Collections

There are two main types of collections: non-generic (ICollection) and generic (ICollection<T>).

Non-generic collections only exist because .NET did not originally have generic data types. They shouldn’t be used because:

• They are untyped at compile time. The frequent casting from object and the actual type is error-prone; it’s easy to put the wrong type in the wrong collection.
• Value types need to be boxed as object, e.g., List<int> stores its data in an int[], which is more performant than using object[] as that requires boxing.

Choosing a Collection Type

In summary, the use cases are:

• Dictionary<TKey, TValue> for quick look-up by key.
• List<T> for accessing items by index.
• Queue<T> for first-in-first-out (FIFO) access where the element is discarded after use.
• Stack<T> for last-in-first-out (LIFO) access where the element is discarded after use.
• LinkedList<T> for sequential access from either the head or the tail.
• ObservableCollection<T> to receive notifications when items are removed/added.
• SortedList<TKey, TValue> for a collection sorted by keys and accessible by either key or index.
• HashSet<T> and SortedSet<T> for a set for mathematical functions.
• KeyedCollection<TKey, TItem> for items that have keys embedded in them; a hybrid between IList<T> and IDictionary<TKey, TValue>.