IEnumerable, List, or IList in C#

The decision between IEnumerable, List, and IList in C# is determined by your individual use case and requirements. Each of these categories serves a distinct purpose, and your selection should be dependent on your performance and optimization requirements. Let’s talk about when to employ each and what it means in terms of time and space complexity.


When working with a succession of elements with no fixed order or enabling postponed execution, use IEnumerable.
It displays a collection with a forward-only, read-only cursor.

IEnumerable<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

foreach (int number in numbers)

Performance and Optimization

  • IEnumerable is suitable when you don’t need to modify the collection, and you can leverage deferred execution to optimize your operations.
  • It may be more memory-efficient because it doesn’t store the entire sequence in memory.
  • The time complexity for operations like enumeration and filtering is O(n), where n is the number of elements.


  • Use List when you need a collection that allows duplicates, and you frequently need to access elements by index.
  • Lists are essentially dynamic arrays, and they provide efficient random access.


List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

Performance and Optimization

  • The list is optimized for indexed access, so retrieving an element by index has a time complexity of O(1).
  • However, if you frequently modify the list by inserting or removing elements in the middle, these operations can be O(n) because elements need to be shifted.
  • Lists have a relatively higher memory overhead due to maintaining a dynamically resizing array.


  • Use IList when you need a collection with more functionality than IEnumerable but don’t want to commit to a specific implementation, allowing flexibility in choosing the concrete type.


IList<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

Performance and Optimization

  • The performance characteristics of an IList depend on the actual implementation you use (e.g., List, ArrayList, or a custom implementation). So, it can vary.

Terms of time and space complexity

  • IEnumerable is generally more memory-efficient when used with deferred execution, as it doesn’t store the entire collection in memory. It has a linear time complexity (O(n)) for enumeration and filtering operations.
  • List offers fast random access (O(1) time complexity) but may consume more memory due to its dynamic array structure.
  • IList is a more general interface, and its performance depends on the underlying implementation. It may offer various trade-offs depending on the specific collection class used.


The choice between IEnumerable, List, and IList depends on the specific needs of your application. Consider the use case and performance requirements to select the most appropriate collection type for your situation.

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