Records: EqualityContract is a Type

[TonyValenti]

Why is the equality contract on records a type? How will it be used?

I kind of expected it to to return a static singleton instance of a synthesized IComparer.

[JustNrik]

It's used to compare the type of the current instance, pretty much like doing typeof(ThisClass) == obj.GetType() on bool Equals(ThisClass obj) but using that property instead.

[TonyValenti]

So would it essentially use an

Activator.CreateInstance(EqualityContract).Equals(x, y)

to do a compare? What would the synthesized compare code look like?

[CyrusNajmabadi]

So would it essentially use an

No.

It will do:

=> this.EqualityContract == other.EqualityContract && ... do the member equality checks ...

[zvrba]

Why is EqualityContract even needed when GetType() exists?

[theunrepentantgeek]

Adding EqualityContract allows for precise control over whether subtypes are equal, or not.

See #3137 for more information

The difficulty with implementing value-based equality across a type hierarchy lies in ensuring symmetry - that the two values agree on the equality being applied. In order to ensure that, types with value-based equality (or any custom equality) that can participate in a hierarchy of mutually comparable objects must essentially agree on who implements their equality. They can do that by declaring a Type-valued virtual property EqualityContract in the root class of the hierarchy, with every derived type that alters equality overriding that to return its own type.

[alrz]

overriding that to return its own type.

Technically that's what GetType returns.

every derived type that alters equality

Is there any case that a derived type doesn't alter equality?

[theunrepentantgeek]

Is there any case that a derived type doesn't alter equality?

Most certainly.

For one example, when NHibernate returns an instance you've loaded from the database, you actually get a subclass of the expected type - so instead of a Person you get (effectively) a PersonProxy where all the properties have been overridden with additional functionality (for dirty tracking, lazy loading, etc). Yet a PersonProxy and a Person can, and should, compare as equal under certain circumstances.

Mads touches on the subject in the blog post Welcome to C# 9.0.