IEnumerable

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Bank bank = new Bank();
            //bank[0] = new Customer(1, "");
            //bank[1] = new Customer(2, "");
            //foreach (Customer c in bank)
            //{
            //    MessageBox.Show(c.ToString());
            //}
            bank.Add(1, "abc");
            bank.Add(2,"erf");
            bank.Add(3, "test");

            foreach (Customer c in bank)
            {
                MessageBox.Show(c.ToString());
            }
        }
    }

    public class Customer
    {
        public int i;
        public string name;
        public Customer(int i, string s)
        {
            this.i = i;
            this.name = s;
        }

        public override string ToString()
        {
            return "CustID:" + i + ";CustName:" + name;
        }
    }

    public class Bank:IEnumerable
    {
        private ArrayList al = new ArrayList();
        public Customer this[int i]
        {
            get { return (Customer)this.al[i]; }
        }

        public int Add(int i, string s)
        {
            return al.Add(new Customer(i, s));
        }

        public int Count
        {
            get { return this.al.Count; }
        }

        #region IEnumerable Members

        public IEnumerator GetEnumerator()
        {
            for (int i = 0; i < this.Count; i++)
            {
                yield return this[i];
            }
        }

        #endregion
    }