Why string or object type don't support nullable reference type? [duplicate]

Question

This question already has answers here:

C# nullable string error (5 answers)

Closed 8 years ago.

Look into following code block:

//Declaring nullable variables.
//Valid for int, char, long...
Nullable<int> _intVar;
Nullable<char> _charVar;

//trying to declare nullable string/object variables
//gives compile time error. 
Nullable<string> _stringVar;
Nullable<object> _objVar;

While compiling code compiler gives following error message:

The type 'string'/'object' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

I read it several times but still unable to understand. Can anyone clarify this? Why object or string dont support nullable reference type?

Answer 1

object and string are reference types, so they're already nullable. For example, this is already valid:

string x = null;

The Nullable<T> generic type is only for cases where T is a non-nullable value type.

In the declaration for Nullable<T> there is a constraint on T:

public struct Nullable<T> where T : struct

That where T : struct is precisely the part that constrains T to be a non-nullable value type.

Answer 2

Nullable<T> is defined as:

public struct Nullable<T> where T : struct

meaning: it only works on value-type T (excluding Nullable<TSomethingElse> itself).

You cannot use Nullable<T> on reference-types (or on Nullable<T>), but you don't need to since all reference-types (including object and string) are already "nullable", in that you can assign null to them.

Answer 3

string and object are reference types, and therefore are "nullable" already. The Nullable<T> type exists as a wrapper around value types that don't support null out of the box.

string myString = null //fine
int myInt = null //compiler error