Удаление шаров через ArrayList при столкновении со специальным объектом - C#

using System;
using System.Threading;
using System.Drawing;
using System.Windows.Forms;
using System.Collections;
 
namespace csMRR
{
    delegate void Del(int x, int y);
 
    class Ball
    {
        int x, y;
        int dx, dy;
        bool life;
        Thread t;
        Window w;
        Color col;
        public ArrayList al_w = new ArrayList();
        public int X { get { return x; } }
        public int Y { get { return y; } }
        public Color Col { get { return col; } }
        
        public Ball(int X, int Y, Window W, int Dx, int Dy, ref ArrayList al)
        {
            al_w = al;
            col = Color.Blue;
            x = X; y = Y; w = W;
            dx = Dx; dy = Dy;
            life = true;
            t = new Thread(new ThreadStart(Run));
            t.Start();
        }
 
        private void Run()
        {
            while (life)
            {
                x += dx;
                y += dy;
                if (x - 10 < 0 || x > w.Width - 50) dx = -dx;
                if (y - 5 < 0 || y > w.Height - 65) dy = -dy;
                Thread.Sleep(100);
                w.Invalidate();
            }
        }
 
        public void Handler(int X, int Y)
        {
            if (Math.Abs(x - X) < 25 && Math.Abs(y - Y) < 25)
            {
                int i = al_w.Count;
                if(al_w.Count!=0)
                    i--;
                while (i < al_w.Count)
                {
                    //Ball ball1 = (Ball)al_w[i];
                    al_w.Remove((Ball)al_w[i]);
                }
                w.Invalidate();
            }
        }
 
        public void Close() { life = false; }
    }
    //--------------------------------------------------------------   
    class Box
    {
        int dx, dy;
        int x1, y1;
        bool life;
        Thread t;
        Window w;
        public event Del ev;//
        public int X { get { return x1; } }
        public int Y { get { return y1; } }
 
        public Box(int X1, int Y1, Window W, int Dx, int Dy)
        {
            x1 = X1; y1 = Y1; w = W;
            dx = Dx; dy = Dy;
            life = true;
            t = new Thread(new ThreadStart(Mov));
            t.Start();
        }
        private void Mov()
        {
            while (life)
            {
                x1 += dx;
                y1 += dy;
                if (x1 - 10 < 0 || x1 > w.Width - 50) dx = -dx;
                if (y1 - 10 < 0 || y1 > w.Height - 70) dy = -dy;
                Thread.Sleep(150);
                if (ev != null) ev(x1,y1);
                w.Invalidate();
            }
        }
 
        public void Close() { life = false; }
    }
    //---------------------------------------------------------------  
    class Window : Form
    {
        Ball ball1;
        Box box;
        Button but;
        Button delete;
        Random rand;
        public ArrayList a;
        public Window()
        {
            box = new Box(160, 90, this, 10, 10);
            rand = new Random();
            this.Text = "Our first window";
            but = new Button();
            but.Location = new Point(0, 0);
            but.Size = new Size(70, 20);
            but.Text = "Добавить";
            but.Click += new EventHandler(ClickBut);
            this.Controls.Add(but);
 
            delete = new Button();
            delete.Location = new Point(0, 20);
            delete.Size = new Size(70, 20);
            delete.Text = "Удалить";
            delete.Click += new EventHandler(ClickDelete);
            this.Controls.Add(delete);
            a = new ArrayList();
        }
 
        private void ClickBut(object obj, EventArgs e)
        {
            Ball ball1 = new Ball(rand.Next(220), rand.Next(220), this, 10, 5, ref a);
            a.Add(ball1);
            for (int i = 0; i < a.Count; i++)
            {
                box.ev += new Del(((Ball)a[i]).Handler);
            }            
            Invalidate();
        }
 
        private void ClickDelete(object obj, EventArgs e)
        {
            int i = a.Count;
            if (i > 0)
                i--;
            while (i < a.Count)
            {
                Ball ball1 = (Ball)a[i];
                a.Remove(ball1);
            }
            Invalidate();
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            int i;
            e.Graphics.DrawRectangle(new Pen(Color.Green, 5), box.X, box.Y, 20, 20);
            for (i = 0; i < a.Count; i++)
                e.Graphics.DrawEllipse(new Pen(((Ball)a[i]).Col, 4),
                      ((Ball)a[i]).X, ((Ball)a[i]).Y, 20, 20);
        }
        protected override void OnClosed(EventArgs e)
        {
            base.OnClosed(e);
            ball1.Close();
            box.Close();
        }
 
        static void Main()
        {
            Application.Run(new Window());
        }
    }
}