Properly implement F# Unit in C#

Question

This question is not about C#/F# compatibility as in this one.

I'd like to know the proper way to implement a type like F# Unit in place of using void.

Obviously I'll discard this result and I'll not expose this type to the outside world.

Wondering if an empty class could impersonate this role.

internal class Unit
{
}

For example in language-ext library, the author used a struct.

Is there any benefit in this choice?

Answer 1

I'm not sure what is the best way to define Unit for usage from C#. It might differ from how this is done in F# (because in F#, the compiler hides the usage in a way).

However, you can actually find the implementation of F# unit in the core library:

• The public interface is defined in prim-types.fsi
• The implementation is provided in prim-types.fs

Here are the key points about the F# unit implementation

• It implements GetHashCode and Equals in the same way to the Rx version
• It is IComparable and all values of the unit type are equal
• Most importantly, it has a private constructor and so you cannot create a new unit value. It also has no default instance (unlike the Rx unit) and so in F#, all unit values are actually represented as null. However, the language/compiler generally hide this fact.

So, it sounds like the only difference in F# is that it uses the null value. If you want to use unit explicitly, this might not be the best choice. However, if you have Unit.Default then you are ultimately defining a type with two possible values, because it can be either Unit.Default or null (and so it is not really a unit!)