Why int can't be null? How does nullable int (int?) work in C#?

Question

I am new to C# and just learned that objects can be null in C# but int can't.

Also how does nullable int (int?) work in C#?

Solution 1

int is a primitive type and only ReferenceTypes (objects) are nullable. You can make an int nullable by wrapping it in an object:

System.Nullable<int> i;

-or-

int? i;

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/nullable-types/using-nullable-types

Solution 2

Value types like int contain direct values rather than references like ReferenceType. A value cannot be null but a reference can be null which means it is pointing to nothing. A value cannot have nothing in it. A value always contain some kind of value

Nullable or Nullable types are special value types which in addition to normal values of value types also support additional null value just like reference types

Solution 3

You can have a nullable int by using int? or Nullable<int> both are exactly the same thing.

Solution 4

An int is a value type. It is really of type Int32. Objects that can be nullable are reference types. At a basic level the difference is that a value type will store the value with the variable where a reference type will store a reference to the value. A reference can be null, meaning that it doesn't point to a value. Where a value type always has a value.

Nullable wraps a value type to allow you to have the option of being null. Using int? is just a compiler shortcut to declare a nullable type.

Solution 5

Objects are reference types and they can reference nothing or NULL, however int is a value type, which can only hold a value, and cannot reference anything.

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Comments

[Thomson]

Someone gives the down vote should show the reason. Nullable<T> add an extra private bool filed hasValue to the structure definition to express null of int, besides, the CLR need to be revised a little to deal with this special structure when comparing it with null. The primitive int could not represent null since all its binary permutations represent value of int, in the condition that it has 4 bytes in the stack.

[B Bulfin]

I didn't downvote it, but may have been due to citing an obscure fact about the memory layout without any supporting documentation. Otherwise, I'm not sure.

[Aaron Franke]

Is int? slower than int is? Does it use more memory?

[Ilia Choly]

@AaronFranke yeah, gets wrapped in an extra object.