How to expose the type as friendly as possible to both C# and F#?

Question

For example, if I have written a module in F#

module Lib

type A =
    member this.x1 x = ...

let helpa x = ...
let helpb x = ...

type B =
    member this.y1 x = ...

let helpc x = ...

typeA with
    member this.x2 x = ...
typeB with
    member this.y2 x = ...

It works well in F# by open Lib, However, if I want to consume it in C# (where I am only interested in types and member functions in Lib), each time I create a type I have to new Lib.A(...). It becomes rather annoying there is no way to omit the module names. Calling a static method like Lib.A.C() is even more of a hassle.

Then I try to replace module with namespace, each time I introduce some helper functions I have to create a new module with a new name. Occasionally I can manage to rearrange all helper functions into 1 module, but that would result in less readable code somehow.

What would be a better structure for this?

Wish I had: Using * = Lib.* for C#.

Do you have a `using Lib` directive at the top of your C# class? http://msdn.microsoft.com/en-US/library/sf0df423(v=vs.80).aspx — Robert Harvey, Jan 22 '13 at 16:46
@RobertHarvey I don't think C# can `using` module, or do u mean alias such as `using A = Lib.A`? — colinfang, Jan 22 '13 at 16:47
You can find other helpful advices here http://stackoverflow.com/questions/10110174/best-approach-for-designing-f-libraries-for-use-from-both-f-and-c-sharp — pad, Jan 22 '13 at 17:49

score 7 · Answer 1 · answered Jan 22 '13 at 17:02

7

F# offers more flexibility than C# here, so I would expose it to C# in the standard way, i.e., enclose types in a namespace. Something like this, I think, offers the best of both worlds:

namespace Lib

type A =
    member this.x1 x = ()

[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module A =
  let helpa x = ()
  let helpb x = ()

type B =
    member this.y1 x = ()

[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module B =
  let helpb x = ()

type A with
    member this.x2 x = ()
type B with
    member this.y2 x = ()

The F# collections follow a similar design. You can use the [<AutoOpen>] and [<RequireQualifiedAccess>] attributes to further control how the modules are used from F#.

answered Jan 22 '13 at 17:02

Daniel

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daniel, any guidance on how to expose f# types such as unions, record types, in a friendly way to c# ? – Alex Gordon Mar 14 '18 at 01:08
@ l--''''''---------'''''''''''' I have been doing some interop recently and found that f# unions are actually exposed really nicely if you use the new expression based switch syntax like you would use match in f#. Records are just read only classes so very natural anyway. – Ryan Apr 13 '18 at 11:12
An example of what I mentioned above: switch (result) { case var checkResult when checkResult.IsOk: HandleOk(checkResult.OkValue); break; case var checkResult when checkResult.IsError: HandleError(checkResult.ErrorValue); break; } – Ryan Apr 13 '18 at 11:14
I just found a great article here: http://connelhooley.uk/blog/2017/04/30/f-sharp-to-c-sharp#cases-with-types – Ryan Apr 13 '18 at 11:25
In addition to my original suggestion for result types, if the union cases have content you can just do switch (unionInstance) { case Union.FirstType content: HandleFirstType(content.Item); break; case Union.SecondType content: HandleSecondType(content.Item); break; } – Ryan Apr 13 '18 at 11:29

score 4 · Accepted Answer · answered Jan 22 '13 at 17:02

I think you already mentioned the best option in your answer - define the file with namespace declaration at the top (this way, you can write just using Lib in C#) and then place all helper functions in modules.

Helper functions that are clearly associated with some type (e.g. with A) could be placed into a module named A (similarly to F# functions in the List module that are associated with the List<'T> type).

This is a bit more work, because you need to mark the module with a special attribute (to avoid name clash), but it will be easy to use from both F# and C# (and I think having nice use is more important than saving a few keystrokes when building the library):

namespace Lib

// Declaration of the 'A' type and helper functions in 'A' module 
type A() =
  member this.x1 x = 10

[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module A = 
  let helpa (x:A) = x.x1
  let helpb (x:A) = x.x1

// Declaration of the 'B' type and helper functions in 'B' module 
type B() =
  member this.y1 x = 10

[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module B = 
  let helpc (x:B) = x.y1

// Member augmentations for easy use from C#
type A with
    member this.x2 x = A.helpa this
type B with
    member this.y2 x = B.helpc this

In C#, A.helpa becomes AModule.helpa. I have to add the functions as static members of type A. — MiloDC, Jun 17 '18 at 18:44

How to expose the type as friendly as possible to both C# and F#?

2 Answers2

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