What does .NET's Equals method really mean?

Question

We know all types inherit Equals from their base class, which is Object.

Per Microsoft docs:

Equals returns true only if the items being compared refer to the same item in memory.

So we use Equals() to compare object references, not the state of the object.

Typically, this method is overridden to return true only if the objects being compared have the same internal state values.

My Question: Can two objects point to the same item in memory but have different states? If not why override Equals?

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Thanks for answers that made this clear. For future readers here is an example of why we override:

Employee A=New Employee();
Employee B=New Employee();

in this case, A and B always point to different memory so Equals is always false.

However if:

A.SSN=B.SSN;
A.LiceneNumber=B.LiceneNumber;

These two Employees are the same person, in this case, we need to check state and therefore override Equals.

So in my case, the point of the matter is: it is possible that two different objects are stored in two different locations but still refer to same entity (in my case Employee).

Answer 1

Can 2 object point to the same item in memory but have different states?

This misunderstands the difference between an object and a variable or reference. The "item in memory" is the object, and the "state" it's talking about is the memory used to hold object. You refer to the object with a variable. The correct way to ask this question is below (and the difference does matter, as it's key to understanding a lot about how C# and similar languages work):

Can two variables refer to the same object in memory but have different states?

To answer this, you can have two variables refer to the same object in memory... but then it really is the same object. There's only one item in memory (it's the object), and that means only one state.

But that brings us to this question:

why override Equals?

You override the Equals() method because sometimes you have two variables refer to two different objects, where those two objects have the same value. For example, I can create two string objects like this:

string a = "hello world";
string b = "hello world";

Ignoring string interning for the moment, those two variables refer to two different string objects in memory. The base Equals() implementation inherited from Object would result in false, but the string type overloads the Equals() method to do a value comparison, so instead a.Equals(b) will result in true.

And, finally, we know enough to answer this question:

Is it possible for two different objects stored in two different locations to still refer to same Entity?

Yes, it is. This, again, is why we overload the Equals() method (along with GetHashCode()). The base Equals() method inherited from Object would see those as two different objects, and the result would be false. You override Equals() for the type so it can return true instead when the two objects represent the same value.

Now it starts to get tricky. An application might end up with two instances of an Employee type for the same person, where a user then changes the e-mail address property on just one of them. Now you have two instances. For the same employee. Where some fields have different values.

What do you want Equals() to do? Think carefully about this; it has been the source for many bugs in many applications over the years. Sometimes you need it one way. Sometimes you need it the other. Sometimes the behavior needs to change depending on context within the same application.

Answer 2

Can 2 [variables] point to the same item in memory but have different states?

No.

if not why override Equals?

To change the default behavior which compares reference equality, to compare the values of the objects.

See also When Should a .NET Class Override Equals()? When Should it Not?.

Answer 3

You override equals so you can test equality for objects that are not the same item in memory.