Nawaz Nawaz -4 years ago 155
C# Question

Why can't I write Nullable<Nullable<int>>?

The definition of


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

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

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

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,
is a value-type, and therefore, it can be type argument to

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 Source

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 (ยง10.1.5), 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.

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