermarkProperty = DependencyProperty.RegisterAttached(
       "Watermark",
       typeof(object),
       typeof(WatermarkService),
       new FrameworkPropertyMetadata((object)null, new PropertyChangedCallback(OnWatermarkChanged)));

    #region Private Fields

    /// <summary>
    /// Dictionary of ItemsControls
    /// </summary>
    private static readonly Dictionary<object, ItemsControl> itemsControls = new Dictionary<object, ItemsControl>();

    #endregion

    /// <summary>
    /// Gets the Watermark property.  This dependency property indicates the watermark for the control.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> to get the property from</param>
    /// <returns>The value of the Watermark property</returns>
    public static object GetWatermark(DependencyObject d)
    {
        return (object)d.GetValue(WatermarkProperty);
    }

    /// <summary>
    /// Sets the Watermark property.  This dependency property indicates the watermark for the control.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> to set the property on</param>
    /// <param name="value">value of the property</param>
    public static void SetWatermark(DependencyObject d, object value)
    {
        d.SetValue(WatermarkProperty, value);
    }

    /// <summary>
    /// Handles changes to the Watermark property.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> that fired the event</param>
    /// <param name="e">A <see cref="DependencyPropertyChangedEventArgs"/> that contains the event data.</param>
    private static void OnWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Control control = (Control)d;
        control.Loaded += Control_Loaded;

        if (d is TextBox || d is PasswordBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
        }
        else if (d is ComboBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
            (d as ComboBox).SelectionChanged += new SelectionChangedEventHandler(SelectionChanged);
        }
        else if (d is ItemsControl)
        {
            ItemsControl i = (ItemsControl)d;

            // for Items property  
            i.ItemContainerGenerator.ItemsChanged += ItemsChanged;
            itemsControls.Add(i.ItemContainerGenerator, i);

            // for ItemsSource property  
            DependencyPropertyDescriptor prop = DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, i.GetType());
            prop.AddValueChanged(i, ItemsSourceChanged);
        }
    }

    /// <summary>
    /// Event handler for the selection changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="ItemsChangedEventArgs"/> that contains the event data.</param>
    private static void SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
        else
        {
            RemoveWatermark(control);
        }
    }

    #region Event Handlers

    /// <summary>
    /// Handle the GotFocus event on the control
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="RoutedEventArgs"/> that contains the event data.</param>
    private static void Control_GotKeyboardFocus(object sender, RoutedEventArgs e)
    {
        Control c = (Control)sender;
        if (ShouldShowWatermark(c))
        {
            RemoveWatermark(c);
        }
    }

    /// <summary>
    /// Handle the Loaded and LostFocus event on the control
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="RoutedEventArgs"/> that contains the event data.</param>
    private static void Control_Loaded(object sender, RoutedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
    }

    /// <summary>
    /// Event handler for the items source changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="EventArgs"/> that contains the event data.</param>
    private static void ItemsSourceChanged(object sender, EventArgs e)
    {
        ItemsControl c = (ItemsControl)sender;
        if (c.ItemsSource != null)
        {
            if (ShouldShowWatermark(c))
            {
                ShowWatermark(c);
            }
            else
            {
                RemoveWatermark(c);
            }
        }
        else
        {
            ShowWatermark(c);
        }
    }

    /// <summary>
    /// Event handler for the items changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="ItemsChangedEventArgs"/> that contains the event data.</param>
    private static void ItemsChanged(object sender, ItemsChangedEventArgs e)
    {
        ItemsControl control;
        if (itemsControls.TryGetValue(sender, out control))
        {
            if (ShouldShowWatermark(control))
            {
                ShowWatermark(control);
            }
            else
            {
                RemoveWatermark(control);
            }
        }
    }

    #endregion

    #region Helper Methods

    /// <summary>
    /// Remove the watermark from the specified element
    /// </summary>
    /// <param name="control">Element to remove the watermark from</param>
    private static void RemoveWatermark(UIElement control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            Adorner[] adorners = layer.GetAdorners(control);
            if (adorners == null)
            {
                return;
            }

            foreach (Adorner adorner in adorners)
            {
                if (adorner is WatermarkAdorner)
                {
                    adorner.Visibility = Visibility.Hidden;
                    layer.Remove(adorner);
                }
            }
        }
    }

    /// <summary>
    /// Show the watermark on the specified control
    /// </summary>
    /// <param name="control">Control to show the watermark on</param>
    private static void ShowWatermark(Control control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            layer.Add(new WatermarkAdorner(control, GetWatermark(control)));
        }
    }

    /// <summary>
    /// Indicates whether or not the watermark should be shown on the specified control
    /// </summary>
    /// <param name="c"><see cref="Control"/> to test</param>
    /// <returns>true if the watermark should be shown; false otherwise</returns>
    private static bool ShouldShowWatermark(Control c)
    {
        if (c is ComboBox)
        {
            return (c as ComboBox).SelectedItem == null;
            //return (c as ComboBox).Text == string.Empty;
        }
        else if (c is TextBoxBase)
        {
            return (c as TextBox).Text == string.Empty;
        }
        else if (c is PasswordBox)
        {
            return (c as PasswordBox).Password == string.Empty;
        }
        else if (c is ItemsControl)
        {
            return (c as ItemsControl).Items.Count == 0;
        }
        else
        {
            return false;
        }
    }

    #endregion
}

现在，一个组合框也可以编辑，密码框也可以添加水印。 别忘了用上面JoanComasFdz的评论来解决保证金问题。

当然，这都是约翰·麦切克的功劳。

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"
                >