Why Arrays Implement the IList Interface

The .NET Array class implements the IList interface to allow fast access to elements by index. IList (and its generic counterpart IList) represent collections that support indexed access.

While arrays provide efficient indexed access, they have limitations as seen in the code snippet:

int[] list = new int[] {};
IList iList = (IList)list;
ilist.Add(1); // exception here

Arrays are inherently fixed-size structures, and methods like Add() or Remove() are not supported. This is because an array's length is determined at creation time and cannot be modified.

The rationale for implementing IList in arrays is that it provides a common interface for collections, regardless of their underlying implementation. This allows developers to work with a variety of collections using a consistent API.

Although arrays have their limitations, their implementation of IList enables programmers to easily cast them to the IList interface and access their elements using indexers, a feature not supported by all collection types.

Some may argue that the current collection interfaces have design flaws, but their contractual properties ensure adherence to the substitution principle, allowing for the use of arrays where an IList is expected.