C# IList Generic Interface

This C# example program shows the IList generic interface. IList is implemented by the List and array types.

IList. Lists and arrays implement IList.

This interface is an abstraction that allows list types to be used with through a single reference type. With it, we can create a single method to receive an int[] or a List<int>.



Example. First, with the IList generic interface, 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.


Next: In this program, we introduce the Display method, which receives an IList<int> parameter.

Note: It accesses the Count property and then uses the enumerator in a foreach-loop.


C# program that uses IList generic interface

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;

	List<int> list = new List<int>();

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


Count: 3
Count: 3

Passing collections to Display method. You can see in the Main method that you can pass an int[] or a List<int> to the Display method. These are implicitly cast to their base interface IList<int>.

Note: Arrays always implement IList<T>. The List type also implements IList<T>.


Discussion. You can also implement IList<T> for a custom class in your C# program. The methods required by this contract are an indexer, the IndexOf method, the Insert method, and the RemoveAt method.

IndexerList InsertRemoveAt



Summary. 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. It uses different syntax.


Tip: With this layer of abstraction, you can formulate more concise ways of acting upon your data.