Implementing equality of reference types by overriding the == operator with C# .NET

In this post we saw how to implement the generic IEquatable interface to make two custom objects equatable using the static Equals method.

You’re probably aware of the equality operator ‘==’ in C#. Let’s see what happens if you try to use it for a custom object:

public class Person
{
	public int Id { get; set; }
	public string Name { get; set; }
	public int Age { get; set; }
}

Person personOne = new Person() { Age = 6, Name = "Eva", Id = 1 };
Person personTwo = new Person() { Age = 6, Name = "Eva", Id = 1 };
Console.WriteLine(personOne == personTwo);

This will print false just like it did in the post referenced above when we first tested the Equals method. The two person objects point to different positions in memory and in this case the Person objects are cast into their memory addresses that are simply integers. As the integers are different the ‘==’ operator and Equals methods will return false.

We can easily make the above code return true. We’ll say that two Person objects are equal if their IDs are equal. Here’s how you can override the ‘==’ operator in the Person object:

public static bool operator ==(Person personOne, Person personTwo)
{
	return personOne.Id == personTwo.Id;
}

You’ll now get a compiler error. As it turns out if the == operator is overridden then there must be a matching override of the != operator. OK, no problem:

public static bool operator !=(Person personOne, Person personTwo)
{
	return personOne.Id != personTwo.Id;
}

Rerun the equality example again…

Person personOne = new Person() { Age = 6, Name = "Eva", Id = 1 };
Person personTwo = new Person() { Age = 6, Name = "Eva", Id = 1 };
Console.WriteLine(personOne == personTwo);

…and will print true.

Note that you’ll get a warning from the compiler. The Person class should also override two other methods: object.GetHashCode and object.Equals. Refer back to the post mentioned above to see how to that.

View all various C# language feature related posts here.

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About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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