HashSet<T>

Syntax

HashSet<Type> set = new HashSet<Type>();

Examples

HashSet<string> tags = new HashSet<string>();
tags.Add("csharp");
tags.Add("dotnet");
tags.Add("csharp");  // duplicate - ignored, Add() returns false

Console.WriteLine(string.Join(", ", tags));  // csharp, dotnet
Console.WriteLine(tags.Count);                // 2

HashSet<int> validIds = new HashSet<int> { 101, 102, 103 };

Console.WriteLine(validIds.Contains(102));  // true
Console.WriteLine(validIds.Contains(999));  // false

HashSet<int> a = new HashSet<int> { 1, 2, 3, 4 };
HashSet<int> b = new HashSet<int> { 3, 4, 5, 6 };

HashSet<int> union = new HashSet<int>(a);
union.UnionWith(b);
Console.WriteLine(string.Join(", ", union));  // 1, 2, 3, 4, 5, 6

HashSet<int> intersection = new HashSet<int>(a);
intersection.IntersectWith(b);
Console.WriteLine(string.Join(", ", intersection));  // 3, 4

HashSet<int> difference = new HashSet<int>(a);
difference.ExceptWith(b);
Console.WriteLine(string.Join(", ", difference));  // 1, 2

List<string> namesWithDupes = new List<string> { "Fola", "Zain", "Fola", "Jamal" };

HashSet<string> uniqueNames = new HashSet<string>(namesWithDupes);
Console.WriteLine(string.Join(", ", uniqueNames));  // Fola, Zain, Jamal

Best practices

• Use HashSet<T> when you need fast membership testing (Contains()) or automatic deduplication and do not care about element order
• Use a SortedSet<T> instead if you need elements kept in sorted order, since HashSet makes no ordering guarantee
• Always override Equals() and GetHashCode() properly (or use a record) for any custom class stored in a HashSet, or duplicate detection will not work correctly
• Prefer a HashSet<T> over a List<T> for repeated Contains() checks on large collections - it is dramatically faster

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation