using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;

namespace pwdwartmask.Attach
{
    public  class AttachProp
    {
        public static readonly DependencyProperty WaterMaskProperty = DependencyProperty.RegisterAttached(
            "WaterMask", typeof(string), typeof(AttachProp), new PropertyMetadata(default(string)));

        public static void SetWaterMask(DependencyObject element, string value)
        {
            element.SetValue(WaterMaskProperty, value);
        }

        public static string GetWaterMask(DependencyObject element)
        {
            return (string) element.GetValue(WaterMaskProperty);
        }

        public static readonly DependencyProperty PasswordLengthProperty = DependencyProperty.RegisterAttached(
            "PasswordLength", typeof(int), typeof(AttachProp), new PropertyMetadata(default(int)));

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

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

        public static readonly DependencyProperty OpenWaterMaskProperty = DependencyProperty.RegisterAttached(
            "OpenWaterMask", typeof(bool), typeof(AttachProp), new PropertyMetadata(false,OpenWaterCallBack));

        public static void SetOpenWaterMask(DependencyObject element, bool value)
        {
            element.SetValue(OpenWaterMaskProperty, value);
        }

        public static int GetOpenWaterMask(DependencyObject element)
        {
            return (int)element.GetValue(PasswordLengthProperty);
        }

        public static void OpenWaterCallBack(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is PasswordBox pwd)
            {
                if ((bool) e.NewValue)
                {
                    pwd.PasswordChanged += Pwd_PasswordChanged;
                }
                else
                {
                    pwd.PasswordChanged -= Pwd_PasswordChanged;
                }
            }
        }

        private static void Pwd_PasswordChanged(object sender, RoutedEventArgs e)
        {
            if (sender is PasswordBox pb)
            {
                SetPasswordLength(pb,pb.Password.Length);
            }
        }
    }
}

 <SolidColorBrush x:Key="TextBox.Static.Border" Color="#FFABAdB3"/>
        <SolidColorBrush x:Key="TextBox.MouseOver.Border" Color="#FF7EB4EA"/>
        <SolidColorBrush x:Key="TextBox.Focus.Border" Color="#FF569DE5"/>
        <Style x:Key="pwd_style" TargetType="{x:Type PasswordBox}">
            <Setter Property="PasswordChar" Value="●"/>
            <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"/>
            <Setter Property="BorderBrush" Value="{StaticResource TextBox.Static.Border}"/>
            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
            <Setter Property="BorderThickness" Value="1"/>
            <Setter Property="KeyboardNavigation.TabNavigation" Value="None"/>
            <Setter Property="HorizontalContentAlignment" Value="Left"/>
            <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
            <Setter Property="AllowDrop" Value="true"/>
            <Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
            <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
            <Setter Property="attach:AttachProp.WaterMask" Value="Test"></Setter>
            <Setter Property="attach:AttachProp.OpenWaterMask" Value="true"></Setter>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type PasswordBox}">
                        <Border x:Name="border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
                            <Grid>
                                <ScrollViewer x:Name="PART_ContentHost" Focusable="false" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
                                <TextBlock  x:Name="PART_WaterMask" Visibility="Collapsed" VerticalAlignment="Center" Foreground="Coral" FontSize="{TemplateBinding FontSize}" Text="{TemplateBinding attach:AttachProp.WaterMask }"></TextBlock>
                            </Grid>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Opacity" TargetName="border" Value="0.56"/>
                            </Trigger>
                            <Trigger Property="IsMouseOver" Value="true">
                                <Setter Property="BorderBrush" TargetName="border" Value="{StaticResource TextBox.MouseOver.Border}"/>
                            </Trigger>
                            <Trigger Property="IsKeyboardFocused" Value="true">
                                <Setter Property="BorderBrush" TargetName="border" Value="{StaticResource TextBox.Focus.Border}"/>
                            </Trigger>
                            <MultiTrigger>
                                <MultiTrigger.Conditions>
                                    <Condition Property="attach:AttachProp.PasswordLength" Value="0"></Condition>
                                </MultiTrigger.Conditions>
                                <MultiTrigger.Setters>
                                    <Setter Property="Visibility" Value="Visible" TargetName="PART_WaterMask"></Setter>
                                </MultiTrigger.Setters>
                            </MultiTrigger>
                            <Trigger Property="IsFocused" Value="True">
                                <Setter Property="Visibility" Value="Collapsed" TargetName="PART_WaterMask"></Setter>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
            <Style.Triggers>
                <MultiTrigger>
                    <MultiTrigger.Conditions>
                        <Condition Property="IsInactiveSelectionHighlightEnabled" Value="true"/>
                        <Condition Property="IsSelectionActive" Value="false"/>
                    </MultiTrigger.Conditions>
                    <Setter Property="SelectionBrush" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightBrushKey}}"/>
                </MultiTrigger>
            </Style.Triggers>
        </Style>