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C# Academy · Lesson

Equality & hashing (value vs reference)

Understand reference vs value equality, override Equals/GetHashCode correctly, implement IEquatable , and use custom comparers with sets/dictionaries.

Equality & hashing (value vs reference) is a free C# Academy lesson on CoddyKit — lesson 3 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Equality basics

Goal: Make equality work as you expect.

  • Reference vs value equality
  • Equals + GetHashCode contract
  • IEquatable<T> for speed
  • Custom comparers when you cannot modify the type

Reference equality pitfall

Classes default to reference equality. Two separate objects with same data are not equal unless you implement value equality.

using System;
using System.Collections.Generic;

public sealed class Point // no overrides
{
  public int X;
  public int Y;
  public Point(int x, int y){ X = x; Y = y; }
}

public class Program
{
  public static void Main(string[] args)
  {
    Point a = new Point(1, 2);
    Point b = new Point(1, 2);

    Console.WriteLine("a == b ? " + (a == b)); // reference equality: False
    Console.WriteLine("a.Equals(b) ? " + a.Equals(b)); // False

    HashSet<Point> set = new HashSet<Point>();
    set.Add(a);
    Console.WriteLine("Contains b? " + set.Contains(b)); // False (unexpected)
  }
}

Value equality implemented

Implement IEquatable<T>, override Equals and GetHashCode. The hash must match equality: equal objects → same hash.

using System;
using System.Collections.Generic;

public sealed class ValuePoint : IEquatable<ValuePoint>
{
  public int X;
  public int Y;
  public ValuePoint(int x, int y){ X = x; Y = y; }

  public bool Equals(ValuePoint other)
  {
    if (ReferenceEquals(other, null)) return false;
    if (ReferenceEquals(this, other)) return true;
    return X == other.X && Y == other.Y;
  }

  public override bool Equals(object obj)
  {
    return Equals(obj as ValuePoint);
  }

  public override int GetHashCode()
  {
    // Simple, stable combination (avoid randomness)
    unchecked
    {
      int hash = 17;
      hash = hash * 31 + X.GetHashCode();
      hash = hash * 31 + Y.GetHashCode();
      return hash;
    }
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    ValuePoint a = new ValuePoint(1, 2);
    ValuePoint b = new ValuePoint(1, 2);

    Console.WriteLine("a.Equals(b)? " + a.Equals(b)); // True

    HashSet<ValuePoint> set = new HashSet<ValuePoint>();
    set.Add(a);
    Console.WriteLine("Contains b? " + set.Contains(b)); // True (value semantics)
  }
}

Custom comparer

Pass a custom IEqualityComparer<T> to HashSet/Dictionary when you cannot change the type itself.

using System;
using System.Collections.Generic;

public sealed class Person // imagine from a library; cannot edit
{
  public string Name;
  public int BirthYear;
  public Person(string name, int year){ Name = name; BirthYear = year; }
}

public sealed class PersonComparer : IEqualityComparer<Person>
{
  public bool Equals(Person a, Person b)
  {
    if (ReferenceEquals(a, b)) return true;
    if (ReferenceEquals(a, null) || ReferenceEquals(b, null)) return false;
    return a.Name == b.Name && a.BirthYear == b.BirthYear;
  }

  public int GetHashCode(Person p)
  {
    if (ReferenceEquals(p, null)) return 0;
    unchecked
    {
      int h = 23;
      h = h * 31 + (p.Name == null ? 0 : p.Name.GetHashCode());
      h = h * 31 + p.BirthYear.GetHashCode();
      return h;
    }
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Person p1 = new Person("Ada", 1815);
    Person p2 = new Person("Ada", 1815);

    HashSet<Person> set = new HashSet<Person>(new PersonComparer());
    set.Add(p1);
    Console.WriteLine("Contains p2? " + set.Contains(p2)); // True via comparer
  }
}

Contract & pitfalls

Rules:

  • If Equals(a,b) is true ⇒ GetHashCode(a) == GetHashCode(b)
  • Equality should be reflexive, symmetric, transitive
  • Use immutable fields for hashing where possible
  • Do not use random values in GetHashCode

Struct vs class note

Structs compare by fields by default (value semantics). Classes compare by reference unless you implement value equality.

using System;

public struct PointS // struct: value type
{
  public int X;
  public int Y;
  public PointS(int x, int y){ X = x; Y = y; }
}

public class Program
{
  public static void Main(string[] args)
  {
    PointS a = new PointS(1, 2);
    PointS b = new PointS(1, 2);
    Console.WriteLine("Struct equality: " + a.Equals(b)); // True by default (field-wise)
  }
}

Equality contract for collections

Quick check: To give a class value equality that works in HashSet and Dictionary, what should you implement?

Recap

Recap: Classes need Equals/GetHashCode (and often IEquatable<T>) for value semantics; use custom comparers when types are not editable.

Frequently asked questions

Is the “Equality & hashing (value vs reference)” lesson free?

Yes — the full text of “Equality & hashing (value vs reference)” is free to read here on the web, and the C# Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.

What will I learn in “Equality & hashing (value vs reference)”?

Understand reference vs value equality, override Equals/GetHashCode correctly, implement IEquatable , and use custom comparers with sets/dictionaries. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Equality & hashing (value vs reference)” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C# Academy lesson?

Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. HashSet , SortedSet , Queue , Stack
  2. ConcurrentDictionary , immutable collections
  3. Equality & hashing (value vs reference)
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