Using F# option types in C#

This post shows a small example of how to use F# option type instances in C#.

Option types

The F# option type provides a very nice way for defining variables that can may or may not hold a value. For example, in the following class definition, the description field is optional .

type Product(name : string, description : string option) =
  class
  
  member this.Name 
    with get() = name

  member this.Description 
    with get() = description
   
end

F# Optional types are constructed using the Some and None constructors. For example:

   let p1 = Product("Foo",Some "A description")
   let p2 = Product("Goo",None)

Optional types are available in many functional languages, for example: Haskell's Maybe, Scala's Option, OCaml's option, among others.

Using option types in C#

Manipulating a option instance from C# requires some considerations since it is defined in F# as a discriminated union. Some useful documentation on how to manipulate discriminated unions from C# includes: Creating and Consuming F# discriminated unions from F#'s manual and this entry from the hubFS.

An instance of the class defined above can be created in C# like this:

Product p = new Product("Foo", Option<string>.Some("A description"));
Product p2 = new Product("Goo", Option<string>.None);

As shown here the Option<A> class (defined in Microsoft.FSharp.Core) provide static methods to construct both cases of the discriminated union.

Several techniques can be used to verify if an instance of Option<A> was created with Some or None. For example, there are static methods called get_IsNone or get_Some which can be used as follows:

if (Option<string>.get_IsNone(p.Description)) 
{
  ...
}
...
if (Option<string>.get_IsSome(p.Description)) 
{
  ...
}

Another alternative is to use the GetTag method as follows:

if(Option<string>.GetTag(p.Description) == Option<string>.tag_Some) {
   ...
}

Extension methods

C# extension methods can be used to avoid calling static methods on the Option<A> class. For example:

static class Utility 
{
   public static bool IsSome(this Option<string> option) 
   {
      return Option<string>.get_IsSome(option);
   }

   public static bool IsNone(this Option<string> option)
   {
      return Option<string>.get_IsNone(option);
   }
}

Having these extension methods the code could be changed to:

if (p.Description.IsSome())
{
   ...
}

if (p2.Description.IsSome())
{
   ...
}

C# extension methods could also be implemented directly in F# . This could be accomplished by applying the System.Runtime.CompilerServices.ExtensionAttribute attribute. For example:

[<System.Runtime.CompilerServices.Extension>]
module Methods =

   [<System.Runtime.CompilerServices.Extension>]
   let Exists(opt : string option) =
                match opt with
                  | Some _ -> true
                  | None -> false

This method could be used in C# only by adding the namespace (using using) to the file where it will be used.

if (p2.Description.Exists()) 
{
   ...
}