`IList vs. List in C#: When Should You Expose an Interface?`

C# Collections: When should you expose an interface? Comparison of IList and List

In the world of C# collections, choosing List or IList is more than just an academic exercise. Understanding the subtle differences between these two constructs can significantly impact the design and flexibility of your code.

When to Prefer IList

If you plan to expose your class for others to use as part of a library, it is wise to choose IList instead of List. The reason for this lies in the principles of interface-based programming. Exposing your classes through interfaces rather than concrete implementations gives you the flexibility to modify the underlying implementation without breaking the contract with users of the library.

For example, suppose you initially select a concrete List implementation in your class. Later, you realize that you need to take advantage of a more efficient data structure, such as a linked list. If your class is already exposed via a List concrete type, this change requires updating your consuming code. However, if you use IList, users of your library will not be affected because the interface remains the same regardless of the underlying implementation.

Precautions for internal use

In the context of using your classes only within your own code base, the choice between List and IList may not be that important. Here, the simplicity and directness of List may be enough, without the additional flexibility provided by IList.

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