Are double and double? interchangeable?

Question

As above - Does anyone know whether a double is implicitly cast to double? (Nullable type)

EDIT: What exactly is going on here?

double d = 5;

double? d2 = d as double?

Answer 1

They're not interchangeable as per your title.

There is an implicit conversion from double to double?.

There is an explicit conversion from double? to double.

The same is true for all nullable value types: there's an implicit conversion from T to Nullable<T>, and an explicit one from Nullable<T> to T.

Interestingly, although Nullable<T> does provide those conversions as user-defined conversions in the normal way, the MS C# compiler doesn't use those - it calls the Nullable<T>(T value) constructor for the implicit conversion, and uses the Value property directly for the explicit conversion the other way round.