WPF Watermark PasswordBox from Watermark TextBox

Question

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?

Answer 1

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