IList

In C# lists and arrays implement IList. This interface is an abstraction that allows list types to be used with through a single reference.

With IList, we can create a single method to receive an int array, or a List of ints. This is powerful—it allows us to combine and reuse code.

Example

With IList, you must specify a type parameter. If you want your method to act upon ints, you can use IList int. Any type (string, object) can be specified.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        int[] array = new int[3];
        array[0] = 1;
        array[1] = 2;
        array[2] = 3;
        Display(array);

        List<int> list = new List<int>();
        list.Add(5);
        list.Add(7);
        list.Add(9);
        Display(list);
    }

    static void Display(IList<int> list)
    {
        Console.WriteLine("Count: {0}", list.Count);
        foreach (int value in list)
        {
            Console.WriteLine(value);
        }
    }
}
Count: 3
1
2
3
Count: 3
5
7
9

IEnumerable use

Consider this alternative program—it uses IEnumerable instead of IList. With IEnumerable we do not have a Count property.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Display(new int[] {1, 2});
        Display(new List<int>() {1, 2});
    }

    static void Display(IEnumerable<int> list)
    {
        foreach (int value in list)
        {
            Console.WriteLine(value);
        }
    }
}
1
2
1
2

Discussion

You can also implement IList T for a custom class. The methods required are an indexer, the IndexOf method, the Insert method, and the RemoveAt method.

We looked at the IList generic interface, which can be used as an abstraction for arrays and Lists. The IList generic interface is separate from the regular IList interface.