Set operations with HashSet (union, intersection, difference)

This lesson covers unions, intersections, and differences with VB.NET's HashSet, with the aim of understanding the basics of set operations.

UnionWith() gives the union (everything in either), IntersectWith() the intersection (what they share), and ExceptWith() the difference (what only one has). The mathematics maps straight onto the code.

The sample code applies each method to the two HashSets {1, 2, 3, 4} and {3, 4, 5, 6} and shows the union, intersection, and difference. Note that it copies first so as not to destroy the originals.

A common early stumble is that these methods change the set they are called on. To keep the original data you first make a copy with New HashSet(Of Integer)(a), as the sample does.

In professional work, set operations are applied constantly — finding the users common to two groups, listing what only one of two lists contains.

📖 Reference code

Imports System.Linq

Module Program
    Sub Main()
        Dim a As New HashSet(Of Integer)({1, 2, 3, 4})
        Dim b As New HashSet(Of Integer)({3, 4, 5, 6})

        Dim union As New HashSet(Of Integer)(a)
        union.UnionWith(b)

        Dim intersection As New HashSet(Of Integer)(a)
        intersection.IntersectWith(b)

        Dim difference As New HashSet(Of Integer)(a)
        difference.ExceptWith(b)

        Console.WriteLine(String.Join(",", union.OrderBy(Function(x) x)))
        Console.WriteLine(String.Join(",", intersection.OrderBy(Function(x) x)))
        Console.WriteLine(String.Join(",", difference.OrderBy(Function(x) x)))
    End Sub
End Module

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