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

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;
}

<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。

下面是我的方法是伟大的MVVM，在那里我也检查文本框是否有焦点，你也可以使用一个常规的触发器只是文本值以及重点是，当值改变时，我只是改变背景图像:

                    <TextBox.Style>
                        <Style TargetType="TextBox">

                            <Style.Triggers>
                                <MultiTrigger>
                                    <MultiTrigger.Conditions>
                                        <Condition Property="IsFocused" Value="True"/>
                                        <Condition Property="Text" Value=""/>
                                    </MultiTrigger.Conditions>
                                    <MultiTrigger.Setters>
                                        <Setter Property="Background">
                                            <Setter.Value>
                                                <ImageBrush ImageSource="/Images/Scan.PNG" Stretch="Uniform" AlignmentX="Left"/>
                                            </Setter.Value>
                                        </Setter>
                                    </MultiTrigger.Setters>
                                </MultiTrigger>

                            </Style.Triggers>
                        </Style>
                    </TextBox.Style>
                </TextBox>

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);
    }
}