FAQ: The Object Class - Object Members

This community-built FAQ covers the " Object Members" exercise from the lesson “The Object Class”.

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Hi. I’m not entirely sure but I think there’s a mistake in this lesson: Object Members

I believe this is incorrect: “referential equality for reference types”.

  • Equals(Object) — returns true if the current instance and the argument are equal (using value equality for value types and referential equality for reference types)

There are two overloads for Equals method (Equals(Object) and Equals(Object, Object)) and they both seem to perform value/type comparison.

For reference comparison one should use Reference.Equals(Object, Object)

See examples from Microsoft docs

public static void Main()
     Point3D point3Da = new Point3D(5, 5, 2);
     Point3D point3Db = new Point3D(5, 5, 2);
     Point3D point3Dc = new Point3D(5, 5, -1);

     Console.WriteLine("{0} = {1}: {2}",
                       point3Da, point3Db, point3Da.Equals(point3Db));
     Console.WriteLine("{0} = {1}: {2}",
                       point3Da, point3Dc, point3Da.Equals(point3Dc));

//Point(5, 5, 2) = Point(5, 5, 2): True
//Point(5, 5, 2) = Point(5, 5, -1): False
      Dog m1 = new Dog("Alaskan Malamute");
      Dog m2 = new Dog("Alaskan Malamute");
      Dog g1 = new Dog("Great Pyrenees");
      Dog g2 = g1;
      Dog d1 = new Dog("Dalmation");

      Console.WriteLine("{0} = {1}: {2}", g1, g2, Object.Equals(g1, g2));
      Console.WriteLine("{0} Reference Equals {1}: {2}\n", g1, g2, Object.ReferenceEquals(g1, g2));

      Console.WriteLine("{0} = {1}: {2}", m1, m2, Object.Equals(m1, m2));
      Console.WriteLine("{0} Reference Equals {1}: {2}\n", m1, m2, Object.ReferenceEquals(m1, m2));

      Console.WriteLine("{0} = {1}: {2}", m1, d1, Object.Equals(m1, d1));
      Console.WriteLine("{0} Reference Equals {1}: {2}", m1, d1, Object.ReferenceEquals(m1, d1));

// Great Pyrenees = Great Pyrenees: True
// Great Pyrenees Reference Equals Great Pyrenees: True

// Alaskan Malamute = Alaskan Malamute: True
// Alaskan Malamute Reference Equals Alaskan Malamute: False

// Alaskan Malamute = Dalmation: False
// Alaskan Malamute Reference Equals Dalmation: False

It is a good question, not a mistake though. There is a semantic difference between Equals and ReferenceEquals(Object,Object):

  1. and most importantly ReferenceEquals(Object,Object) is a STATIC method - as your example from documentation shows it’s called with a class Object, not with an instance.
  2. ReferenceEquals(Object,Object) CANNOT be overriden.
  3. ReferenceEquals(Object,Object) CAN ONLY have two arguments, however - they CAN be null and it will never throw NullReferenceException.
  4. Equals works ONLY on object instances
  5. Equals CANNOT be called on null or unassigned reference.
  6. Equals can be overriden.

Basically ReferenceEquality is the same thing as == and yes it should be used with references. HOWEVER, this will not always be possible. You will see Equals more often since it’s one of the most commonly overriden methods…

So I wouldn’t say it was a mistake on their part, but I agree it should be explicitly pointed out to seek for ReferenceEquals inside documentation.

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