Utilizing the Attached Dependency Property in WPF using C#

<Window x:Class="AttachedDependencyExample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:AttachedDependencyExample"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <TextBox Padding="2,10,0,0" HorizontalContentAlignment="Left" x:Name="TxtbxWatermark" local:AttachedTextBoxProperty.WatermarkText="Enter text here...." Height="40" Margin="20"/>
        <Button Content="Get TextBox Value" Height="30" Margin="20,100,20,0" Click="Button_Click"/>
    </Grid>
</Window>

Code Behind implementation

using System.Windows;

namespace AttachedDependencyExample
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            // Access the attached property value programmatically
            string propertyValue = AttachedTextBoxProperty.GetWatermarkText(TxtbxWatermark);
            MessageBox.Show(propertyValue);
        }
    }
}

Step 2. Create a custom class for an attached property

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

namespace AttachedDependencyExample
{
    public static class AttachedTextBoxProperty
    {
        public static readonly DependencyProperty WatermarkTextProperty =
            DependencyProperty.RegisterAttached(
                "WatermarkText",
                typeof(string),
                typeof(AttachedTextBoxProperty),
                new FrameworkPropertyMetadata(string.Empty, OnWatermarkTextChanged));

        public static string GetWatermarkText(DependencyObject obj)
        {
            if (obj is TextBox textBox)
            {
                return textBox.Text;
            }
            return null;
        }

        public static void SetWatermarkText(DependencyObject obj, string value)
        {
            obj.SetValue(WatermarkTextProperty, value);
        }

        private static void OnWatermarkTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            TextBox textBox = d as TextBox;
            if (textBox == null) return;

            string watermarkText = e.NewValue as string;
            if (!string.IsNullOrEmpty(watermarkText))
            {
                textBox.GotFocus += (sender, args) =>
                {
                    if (textBox.Text == watermarkText)
                    {
                        textBox.Text = string.Empty;
                    }
                };

                textBox.LostFocus += (sender, args) =>
                {
                    if (string.IsNullOrEmpty(textBox.Text))
                    {
                        textBox.Text = watermarkText;
                    }
                };

                textBox.TextChanged += (sender, args) =>
                {
                    string newText = textBox.Text;
                    Console.WriteLine("New text entered: " + newText);
                };

                textBox.Text = watermarkText;
            }
        }
    }
}

Step 3. Final Result View