In the msdn guidance on Equals override, why the cast to object in the null check?

Question

I was just looking at the Guidelines for Overloading Equals() on msdn (see code below); most of it is clear to me, but there is one line I don't get.

if ((System.Object)p == null)

or, in the second override

if ((object)p == null)

Why not simply

 if (p == null)

What is the cast to object buying us?

public override bool Equals(System.Object obj)
{
    // If parameter is null return false.
    if (obj == null)
    {
        return false;
    }

    // If parameter cannot be cast to Point return false.
    TwoDPoint p = obj as TwoDPoint;
    if ((System.Object)p == null)
    {
        return false;
    }

    // Return true if the fields match:
    return (x == p.x) && (y == p.y);
}

public bool Equals(TwoDPoint p)
{
    // If parameter is null return false:
    if ((object)p == null)
    {
        return false;
    }

    // Return true if the fields match:
    return (x == p.x) && (y == p.y);
}

Answer 1

I suppose, since the article also talks about overriding operator==, that it's forcing it to use the == operator defined on Object rather than any overloaded operator in the current class.

Answer 2

The == operator may be overridden, and if it is, the default reference comparison may not be what you get. Casting to System.Object ensures that calling == performs a reference equality test.

public static bool operator ==(MyObj a, MyObj b)
{
  // don't do this!
  return true;
}

...
MyObj a = new MyObj();
MyObj b = null;
Console.WriteLine(a == b); // prints true
Console.WriteLine((object)a == (object)b); // prints false

Answer 3

I prefer using object.ReferenceEquals(a, b) in this ambiguous context to force reference comparison because it makes the intent clear while preserving the semantics precisely (in fact, ReferenceEquals is implemented like that).