If null.Equals(null) why do I get a NullReferenceException

Question

I have the following line of code:

var selectedDomainID = lkuDomainType.EditValue.Equals(null) 
    ? string.Empty 
    : lkuDomainType.EditValue;

Sometimes this generates a NullReferenceException. What I don't understand is why. Isn't the whole point of my code to check for null and if so assign string.empty? When I check in DEBUG it is stating that EditValue == null so what am I missing?

can't lkuDomkainType be null itself? When you call lkuDomainType.EditValue , it throws the NullReferenceException. — Saher Ahwal, Aug 11 '10 at 15:03
Because of your title: `a == b` is not the same as `a.Equals(b)`. — Stefan Steinegger, Aug 11 '10 at 15:12
@Stefan: Great point! Changed to be more accurate for future searchers...Thanks — Refracted Paladin, Aug 11 '10 at 15:15
Possible duplicate of [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — user247702, Sep 04 '17 at 15:01

score 32 · Accepted Answer · answered Aug 11 '10 at 15:03

32

Use lkuDomainType.EditValue == null, otherwise you are trying to call an instance method on a null object. But the better option might be lkuDomainType.EditValue ?? String.Empty. Also watch out for lkuDomainType being null, unless it is a class not an object.

answered Aug 11 '10 at 15:03

Yuriy Faktorovich

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Thanks, seems pretty obvious in hindsight, definitely a 'duh' moment. Thanks. – Refracted Paladin Aug 11 '10 at 15:08
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+2 (unfortunately not possible) for the correct answer and the recommendation to use `??`. – Stefan Steinegger Aug 11 '10 at 15:10
+1: The null coalescing (or `??`) operator was a great addition to .NET and something I sorely miss when using Java. – Powerlord Aug 11 '10 at 15:19

score 16 · Answer 2 · answered Aug 11 '10 at 15:03

16

When you use Object.Property and Object is undefined, you are dereferencing a null pointer and that's why you get the exception. Instead, use:

var selectedDomainID = lkuDomainType.EditValue == null ? string.Empty : lkuDomainType.EditValue;

answered Aug 11 '10 at 15:03

NullUserException

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I had to upvote this answer a) for the correct answer, b) for using *dereference* in the sentence, and b) for the almost appropriate user name – Giuseppe Accaputo Aug 11 '10 at 15:07
Great answer, +1, but SO says that @Yuriy's answer was first. Thanks though. – Refracted Paladin Aug 11 '10 at 15:12
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@Refracted Yes it was, by 4 seconds :) I like his/her suggestion of using `??` – NullUserException Aug 11 '10 at 15:12
@Refracted Yes, you're right. I've forgot to mention that I upvoted Yuriy's answer, too, for using the `??` operator – Giuseppe Accaputo Aug 11 '10 at 15:15
In this case, it's `Object.Property.Method`, and if either `Object` or `Property` are undefined, you'd be dereferencing a null pointer. – Powerlord Aug 11 '10 at 15:17

score 5 · Answer 3 · answered Aug 11 '10 at 15:04

If EditValue is null then you can't call Equals. In this cas you would have to do:

var selectedDomainID = lkuDomainType.EditValue == null ? string.Empty : lkuDomainType.EditValue;

Or you can simplify it by doing:

var selectedDomainID = lkuDomainType.EditValue ?? string.Empty;

score 3 · Answer 4 · answered Aug 11 '10 at 15:04

3

you are trying to call the Equals method on a null object - do it like this instead:

lkuDomainType.EditValue == null

answered Aug 11 '10 at 15:04

Ray

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score 3 · Answer 5 · answered Aug 11 '10 at 15:05

The problem is that you are using the object before checking if it's null. You are calling the Equals method of the object, which fails if the reference is null.

You have to exchange your lkuDomainType.EditValue.Equals(null) for lkuDomainType.EditValue == null.

score 2 · Answer 6 · answered Aug 11 '10 at 15:04

2

EditValue == null. That means that there is no object there. You cannot call functions on null objects, even if the function is .Equals().

You're better off just saying "(lkuDomainType.EditValue == null)" in this case.

answered Aug 11 '10 at 15:04

GWLlosa

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score 2 · Answer 7 · answered Aug 11 '10 at 15:05

2

You should use String.IsNullOrEmpty here. Like this:

var selectedDomainID = String.IsNullOrEmpty(lkuDomainType.EditValue) ? string.Empty : lkuDomainType.EditValue;

Equals is a method, you're trying to call a method on a null object which is throwing an exception.

answered Aug 11 '10 at 15:05

brendan

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I don't understand, isn't `String.IsNullOrEmpty` also a *method*? – Refracted Paladin Aug 11 '10 at 15:09
It's a static method, it's not a method of the lkuDomainType.EditValue object. – brendan Aug 11 '10 at 15:26

score 2 · Answer 8 · answered Aug 11 '10 at 15:05

2

when EditValue is null you cannot call the Equals method on it so the best way to check is to use

lkuDomainType.EditValue == null ? string.Empty : lkuDomainType.EditValue;

answered Aug 11 '10 at 15:05

Rony

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score 1 · Answer 9 · answered Feb 18 '21 at 19:14

All current answers fail to address a critical point: the difference between calling Equals on a Nullable type (which is a struct) vs. class type.

Calling Equals on a Nullable typed object whose value is null will not issue an exception, while doing the same on a class typed object will.

Consider the following code:

int? num = null; // int? is Nullable<int>
String s = null; // String is a class

var equalsNullable = num.Equals(null); // works
var equalsClass = s.Equals(null);      // throws

The reason is that Nullable's HasValue method is called in such case, see Jon Skeet's answer for more info.

As other people said, the way to go in the OP's case is to use == or ??.

score 0 · Answer 10 · answered Aug 11 '10 at 15:16

If lkuDomainType.EditValue is null, then "lkuDomainType.EditValue.Equals(someObject)" is the same as coding "null.Equals(someObject)". Well, obviously "null" doesn't have any members or methods (it wouldn't be null if it did). That's why you get a NullReferenceException.

Most of the examples from the other posts will work, including String.IsNullOrEmpty, which is a method that returns a boolean value.

If null.Equals(null) why do I get a NullReferenceException

10 Answers10