为什么在C＃中使用something.Equals（null）时会出现空引用错误

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 ie there is no value, shouldn't Equals(null) return true?

Answer 1

Since indvUser is null , and indvUser.Equals is an instance method on your User object (ie, 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

Answer 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)

Answer 3

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

Answer 4

如果indUser == null ，则将执行null.Equals(null) ，这将引发异常。