Why Do C# Arrays Implement the IList Interface Despite Being Immutable?

Why Does Array Implement IList?

In the .NET framework, the Array class implements the IList interface. This allows arrays to be treated as lists, providing access to properties and methods like Add, Remove, Count, and IndexOf.

While arrays provide efficient indexed access, the IList interface defines a common set of operations for collections that support item insertion, deletion, and retrieval. By implementing IList, arrays can be used in situations where a generic IList is expected.

Theoretically, one could assign an array to an IList variable:

int[] list = new int[] {};
IList iList = (IList)list;

However, attempting to add an item to the IList would result in an exception. This is because arrays in C# are immutable; they cannot be modified after their elements have been initialized.

The reason for implementing IList on arrays is to provide a consistent interface for working with different types of collections. By having a common interface, developers can use the same methods and properties on a variety of collection types, regardless of their specific implementation.

While there is no interface specifically designed for constant collections with indexers, the IList interface provides a reasonable approximation. It provides basic operations like Add and Remove, as well as indexed access. Additionally, arrays are efficient data structures that can provide fast access to indexed elements.

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