可空对象的类别

Question

This question is more a curiosity then anything...

Say there is a class like:

public class Foo{
public int? x {get; set;}
public int? y {get; set;}
}

And somewhere in the project instances were created:

var foo1= new Foo
         {
           x= 1;
           y= 1;
         };

var foo2= new Foo
         {
           x= 1;
           y= 1;
         };

If for some reason they wanted to check to see if the equal each other and set them to NULL if they are and print to screen...

ex:

if(foo1.x == foo2.x)
   foo1.x = null;
if(foo1.y == foo2.y)
   foo1.y = null;

if(foo1 == null){
   Console.WriteLine("foo1 is NULL");
}else{
   Console.WriteLine("foo1 is not NULL");
}

Which would print?

The instance of foo1 exists, but all it's objects are set to NULL . I'm new to the concept of nullable types so this struck a curiosity in me! (My Visual Studio is on the fritz or I'd test myself)

Answer 1

I have two hands. If my two hands are empty, does that mean I do not exist?

An variable's null state does not depend on any of the properties of the instance it points at.

Answer 2

如您所说，foo本身仍然是一个实例化的对象，无论其成员数据的值如何，因此else语句都会输出。

Answer 3

You will have the else exucuted foo1 is not NULL , the object itself is not null, but the properties are.

you can set foo1 to null by foo1 = null

Answer 4

foo1 is still an instantiated object and so would not be null so Console.WriteLine("foo1 is not NULL"); would be executed.

If you really needed foo1 == null to return true if x and y are null then you could override the == operator and the Equals() method.