Set Operations with HashSet (Union, Intersection, Difference)

In this lesson you will learn how to compute unions, intersections, and differences with HashSet in C#, covering the basics of set operations. This is for anyone searching for "C# HashSet set operations".

The UnionWith, IntersectWith, and ExceptWith methods of HashSet<T> compute the union, intersection, and difference of two sets.

The sample code applies each method to the two sets {1, 2, 3, 4} and {3, 4, 5, 6}, then sorts each result with OrderBy() so it reads clearly.

A common stumbling block for beginners is that these methods modify the HashSet they are called on. If you need to keep the original data, make a copy first as the sample does.

Set operations come from mathematics, and the ideas behind intersection and difference carry over to understanding join operations in SQL.

📖 Reference code

using System;
using System.Linq;
using System.Collections.Generic;

class Program {
    static void Main() {
        var a = new HashSet<int> { 1, 2, 3, 4 };
        var b = new HashSet<int> { 3, 4, 5, 6 };

        var union = new HashSet<int>(a);
        union.UnionWith(b);

        var intersection = new HashSet<int>(a);
        intersection.IntersectWith(b);

        var difference = new HashSet<int>(a);
        difference.ExceptWith(b);

        Console.WriteLine(string.Join(",", union.OrderBy(x => x)));
        Console.WriteLine(string.Join(",", intersection.OrderBy(x => x)));
        Console.WriteLine(string.Join(",", difference.OrderBy(x => x)));
    }
}

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run C# directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).