Is Scala Option the same as a C# Nullable type?

Question

In Scala you have Option[Int] which will be Some(number) or None and has a property isDefined which returns true in the first case and false in the other.

In C# you have int? which will be a number or null and has a property HasValue which returns true in the first case and false in the other.

Is Scala's Option the same as a C#'s Nullable type and are there any differences?

Answer 1

No, it isn't at all the same. In addition to other answers:

1. Scala has Option[Option[Int]]; C# doesn't have int??.

2. Scala has Option[String]; C# doesn't have string?.

3. In C#, there is an implicit conversion from T to T?; in Scala, there is no implicit conversion from T to Option[T] (Scala allows you to define such a conversion manually, but this isn't recommended).

This means Option is a lot more uniform and composable than ? and allows it to be used much more widely. E.g. Scala's Map[K, V].get(K) returns Option[V]; but in .Net, Dictionary<TKey, TValue>[TKey] can't return TValue?, because TValue can be a reference type. Even an equivalent of Option.map would need to have separate implementations for functions returning value types and reference types.

I'd also like to reiterate that get should be avoided! isDefined when not followed by get is perfectly fine, though.

Answer 2

C#'s Nullable type is meant to allow value types to have null values while Scala's Option is meant to get rid of null entirely. Instead, you pattern match on Option[A] to get a Some or a None. You can think of Option as a generalized version of Nullable type.

However, the problems dealt with aren't exactly the same: C# is trying to add object-like behavior to value types while Scala is trying to provide a safe alternative to null. However, Scala's Option does also sort of solve the value type problem Nullable type was designed for:

scala> case class MyInt(val i: Int) extends AnyVal // scala value type

scala> null : MyInt
<console>:14: error: type mismatch;
  found   : Null(null)
  required: MyInt
        null : MyInt
        ^

scala> None : Option[MyInt]
res1: Option[MyInt] = None

scala> Some(MyInt(1)) : Option[MyInt]
res2: Option[MyInt] = Some(MyInt(1)) 

I should also mention that Nullable type is built into the C# language, while Option is just a regular Scala class in the standard library.