Question

Why does the following DisplayContents not work(wont compile) for an ArrayList as it inherits form IEnumerable)

public class Program
    {
        static void Main(string[] args)
        {
            List<int> l = new List<int>(){1,2,3};
            DisplayContents(l);

            string[] l2 = new string[] {"ss", "ee"};
            DisplayContents(l2);

            ArrayList l3 = new ArrayList() { "ss", "ee" };
            DisplayContents < ArrayList>(l3);

            Console.ReadLine();
        }

        public static void DisplayContents<T>(IEnumerable<T> collection)
        {
            foreach (var _item in collection)
            {
                Console.WriteLine(_item);
            }
        }
    }

Answer 1

ArrayList implements IEnumerable, but not the generic IEnumerable<T>. This is to be expected, since ArrayList is neither generic nor bound to any specific type.

You need to change the parameter type of your DisplayContents method from IEnumerable<T> to IEnumerable and remove its type parameter. The items of your collection are passed to Console.WriteLine, which can accept any object.

public static void DisplayContents(IEnumerable collection)
{
    foreach (var _item in collection)
    {
        Console.WriteLine(_item);
    }
}