IList

IList is the name of interfaces in the .NET framework that describe a list data structure with indexed access and mutability. In .NET there are two closely related interfaces with this name: System.Collections.Generic.IList<T>, a generic interface for type-safe lists, and System.Collections.IList, a non-generic version that operates on objects. Both define an ordered collection that supports insertion and removal at arbitrary positions and provides indexed access.

IList<T> represents a strongly typed list and is commonly implemented by List<T>, which provides fast indexed access and dynamic resizing. Core members include the indexer T this[int index] { get; set; }, int IndexOf(T item), void Insert(int index, T item), void RemoveAt(int index), void Add(T item), void Clear(), bool Contains(T item), int Count { get; }, bool IsReadOnly { get; }, and CopyTo(T[] array, int arrayIndex) along with enumeration via IEnumerable<T>. Implementations typically support efficient random access and are optimized for use with value types or reference types of a specific T.

IList (non-generic) represents a list of objects and is typically implemented by ArrayList and other legacy collections. Its members include int Add(object value); bool Contains(object value); int IndexOf(object value); void Insert(int index, object value); void Remove(object value); void RemoveAt(int index); object this[int index] { get; set; }; int Count { get; }; bool IsFixedSize { get; }; bool IsReadOnly { get; }; void CopyTo(Array array, int index); IEnumerator GetEnumerator(); in addition to synchronization properties from ICollection such as IsSynchronized and SyncRoot in some implementations.

Both interfaces enable common list operations: accessing elements by position, inserting and removing items, and iterating over the collection, with the generic version offering type safety and better performance in modern code.