Please Refer the below code:
Namespace
using System.Collections;
C#
protected void Page_Load(object sender, EventArgs e)
{
ArrayList arrayList1= new ArrayList();
arrayList1.Add("1");
arrayList1.Add("2");
arrayList1.Add("3");
ArrayList arrayList2 = new ArrayList();
arrayList2.AddRange(arrayList1);
ArrayList arrayList3 = new ArrayList();
arrayList3.AddRange(arrayList2);
string Number = arrayList3[1].ToString();
}