Why do I get a null reference error when using something.Equals(null) in C#

Question

I keep getting a null reference error when I am trying to check if something is null. I have a class called User and I initialize the variable indvUser like so

User indvUser = api.Users.SearchByExternalId(session.UserInfo.UserId.ToString())
                   .Users.FirstOrDefault();

Then I want to check if indvUser is null like this

if (indvUser.Equals(null))
{
    int a = 1;
}

But I get a null reference error when using Equals(null) which I don't understand. If it actually is null i.e. there is no value, shouldn't Equals(null) return true?

That is what I ended up doing, but I was curious why this wouldn't work. — Adam
– Adam, Commented Dec 17, 2013 at 21:33

valverij · Accepted Answer · 2013-12-17 19:26:19Z

6

Since indvUser is null, and indvUser.Equals is an instance method on your User object (i.e., it requires an non-null instance of your object), .NET will throw the error you attempt to use it.

For something like this, you could use this:

Object.ReferenceEquals(indvUser, null)

Or simply:

indvUser == null

Since neither of these approaches actually attempt to access methods or properties on the indvUser object itself, you should be safe from NullReferenceExceptions

edited Dec 17, 2013 at 19:26

answered Dec 17, 2013 at 19:11

valverij

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1 Comment

DotNetRussell Over a year ago

Object.equals already calls ReferenceEquals msdn.microsoft.com/en-us/library/bsc2ak47(v=vs.110).aspx

Konrad Kokosa · Accepted Answer · 2013-12-17 19:11:01Z

2

In line:

indvUser.Equals(null)

if indvUser is null, how Equals method could be called on it? It simply can be seen as:

null.Equals(null)

You should compare it as indvUser == null or object.ReferenceEquals(indvUser, null)

answered Dec 17, 2013 at 19:11

Konrad Kokosa

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1 Comment

Pete Garafano Over a year ago

Good explanation as to WHY the NullReferenceException is occurring.

AFrieze · Accepted Answer · 2013-12-17 19:11:43Z

1

Object.Equals is a method which is not static. In order to call the Equals method, you must have an instance of the class.

answered Dec 17, 2013 at 19:11

AFrieze

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Comments

AlG · Accepted Answer · 2013-12-17 19:11:21Z

0

If indUser == null you'd be doing null.Equals(null) which would throw an exception.

answered Dec 17, 2013 at 19:11

AlG

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