using System;

public class Program

{

    public abstract class DialogBox

    {

        public abstract void Changed(UIControl control);

    }

​

    public class UIControl

    {

        protected DialogBox owner;

        public UIControl(DialogBox owner)

        {

            this.owner = owner;

        }

    }

    public class ListBox: UIControl

    {

        private string selection;

        public string Selection

        {

            get {

                return selection;

            }

            set

            {

                selection = value;

                this.owner.Changed(this);

            }

        }

        public ListBox(DialogBox owner): base(owner)

        {

        }

    }

    public class TextBox: UIControl

    {

        private string content;

        public string Content

        {

            get {

                return content;

            }

            set

            {

                content = value;

                this.owner.Changed(this);

            }

        }

        public TextBox(DialogBox owner): base(owner)

        {

        }

    }

    public class Button: UIControl

    {

        private bool enabled;

        public bool Enabled

        {

            get {

                return enabled;

            }

            set

            {

                enabled = value;

                this.owner.Changed(this);

            }

        }

        public Button(DialogBox owner): base(owner)

        {

        }

    }

    public class ArticleDialogBox: DialogBox

    {

        ListBox articleListBox;

        TextBox titleTextBox;

        Button saveButton;

        public ArticleDialogBox()

        {

            articleListBox = new ListBox(this);

            titleTextBox = new TextBox(this);

            saveButton = new Button(this);

        }

        public override void Changed(UIControl control)

        {

            if(control is ListBox)

            {

                titleTextBox.Content = articleListBox.Selection;

                saveButton.Enabled = true;

            }

            else if(control is TextBox)

            {

                saveButton.Enabled = String.IsNullOrEmpty(titleTextBox.Content);

            }

        }

        public void SimulateUserInteraction()

        {

            articleListBox.Selection = "Article 1";

            Console.WriteLine("TextBox: " + titleTextBox.Content);

            Console.WriteLine("Button: " + saveButton.Enabled);

        }

    }

    public static void Main()

    {

        var dialog = new ArticleDialogBox();

        dialog.SimulateUserInteraction();

    }

}