string [] arr ={"a","b","a","c"};
ArrayList al1=new ArrayList(arr);
ArrayList al2=new ArrayList();
foreach(string str in al1)
{
if(!al2.Contains(str))
{
al2.Add(str);
}
}
Console.WriteLine(al2.Count.ToString());
Console.ReadLine();
atlast u will have in al2 all the distinct values