HashSet (C#)

HashSet

A generic collection in System.Collections.Generic that stores unique elements and provides efficient membership testing and set-based operations. Domain: C# → .NET → System.Collections.Generic → Sets

T in HashSet

The generic type parameter that specifies the type of elements stored in the HashSet.

new HashSet()

Creates an empty HashSet using the default equality comparer for T.

new HashSet(collection)

Creates a HashSet initialized with the unique elements from the specified collection.

new HashSet(comparer)

Creates an empty HashSet that uses the specified equality comparer to determine whether elements are equal.

HashSet Initializer

Syntax that creates a HashSet and supplies its initial elements inside braces.

set.Count

Gets the number of unique elements currently contained in the HashSet.

set.Add(item)

Attempts to add an element to the HashSet and returns true if the element was added or false if an equivalent element was already present.

set.Remove(item)

Attempts to remove the specified element and returns whether the element was found and removed.

set.Contains(item)

Determines whether the HashSet contains an element equivalent to the specified item.

set.Clear()

Removes all elements from the HashSet.

HashSet Uniqueness

A HashSet stores at most one element from each equality-equivalent group; attempts to add an equivalent duplicate do not create another entry.

HashSet Equality Comparer

The comparer used by a HashSet to determine whether two elements should be treated as equal.

set.Comparer

Gets the IEqualityComparer used to determine element equality within the HashSet.

set.UnionWith(other)

Modifies the current HashSet so that it contains every element present in either the current set or the specified collection.

set.IntersectWith(other)

Modifies the current HashSet so that it contains only elements present in both the current set and the specified collection.

set.ExceptWith(other)

Modifies the current HashSet by removing elements that also occur in the specified collection.

set.SymmetricExceptWith(other)

Modifies the current HashSet so that it contains elements present in either set but not in both.

set.IsSubsetOf(other)

Determines whether every element in the current HashSet is contained in the specified collection.

set.IsProperSubsetOf(other)

Determines whether the current HashSet is a subset of the specified collection and the specified collection contains at least one additional element.

set.IsSupersetOf(other)

Determines whether the current HashSet contains every element in the specified collection.

set.IsProperSupersetOf(other)

Determines whether the current HashSet is a superset of the specified collection and contains at least one additional element.

set.SetEquals(other)

Determines whether the current HashSet and the specified collection contain the same elements.

set.Overlaps(other)

Determines whether the current HashSet and specified collection share at least one common element.

HashSet vs List

List is an ordered, index-accessible collection that permits duplicates, whereas HashSet focuses on unique elements and membership rather than positional indexing.