.NET Collections

Updated on May 9th 2024

Sometimes, the proper choice of a collection can greatly impact the performance of your application. For example, there are collection types that are more appropriate for insertions, others that allow faster lookups, and so on. Plus, you must decide if you want indexed collections or not, and if you want to have generic collections, in order to have compile-time type checking. The .NET BCL comes with several general-purpose collection classes. Here is a list of all the collection classes in .NET 8 and their intended use.

Namespace Purpose
System.Collections Basic types, non-generic collections and interfaces
System.Collections.Generic Generic interfaces and collections
System.Collections.ObjectModel Observable and read-only collections
System.Collections.Specialized Special collections
System.Collections.Concurrent Concurrent (thread-safe) collections
System.Collections.Immutable Immutable collections
System.Collection.Frozen Frozen collections

PurposeTypesDescription
Fixed size

System.Array,

System.Collections.Immutable.ImmutableArray<T>

Can have multiple dimensions
FIFOs (Queues)

System.Collections.Queue,

System.Collections.Generic.Queue<T>,

System.Collections.Concurrent.ConcurrentQueue<T>,

System.Collections.Immutable.ImmutableQueue<T>

First in, first out
Priority Queue

System.Collections.Generic.PriorityQueue<TElement,TPriority>

A queue that respects the given priority. Does not implement any of the standard interfaces, but is similar to the queue interface, except that it requires a priority
LIFOs (Stacks)

System.Collections.Stack,

System.Collections.Generic.Stack<T>,

System.Collections.Concurrent.ConcurrentStack<T>

System.Collections.Immutable.ImmutableStack<T>

Last in, first out
Linked lists System.Collections.Generic.LinkedList<T> Random inserts and deletes at any point
Array-based

System.Collections.ArrayList,

System.Collections.Generic.List<T>,

System.Collections.Concurrent.ConcurrentBag<T>

System.Collections.Immutable.ImmutableList<T>

Customizable System.Collections.ObjectModel.Collection<T> Can override InsertItem, RemoveItem, SetItem and ClearItems to implement custom algorithms
Thread-safe

System.Collections.Generic.SynchronizedCollection<T>,

System.Collections.Generic.SynchronizedReadOnlyCollection<T>

Thread-safe
Read-only

System.Collections.ObjectModel.ReadOnlyCollection<T>

System.Collections.Generic.SynchronizedReadOnlyCollection<T>,

System.Collections.ObjectModel.ReadOnlyObservableCollection<T>

Wrap existing (not read-only collections)
Sorted

System.Collections.SortedList,

System.Collections.Generic.SortedList<K, V>,

System.Collections.Generic.SortedDictionary<K, V>

System.Collections.Generic.SortedSet<T>

Sorted by key
Sets

System.Collections.Generic.HashSet<T>

System.Collections.SortedSet<T>

System.Collections.Immutable.ImmutableSortedSet<T>,

System.Collections.Immutable.ImmutableHashSet<T>

System.Collections.Frozen.FrozenSet<T>

No repetition of elements
Keyed

System.Collections.Specialized.ListDictionary (for <=10 items),

System.Collections.Generic.Dictionary<K, V>,

System.Collections.Hashtable (index by hash),

System.Collections.Generic.KeyedByTypeCollection<T> (index by type), 

System.Collections.Specialized.HybridDictionary (changes depending on number of elements, ListDictionary -> Hashtable),

System.Collections.Specialized.OrderedDictionary (access by key or index), 

System.Collections.Immutable.ImmutableDictionary<K, V>,

System.Collections.Concurrent.ConcurrentDictionary<K, V>

System.Collections.Immutable.ImmutableSortedDictionary<K, V>

System.Collections.Frozen.FrozenDictionary<K, V>

Single value per key
Multiple values for a single key System.Collections.Specialized.NameValueCollection Can store multiple values per key
Bits

System.Collections.BitArray,

System.Collections.BitVector32 (up to 32 bits only)

Bit operations
Strings

System.Collections.Specialized.StringDictionary,

System.Collections.Specialized.StringCollection

Strings
Observable

System.Collections.ObjectModel.ObservableCollection<T>,

System.Collections.ObjectModel.ReadOnlyObservableCollection<T>

Identical to System.Collections.ObjectModel.Collection<T>, fires events upon adding, removing, modifying or clearing.
Concurrent

System.Collections.Concurrent.BlockingCollection<T>

System.Collections.Concurrent.ConcurrentBag<T>

System.Collections.Concurrent.ConcurrentDictionary<K, V>

System.Collections.Concurrent.ConcurrentQueue<T>

System.Collections.Concurrent.ConcurrentStack<T>

Thread-safe, lock-free
Immutable

System.Collections.Immutable.ImmutableDictionary<K, V>

System.Collections.Immutable.ImmutableHashSet<T>

System.Collections.Immutable.ImmutableList<T>

System.Collections.Immutable.ImmutableQueue<T>

System.Collections.Immutable.ImmutableSortedDictionary<K, V>

System.Collections.Immutable.ImmutableSortedSet<T>

System.Collections.Immutable.ImmutableStack<T>

System.Collections.Immutable.ImmutableArray<T>

Immutable collections (see http://msdn.microsoft.com/en-us/library/dn385366.aspx)
Frozen

System.Collections.Frozen.FrozenSet<T>

System.Collections.Frozen.FrozenDictionary<K, V>

Frozen collections


Of course, you should not expose a collection class directly because it couples you to a an implementation, instead you should use collection interfaces, all these classes implement (at least) one of them). These interfaces are:

PurposeTypes
Enumerate only

System.Collections.IEnumerable,

System.Collections.IEnumerable<T>,

System.Linq.IOrderedEnumerable<T>

System.Linq.ILookup<K, E> (multiple values per key)

Enumerate, count

System.Collections.ICollection,

System.Collections.Generic.ICollection<T>

Indexed access, add/remove (where not read-only or immutable)

System.Collections.IList,

System.Collections.Generic.IList<T>,

System.Collections.Immutable.IImmutableList<T>

System.Collections.Generic.IReadOnlyList<T>

System.Collections.Generic.IReadOnlyCollection<T>

Keyed

System.Collections.IDictionary,

System.Collections.Specialized.IOrderedDictionary,

System.Collections.Generic.IDictionary<K, V>,

System.Collections.Generic.IReadOnlyDictionary<K, V>,

System.Collections.Immutable.IImmutableDictionary<K, V>

FIFOs (Queues) System.Collections.Immutable.IImmutableQueue<T>
LIFOs (Stacks) System.Collections.Immutable.IImmutableStack<T>
Sets

System.Collections.Generic.ISet<T>,

System.Collections.Immutable.IImmutableSet<T>

System.Collections.Generic.IReadOnlySet<T>

Some recommendations for using collections:

  • Always prefer generic collections (should be obvious by now!);
  • Always expose collections in properties, not arrays, and always collections with the less access possible (enumerate only, or read-only access to elements);
  • All collections except synchronized and concurrent are not thread-safe;
  • When using a collection where you need random inserts/removes at specific positions, prefer a linked list over an array-based implementation, as the latter requires constant allocations and copies;
  • Choose a collection that better suits your intent (no repetition of items, indexed, always sorted, ...);
  • Collections that rely on object identity (hashed/keyed collections) may need proper implementation of Equals or GetHashCode methods; the implementation of the two should be consistent;
  • For customized object equality comparison, a custom implementation of System.Collections.Generic.IEqualityComparer<T> or System.Collections.IEqualityComparer may be passed on the constructor;
  • For sorted collections, a custom implementation of System.Collections.Generic.IComparer<T> or System.Collections.IComparer may be passed on the constructor (System.Collections.Comparer, System.Collections.CaseInsensitiveComparer are default implementations);
  • Indexed and sorted collections rely on a proper, consistent and stable implementation of GetHashCode. It cannot ever change for the same object, or things may go wrong with the collection - object may not be found, for example;
  • If number of elements is known beforehand, use the constructor that specifies the initial capacity;
  • Immutable collections always return new objects when adding or removing from an existing one;
  • Cannot add or remove from a frozen collection;
  • Avoid using value types for elements, except for keys, because they need to be copied bit by bit, unlike with reference types, where only their pointer is copied.

And that's it.

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