Why Equals(null) fail on object array null check

Question

I have an object array as follows:

private object[] _yazdirmaBilgisi = new object[5];

On some point I want to access the first element but of course I wanna check if it is null first

At some point [0] is null but [1] is not null.

When I check if the value is null with Equals it throws exception:

if (!_yazdirmaBilgisi[0].Equals(null))  //Throws exception

But if i check with != null ne exception

if (_yazdirmaBilgisi[0] != null) // No exception

Why does it differ? Why Equals(null) throws exception but != null doesn't

`_yazdirmaBilgisi[0].Equals` means you are trying to call instance method `Equals` and since `_yazdirmaBilgisi[0]` is null, you get NRE — Habib, Nov 13 '14 at 14:52
and what exception does it throw? So you can answer your own question. — Jodrell, Nov 13 '14 at 14:54
Be clear please, we have to guess what is wrong with your code to answer. — Patrick Hofman, Nov 13 '14 at 14:56
Duplicate of http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it. — Patrick Hofman, Nov 13 '14 at 14:57
@PatrickHofman I don't think he's saying something is wrong with his code, he's just asking WHY the two different checks produce different results. Seems like more of a newbie question. — Andy, Nov 14 '14 at 14:55

score 3 · Answer 1 · answered Nov 13 '14 at 14:55

Because == null isn't the same as .Equals(null). If you use Equals, you will obviously get an exception because of trying to access null reference. == works different, similar to Object.Equals(a,b) in that it checks if both values are null first, then it does more comparing.

score 1 · Answer 2 · answered Nov 13 '14 at 14:54

1

The first throws because the first element in the list is a null reference. This is what your code is doing:

var element = _yazdirmaBilgisi[0];
if (element.Equals(null)) // element is null, and null.Equals is calling a method on a null reference

The == operator is defined to do something similar to this:

var element = _yazdirmaBilgisi[0]; // element is reference who's value is null
if (object.ReferenceEquals(element, null)) // object.Equals is a static method which doesn't require an object instance

answered Nov 13 '14 at 14:54

Andy

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*The == operator is overloaded to do this* is misleading. – Sriram Sakthivel Nov 13 '14 at 14:54
@SriramSakthivel I removed the term overload, but otherwise how is it misleading? – Andy Nov 13 '14 at 23:15
Now that's fine. Not only the term *overloaded* was misleading, you said it is overloaded to do `object.Equals`, that's wrong. Later you changed it to `ReferenceEquals`. – Sriram Sakthivel Nov 14 '14 at 06:39
@SriramSakthivel I'm not sure I like ReferenceEquals either (which is why I said my code is similar). In the question he's explicitly using System.Objects, but Strings do define the == operator and it'd be a bit more complicated than my sample. – Andy Nov 14 '14 at 14:58

score 1 · Accepted Answer · answered Nov 13 '14 at 14:54

This line:

if (!_yazdirmaBilgisi[0].Equals(null))

Extracts a reference from _yazdirmaBilgisi[0]
Uses that reference as the object on which to call .Equals().

But because that reference is null, it throws a NullReferenceException.

Whereas:

if (_yazdirmaBilgisi[0] != null)

is directly comparing the reference in _yazdirmaBilgisi[0] with null, so it doesn't throw an exception - it is not trying to call any methods of the referenced object.

Why Equals(null) fail on object array null check

3 Answers3