WPF Watermark PasswordBox from Watermark TextBox

I am using a Watermark textbox as in Watermark TextBox in WPF

 <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>

How can I apply this for a PasswordBox?

The general approach is the same: write a custom control style, and show the watermark whenever the password box is empty. The only problem here is that PasswordBox.Password is not a dependency property, and you can't use it in a trigger. Also PasswordBox is sealed, so you can't override this notification behavior, but you can use attached properties here.

The following code demonstrates how.

XAML:

<Window x:Class="WpfTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:WpfTest="clr-namespace:WpfTest"
    Title="Password Box Sample" Height="300" Width="300">
  <Window.Resources>
    <Style x:Key="{x:Type PasswordBox}"
        TargetType="{x:Type PasswordBox}">
      <Setter Property="WpfTest:PasswordBoxMonitor.IsMonitoring"
              Value="True"/>
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="{x:Type PasswordBox}">
            <Border Name="Bd"
                    Background="{TemplateBinding Background}"
                    BorderThickness="{TemplateBinding BorderThickness}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    SnapsToDevicePixels="true">
              <Grid>
                <ScrollViewer x:Name="PART_ContentHost"
                              SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                <TextBlock Text="Please enter your password" 
                           Margin="4, 2, 0, 0"
                           Foreground="Gray" 
                           Visibility="Collapsed"
                           Name="txtPrompt" />
              </Grid>
            </Border>
            <ControlTemplate.Triggers>
              <Trigger Property="IsEnabled"
                                         Value="false">
                <Setter TargetName="Bd"
                                            Property="Background"
                                            Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
                <Setter Property="Foreground"
                                            Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
              </Trigger>
              <Trigger Property="WpfTest:PasswordBoxMonitor.PasswordLength" Value="0">
                <Setter Property="Visibility" TargetName="txtPrompt" Value="Visible"/>
              </Trigger>
            </ControlTemplate.Triggers>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>
  </Window.Resources>
  <Grid>
    <PasswordBox VerticalAlignment="Top"/>
  </Grid>
</Window>

C#:

using System.Windows;
using System.Windows.Controls;

namespace WpfTest {
    public partial class Window1 : Window {
        public Window1() {
            InitializeComponent();
        }
    }

  public class PasswordBoxMonitor : DependencyObject {
    public static bool GetIsMonitoring(DependencyObject obj) {
      return (bool)obj.GetValue(IsMonitoringProperty);
    }

    public static void SetIsMonitoring(DependencyObject obj, bool value) {
      obj.SetValue(IsMonitoringProperty, value);
    }

    public static readonly DependencyProperty IsMonitoringProperty =
        DependencyProperty.RegisterAttached("IsMonitoring", typeof(bool), typeof(PasswordBoxMonitor), new UIPropertyMetadata(false, OnIsMonitoringChanged));

    public static int GetPasswordLength(DependencyObject obj) {
      return (int)obj.GetValue(PasswordLengthProperty);
    }

    public static void SetPasswordLength(DependencyObject obj, int value) {
      obj.SetValue(PasswordLengthProperty, value);
    }

    public static readonly DependencyProperty PasswordLengthProperty =
        DependencyProperty.RegisterAttached("PasswordLength", typeof(int), typeof(PasswordBoxMonitor), new UIPropertyMetadata(0));

    private static void OnIsMonitoringChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
      var pb = d as PasswordBox;
      if (pb == null) {
        return;
      }
      if ((bool) e.NewValue) {
        pb.PasswordChanged += PasswordChanged;
      } else {
        pb.PasswordChanged -= PasswordChanged;
      }
    }

    static void PasswordChanged(object sender, RoutedEventArgs e) {
      var pb = sender as PasswordBox;
      if (pb == null) {
        return;
      }
      SetPasswordLength(pb, pb.Password.Length);
    }
  }
}

Please notice PasswordBoxMonitor in XAML code.