List vs. IList in C#: When Should I Use Which?

Choice between List and IList in C#: When to use which?

In C#, both List<T> and IList<T> are interfaces that represent collections of elements. However, there are situations where it is more advantageous to use IList<T> than List<T>.

When to use IList?

• Interface abstraction: It is recommended to expose classes in a class library through interfaces rather than concrete implementations. This approach provides flexibility if you later need to change the underlying implementation without affecting consumers of the library. IList<T> provides a standard interface that different concrete implementations (such as List<T> or LinkedList<T>) can adhere to.
• Encapsulation: IList<T> hides the implementation details of the underlying collection. This is useful when you want to ensure that callers can only interact with a collection through specified methods defined in the interface.
• Legacy code: If you are dealing with legacy code that uses IList<T> as a collection type, you may want to continue using it to maintain compatibility.

When to use List?

• Use directly: If you are not exposing your class to external consumers and are only using it internally, using List<T> can be a simpler and more efficient option. It has direct access to the underlying implementation, allowing you to take advantage of the full power of the List<T> class.
• Performance: In some cases, using List<T> directly can achieve better performance than via an interface. This is because interface abstraction introduces an extra layer that may cause some overhead.
• Convenience: List<T> provides additional implementation-specific functionality and methods, such as Sort() or BinarySearch(), that may not be available through IList<T>.

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