Equals Method

The Equals method is used to determine whether 2 objects of a class are the same.

Syntax

Objects equality

Let's look at the following example:

using System;

namespace ConsoleApp1
{
    class Smartphone : IEquatable<Smartphone>
    {
        public string model;
        public double price;

public Smartphone(string model, double price)
        {
            this.model = model;
            this.price = price;
        }

public bool Equals(Smartphone smartphone)
        {
            // two smartphones are considered identical if
            // they are the same model and have the same price:
            return this.model == smartphone.model &&
                    this.price == smartphone.price;
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            Smartphone phone_1 = new Smartphone("Iphone 11", 2500);
            Smartphone phone_2 = new Smartphone("Iphone 11", 2600);
            Smartphone phone_3 = new Smartphone("Iphone 11", 2600);
            Smartphone phone_4 = new Smartphone("Samsung Galaxy Fold", 2600);

if (phone_1.Equals(phone_2))
            {
                Console.WriteLine("Phone 1 and Phone 2 are the same.");
            }

if (phone_2.Equals(phone_3))
            {
                Console.WriteLine("Phone 2 and Phone 3 are the same.");
            }

if (phone_3.Equals(phone_4))
            {
                Console.WriteLine("Phone 3 and Phone 4 are the same.");
            }
        }
    }
}

Output:

Explanation:

At lines 16 - 22 the Equals method is defined. Two Smartphone objects are considered identical if they have the same model and price.
That's why only phone_2 and phone_3 are considered identical.

Equals And Contains Methods

It can be checked whether an object is contained in List collections via the Equals and Contains methods.

The rule whether an object is contained in the array is specified in the Equals method.

Example:

using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
    class Smartphone : IEquatable<Smartphone>
    {
        public string model;
        public double price;

public Smartphone(string model, double price)
        {
            this.model = model;
            this.price = price;
        }

public bool Equals(Smartphone smartphone)
        {
            // two smartphones are considered identical if
            // they are the same model.
            return this.model == smartphone.model;
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            Smartphone phone_1 = new Smartphone("Iphone 11", 2500);
            Smartphone phone_2 = new Smartphone("Iphone 11", 2600);
            Smartphone phone_3 = new Smartphone("Oneplus 7", 1200);
            Smartphone phone_4 = new Smartphone("Samsung Galaxy Fold", 2600);
            Smartphone phone_5 = new Smartphone("Samsung Galaxy S20", 1700);

List<Smartphone> smartphones = new List<Smartphone>();
            smartphones.Add(phone_1);
            smartphones.Add(phone_4);

if (smartphones.Contains(phone_1))
            {
                Console.WriteLine("phone_1's model is contained in the array");
            }

if (smartphones.Contains(phone_2))
            {
                Console.WriteLine("phone_2's model is contained in the array");
            }

if (smartphones.Contains(phone_3))
            {
                Console.WriteLine("phone_3's model is contained in the array");
            }

if (smartphones.Contains(phone_4))
            {
                Console.WriteLine("phone_4's model is contained in the array");
            }

if (smartphones.Contains(phone_5))
            {
                Console.WriteLine("phone_5's model is contained in the array");
            }
        }
    }
}

Output:

Explanation:

At line 21 the Equals method states that two Smartphone objects are considered identical if their model is the same.
The Contains method at lines 40, 45, 50, 55, 60 relies on the Equals method.
This means that a Smartphone object is considered to be contained in the list, if the list has an object with the same model.