Collection with multiple keys

I give a code

supose that you want to create your customized container object witch called MyCollection that contains ints
If you are using the classic way (without generics)
using System.Collection;
public class MyCollection
    {
        private ArrayList list;
        public MyCollection() { list = new ArrayList(); }
        public int this[int Index]
        {
            get { return (int)list[Index]; }
            set { list[Index] = value; }
        }
        public int this[string ID]
        {
            get { return (int)list[ID]; }
            set { list[ID] = value; }
        }
        ............
    }
If you are using the new way (with generics) introduced within the 2.0 version here you is the code
class MyCollection
    {
        private T[] arr;
        public MyCollection(int Dimension)
        {
            arr = new T[Dimension];
        }
        public T this[int Index]
        {
            get
            {
                return arr[Index];
            }
            set
            {
                arr[Index] = value;
            }
        }
        public T this[string ID]
        {
            get
            {
                return arr[ID];
            }
            set
            {
                arr[ID] = value;
            }
        }
        ....
    }