HashSet in C# with Examples

In C#, HashSet is an unordered collection of unique elements. This collection is introduced in .NET 3.5. It provides fast lookups and high-performance set operations and uses the hash table for storage. This collection is of the generic type collection and it is part of the System.Collections.Generic namespace. It is generally used when we want to prevent duplicate elements from being placed in the collection. The performance of the HashSet is much better in comparison to the list.

Important Points Related To HashSet in C#

• The HashSet class implements the ICollection, IEnumerable, IReadOnlyCollection, ISet, IEnumerable, IDeserializationCallback, and ISerializable interfaces.
• In HashSet, the order of the element is not defined. You cannot sort the elements of HashSet.
• HashSet elements are always unique. In oder word, duplicate elements are not allowed
• It provides many mathematical set operations, such as intersection, union, and difference.
• A HashSet is a dynamic collection means the size of the HashSet is automatically increased when the new elements are added.
• In HashSet, you can only store the same type of elements.

Example:

// C# program to illustrate how to create hashset
using System;
using System.Collections.Generic;

class HashsetTest {

	// Main Method
	static public void Main()
	{

		// Creating HashSet
		HashSet<string> myhash1 = new ();

		// Add the elements in HashSet
		myhash1.Add("C");
		myhash1.Add("C++");
		myhash1.Add("C#");
		myhash1.Add("Java");
		myhash1.Add("Ruby");
		Console.WriteLine("Elements of myhash1:");

		// Accessing elements of HashSet
		foreach(var val in myhash1)
		{
			Console.WriteLine(val);
		}

		// Creating another HashSet
		// using collection initializer to initialize HashSet
		HashSet<int> myhash2 = new () 
        {   10,
		    100,
            1000,
            10000,
            100000
        };
				
		// Display elements of myhash2
		Console.WriteLine("Elements of myhash2:");
		foreach(var value in myhash2)
		{
			Console.WriteLine(value);
		}
	}
}