EqualityComparer<T>.Default misunderstanding?

Question

I have a class Person, it implements Equals() method from IEquatable<Person> (also overrides Object.Equals method, lets ignore the GetHashcode() method for now)

class Person : IEquatable<Person>
{
    public string Name { get; set; }

    public bool Equals(Person other)
    {
        return this.Name == other.Name;
    }
    public override bool Equals(object obj)
    {
        var person = obj as Person;
        return person != null && person.Name == Name;
    }
}

Ok, lets start:

Person p1 = new Person() { Name = "a" };
Person p2 = new Person() { Name = "a" };

List<Person> lst1 = new List<Person>() { p1 };
List<Person> lst2 = new List<Person>() { p2 };

Lets talk about this line :

 bool b = lst1.SequenceEqual(lst2, EqualityComparer<Person>.Default);

I have a problem understanding this part :

EqualityComparer<Person>.Default

I've heard that EqualityComparer<Person>.Default will check if the class is implementing IEquatable - it will take the Equals(Person other) Method and not the Equals(object obj). it has the advantage of avoiding boxing

but

the Equals(Person other) will run with or withOUT EqualityComparer<Person>.Default (because it's implementing IEquatable)

So what Boxing are we talking about ? there isn't !

The only time that Equals(object obj) will run is when :

bool b = lst1.SequenceEqual(lst2,EqualityComparer<Object>.Default);

But I'm a programmer! I'll never Send an object when its actually a Person!

What am I missing? I'm having trouble understanding the benefit here of EqualityComparer<Object>.Default. Can someone please give me an example to prove me I'm wrong ?

Answer 1

That you can pass in IEqualityComparer<object>.Default is an effect of generic contravariance, added in .NET 4.

Essentially, an IEqualityComparer<BaseType> can be used whenever an IEqualityComparer<DerivedType> is required, where DerivedType : BaseType. Since Person derives from Object, this means that an IEqualityComparer<Object> can be used wherever an IEqualityComparer<Person> is required.

Answer 2

If you pass in null as the second parameter or if you don't pass in a second argument at all (which is basically the same), the implementation of SequenceEquals will call EqualityComparer<T>.Default itself (decompile Enumerable to see this). That explains why you don't see a difference whether you provide EqualityComparer<T>.Default or not.

So in the end the second parameter only makes sense if you want to use an equality comparer other than EqualityComparer<T>.Default.