IReadOnlyListT

IReadOnlyList<T> is a generic interface in the .NET Framework that represents a read-only collection of elements of a specific type, denoted by the type parameter T. Introduced in .NET 4.5, it is part of the System.Collections.Generic namespace and serves as a more specialized alternative to the non-generic IReadOnlyCollection<T> interface. While IReadOnlyCollection<T> only guarantees random access to elements by index, IReadOnlyList<T> extends this by providing indexed access to elements, allowing for efficient iteration and lookup by position.

The interface defines a single method, GetItem(int index), which retrieves the element at the specified zero-based index. This ensures that the collection supports indexed access while maintaining immutability, meaning the list cannot be modified after creation. This immutability is enforced through the absence of methods like Add, Remove, or Clear, which are present in mutable list interfaces such as IList<T>.

IReadOnlyList<T> is commonly used in scenarios where a read-only view of a list is required, such as passing a collection to a method that expects immutability or when working with asynchronous programming patterns like Task<T> or ValueTask<T>, where the result is expected to be a read-only collection. It is also useful in functional programming paradigms, where immutability is a core principle.

Implementing IReadOnlyList<T> allows developers to expose a list in a way that prevents unintended modifications while still providing full read access. This is particularly useful in scenarios where thread safety is a concern, as it eliminates the risk of concurrent modifications. The interface is widely adopted in modern .NET libraries, including those for reactive programming, such as System.Reactive, and in APIs that require immutable data structures.