Question

The definition of Nullable<T> is:

[SerializableAttribute]
public struct Nullable<T> where T : struct, new()

The constraint where T : struct implies that T can only be a value type. So I very well understand that I cannot write:

Nullable<string> a; //error. makes sense to me

Because string is a reference type, not a value type. But I don’t really understand why can’t I write

Nullable<Nullable<int>> b; //error. but why?

Why is it not allowed? After all, Nullable<int> is a value-type, and therefore, it can be type argument to Nullablle<T>.

When I compiled it on ideone, it gives this error (ideone):

error CS0453: The type ‘int?’ must be a non-nullable value type in order to use it as type parameter ‘T’ in the generic type or method ‘System.Nullable’
Compilation failed: 1 error(s), 0 warnings

Answer 1

From section 4.1.10 of the C# language spec:

A non-nullable value type conversely is any value type other than System.Nullable<T> and its shorthand T? (for any T), plus any type parameter that is constrained to be a non-nullable value type (that is, any type parameter with a struct constraint). The System.Nullable<T> type specifies the value type constraint for T (§10.1.5), which means that the underlying type of a nullable type can be any non-nullable value type. The underlying type of a nullable type cannot be a nullable type or a reference type. For example, int?? and string? are invalid types.