When Should You Prefer `IList` over `List` in C#?

Understanding the Advantages of IList<T> over List<T> in C#

When managing collections in C#, the decision between using the List<T> class and its interface, IList<T>, is a common dilemma. While List<T> offers a concrete implementation, IList<T> provides greater flexibility, enhancing code decoupling and maintainability.

Why Choose IList<T>?

Employing IList<T> instead of List<T> offers several key benefits:

• Enhanced Flexibility: IList<T> defines fundamental list operations: adding, removing, and accessing elements by index. Using the interface rather than the concrete class makes your code independent of the list's specific implementation. This simplifies switching between different implementations without impacting other code sections.
• Improved Extensibility: IList<T>'s extensibility surpasses that of List<T>. You can create custom classes implementing the IList<T> interface, offering specialized behaviors or functionalities beyond the standard List<T> capabilities.
• Increased Security: In scenarios requiring interface-based object exposure for security, IList<T> is preferable. Exposing the interface restricts client access to the collection's underlying implementation details.

Internal vs. External Usage

The choice between List<T> and IList<T> depends on the context. For internal use within your project, List<T> may suffice. However, when developing libraries or frameworks for external use, exposing collections through interfaces like IList<T> is best practice.

Addressing the Issue of Exposing List<T> in Libraries

Exposing List<T> directly in libraries presents potential problems. Tight coupling to a specific implementation arises, requiring client code updates if the implementation changes. Using interfaces like IList<T> mitigates this risk, allowing implementation modifications without affecting client code.

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