Why Does the C# `System.Array` Class Implement the `IList` Interface?

Delving into Array's Implementation of IList: Understanding the Rationale

The System.Array class in C# exhibits a unique characteristic: it implements the IList interface. While this concept may seem straightforward on the surface, the practicality of calling IList methods on an array might raise questions. This article aims to address the underlying reason behind this implementation, rather than focusing on the exception that arises when invoking certain methods.

Unveiling the Rationale

The answer to why Array implements IList lies in the inherent capabilities of an array. An array offers remarkably swift access through indexing, a feature not provided by other collection interfaces. Consequently, IList/IList stand out as the sole collection interfaces that support this functionality. Hence, the implementation of IList in Array serves a pragmatic purpose.

Questioning Interface Design

The lack of an interface specifically tailored for constant collections with indexers provokes further inquiry. Unfortunately, no definitive response can be offered at this time. Moreover, the absence of readonly interfaces for collections is also a notable omission that warrants consideration.

Re-evaluating Interface Design

In an ideal scenario, a comprehensive suite of (generic) collection interfaces would exist, each catering to specific collection features. The nomenclature should also be revised, as employing the term "List" for an indexed collection seems counterintuitive. A potential alternative hierarchy could include:

• Just Enumeration: IEnumerable
• Readonly with No Indexer: IReadOnlyCollection
• Resizable with No Indexer: ICollection
• Readonly with Indexer: IReadOnlyList
• Constant Size with Indexer: IReadOnlyIndexedList
• Variable Size with Indexer: IList

Addressing Interface Limitations

While the current collection interfaces may fall short of ideal, they adhere to the substitution principle by declaring the valid methods through properties, thereby preserving the integrity of interfaces.

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