ICollection

ICollection is a non-generic interface in the .NET ecosystem, defined in the System.Collections namespace. It represents a collection of objects that can be counted, enumerated, and copied to an array. It is implemented by many classic non-generic collections such as ArrayList, Hashtable, Queue, and Stack. The interface is part of the broader collection framework that precedes generics and provides a common contract for various collection types.

Key members and characteristics include:

- Count: a property that returns the number of elements in the collection.

- CopyTo: a method that copies the elements of the collection into a one-dimensional array, starting at a specified index.

- IsSynchronized: a property indicating whether access to the collection is synchronized (thread-safe).

- SyncRoot: a property that returns an object used to synchronize access to the collection.

Enumeration is provided through the IEnumerable interface, which ICollection extends. This means that a collection implementing ICollection can be iterated using a foreach-style construct, with iteration performed by obtaining an IEnumerator from the collection.

Relationship to generic variants: In modern .NET development, a generic counterpart, ICollection<T> in System.Collections.Generic, provides a type-safe contract with a similar purpose. ICollection<T> extends IEnumerable<T> and adds type-specific members. ICollection (non-generic) is retained primarily for backward compatibility and for interoperability with older APIs that do not use generics.

Usage notes: Developers typically use ICollection when implementing or consuming non-generic collections or when maintaining compatibility with legacy code. For new development, generic interfaces such as ICollection<T>, IList<T>, or IReadOnlyCollection<T> are generally preferred for type safety and performance.