Overriding == operator. How to compare to null?

Question

Possible Duplicate:
How do I check for nulls in an ‘==’ operator overload without infinite recursion?

There is probably an easy answer to this...but it seems to be eluding me. Here is a simplified example:

public class Person
{
   public string SocialSecurityNumber;
   public string FirstName;
   public string LastName;
}

Let's say that for this particular application, it is valid to say that if the social security numbers match, and both names match, then we are referring to the same "person".

public override bool Equals(object Obj)
{
    Person other = (Person)Obj;
    return (this.SocialSecurityNumber == other.SocialSecurityNumber &&
        this.FirstName == other.FirstName &&
        this.LastName == other.LastName);
}

To keep things consistent, we override the == and != operators, too, for the developers on the team who don't use the .Equals method.

public static bool operator !=(Person person1, Person person2)
{
    return ! person1.Equals(person2);
}

public static bool operator ==(Person person1, Person person2)
{
    return person1.Equals(person2);
}

Fine and dandy, right?

However, what happens when a Person object is null?

You can't write:

if (person == null)
{
    //fail!
}

Since this will cause the == operator override to run, and the code will fail on the:

person.Equals()

method call, since you can't call a method on a null instance.

On the other hand, you can't explicitly check for this condition inside the == override, since it would cause an infinite recursion (and a Stack Overflow [dot com])

public static bool operator ==(Person person1, Person person2)
{
    if (person1 == null)
    {
         //any code here never gets executed!  We first die a slow painful death.
    }
    return person1.Equals(person2);
}

So, how do you override the == and != operators for value equality and still account for null objects?

I hope that the answer is not painfully simple. :-)

Answer 1

Use object.ReferenceEquals(person1, null) instead of the == operator:

public static bool operator ==(Person person1, Person person2)
{
    if (object.ReferenceEquals(person1, null))
    {
         return object.ReferenceEquals(person2, null);
    }

    return person1.Equals(person2);
}