IEnumerable vs ICollection vs IList in C#: Key Differences and Usage

IEnumerable provides simple iteration over a collection. ICollection extends IEnumerable by adding size, add, and remove capabilities. IList further extends ICollection by supporting indexed access and insertion.

Quick Comparison

Here is a quick table comparing IEnumerable, ICollection, and IList based on key features and usage.

Feature	IEnumerable	ICollection	IList
Namespace	System.Collections	System.Collections	System.Collections
Supports iteration	Yes	Yes	Yes
Supports Count property	No	Yes	Yes
Supports Add/Remove methods	No	Yes	Yes
Supports indexed access	No	No	Yes
Supports insertion at index	No	No	Yes

Key Differences

IEnumerable is the simplest interface that allows you to loop through a collection using foreach. It only guarantees that you can read items one by one, but it does not provide any information about the collection size or allow modification.

ICollection extends IEnumerable by adding the Count property and methods like Add, Remove, and Clear. This means you can check how many items are in the collection and modify it, but you cannot access items by position.

IList extends ICollection by adding indexed access through the this[int index] property. This lets you get or set items at specific positions and insert or remove items at any index, making it suitable for list-like collections.

Code Comparison

Using IEnumerable to iterate over a collection:

csharp

using System;
using System.Collections;

class Program
{
    static void Main()
    {
        IEnumerable numbers = new int[] {1, 2, 3};
        foreach (int num in numbers)
        {
            Console.Write(num + " ");
        }
    }
}

Output

1 2 3 

IList Equivalent

Using IList to access and modify a collection by index:

csharp

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        IList<int> numbers = new List<int> {1, 2, 3};
        numbers[1] = 20; // Modify element at index 1
        numbers.Insert(2, 30); // Insert 30 at index 2
        foreach (int num in numbers)
        {
            Console.Write(num + " ");
        }
    }
}

Output

1 20 30 3 

When to Use Which

Choose IEnumerable when you only need to read or loop through a collection without modifying it or knowing its size. It is the most basic and flexible interface for iteration.

Choose ICollection when you need to know the number of items or add and remove elements but do not require indexed access.

Choose IList when you need full list functionality, including accessing, inserting, or removing items by their position in the collection.

Key Takeaways

IEnumerable is for simple read-only iteration.

ICollection adds size and modification capabilities.

IList supports indexed access and insertion.

Use the simplest interface that meets your needs for better code clarity.

All three interfaces are part of System.Collections and build on each other.