IList<T> Interface

The System.Collections.Generic.IList<T> interface is a generic interface that represents an ordered collection of objects that can be accessed by index. It inherits from ICollection<T>, which in turn inherits from IEnumerable<T>, providing additional functionality beyond simple iteration and counting.

Syntax

public interface IList<out T> : ICollection<T>

Remarks

An IList<T> provides capabilities for:

  • Accessing elements by their zero-based index.
  • Inserting elements at a specific index.
  • Removing elements from a specific index.
  • Searching for elements within the list.
  • Getting or setting an element at a specific index.

Implementations of IList<T> can vary in performance characteristics. For example, List<T> offers O(1) time complexity for most operations (add, remove at index, access by index), while other implementations might have different performance profiles.

Methods

Method Description
int Add(T element) Adds an item to the end of the IList<T>.
void Clear() Removes all items from the IList<T>.
bool Contains(T element) Determines whether an element is in the IList<T>.
int IndexOf(T element) Searches for the specified object and returns the zero-based index of the first occurrence within the entire IList<T>.
void Insert(int index, T element) Inserts an item at the specified index.
void Remove(T element) Removes the first occurrence of a specific object from the IList<T>.
void RemoveAt(int index) Removes the element at the specified index.

Properties

Property Description
T this[int index] { get; set; } Gets or sets the element at the specified index.
int Count { get; } Gets the number of elements contained in the IList<T>. (Inherited from ICollection<T>)
bool IsReadOnly { get; } Gets a value indicating whether the IList<T> has a fixed size. (Inherited from ICollection<T>)

Example

C# Example


using System;
using System.Collections.Generic;

public class Example
{
    public static void Main(string[] args)
    {
        // Create a List, which implements IList
        IList<string> fruits = new List<string>();

        // Add elements
        fruits.Add("Apple");
        fruits.Add("Banana");
        fruits.Add("Cherry");

        Console.WriteLine("Initial list:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine($"- {fruit}");
        }

        // Access and modify an element
        Console.WriteLine($"\nElement at index 1: {fruits[1]}");
        fruits[1] = "Blueberry";
        Console.WriteLine($"Updated element at index 1: {fruits[1]}");

        // Insert an element
        fruits.Insert(1, "Apricot");
        Console.WriteLine("\nList after inserting 'Apricot' at index 1:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine($"- {fruit}");
        }

        // Check if an element exists
        Console.WriteLine($"\nContains 'Banana'? {fruits.Contains("Banana")}");

        // Find the index of an element
        Console.WriteLine($"Index of 'Cherry': {fruits.IndexOf("Cherry")}");

        // Remove an element
        fruits.Remove("Apple");
        Console.WriteLine("\nList after removing 'Apple':");
        foreach (var fruit in fruits)
        {
            Console.WriteLine($"- {fruit}");
        }

        // Remove an element by index
        fruits.RemoveAt(0); // Removes 'Apricot'
        Console.WriteLine("\nList after removing element at index 0:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine($"- {fruit}");
        }

        Console.WriteLine($"\nTotal elements: {fruits.Count}");
    }
}

See Also