Nullable is not possible, Why not?

Question

Excuse me if its a silly question, I am trying to get a better understanding of Nullable types in .Net.

From what i notice from Microsoft source code (using ReSharper), I understand that Nullable is a struct and T needs to be a struct

public struct Nullable<T> where T : struct

Now, I tried to do something like this

public struct CustomNullable<T> where T : struct
{
}
public class CustomNullableClass<T> where T : struct
{
}

And I get an error when I compile:

  Nullable<int?> a = null;
  Nullable<Nullable<int>> a1 = null;

For the above mentioned code I get an error 'Only non-nullable value types could be underlying of System.Nullable', but how is this enforced in the Nullable type ?

And for

   CustomNullable<int?> a2 = null;
   CustomNullableClass<int?> a3 = null;

I get an error 'The type System.Nullable must be non-nullable value type in order to use it as parameter T '.

I am bit confused now, can some one help me understand whats going on or have I not understood something ?

Thanks a lot in advance.

EDIT: If structs are value types and value types can't be null, how can a Nullable be a struct?

Credit : spender

I'm sure there is a duplicate but I can't find it. Edit: done! — Soner Gönül, Jul 07 '14 at 14:02
Wouldn't a better question be, if structs are value types and value types can't be null, how can a `Nullable` be a struct? — spender, Jul 07 '14 at 14:03
I dont want to make a nullable nullable type, I am just trying to understand, playing about really — RaM, Jul 07 '14 at 14:04
See here also: http://stackoverflow.com/questions/12476590/why-is-nullablet-nullable-why-it-cannot-be-reproduced?rq=1 — spender, Jul 07 '14 at 14:07
@spender Actually, `Nullable` isn't really capable of being `null`, it "just" has a special state that when compared to `null` produces `true`. This is nicely evident through the fact that on a `null` `Nullable` you can still retrieve the property `HasValue`. Interestingly, once you *box* the nullable `null` value, it becomes *actual* `null` and calling any method on that results in `NullReferenceException` - yet another hack in the type system (comment in reference source: `we have special type system support that says a a boxed Nullable can be used where a boxed is used`). — Luaan, Jul 07 '14 at 14:11
@Luuaan: I knew that, but it seemed like a pertinent question to get the OP to question more about the magical qualities of a `Nullable`. Thanks for stepping up to the mark :) — spender, Jul 07 '14 at 14:44

score 9 · Accepted Answer · answered Jul 07 '14 at 14:03

9

Actually, it's an internal hack somewhere in the C# compiler (guessing). You cannot replicate it in your own classes. If you make your own type, using the exact same IL, it will not enforce that additional hidden constraint.

answered Jul 07 '14 at 14:03

Luaan

59,957
7
91
109

score 4 · Answer 2 · answered Jul 07 '14 at 14:04

There is special compiler support specifically for Nullable. You're not capable of reproducing a number of different possible behaviors of Nullable with a custom struct. One of those behaviors is the one that you've mentioned here, that Nullable doesn't meet the generic constraint for a struct despite being a struct.

Nullable is not possible, Why not?

2 Answers2

Linked