丹尼尔已经解释如何定义你写的F＃函数的C＃ - 友好的版本，所以我会添加一些更高层次的意见。首先，你应该阅读 F＃组件设计准则（引用已经gradbot）。这是一个文件，说明如何使用F＃来设计F＃和.NET库，它应该回答你的很多问题。

在使用F＃中，有基本上有两种;库，你可以这样写：

• F＃库以使用设计的唯一从F＃，所以它的公共接口是用一个函数式风格（使用F＃函数类型，元组，歧视工会等）




• 。 NET库的设计是从的​​任何的.NET语言（包括C＃和F＃），它通常遵循.NET的面向对象式的使用。这意味着你暴露的大部分功能与方法的类（有时扩展方法或静态方法，但大多代码应该于二OO设计写）。

在你的问题，你问如何公开函数组合作为.NET库，但我觉得喜欢你的撰写$函数C $ C>从的角度.NET库点太低层次的概念。你可以将它们公开为与函数功能和动作工作方法，但是这可能不是你会如何设计一个正常摆在首位.NET库（也许你会使用Builder模式，而不是或类似的东西）。

在某些情况下（即设计数字库，别当真正的.NET库风格合身），它使一个良好的意识，设计出两种混合图书馆F＃和 .NET 样式单个库中。要做到这一点的***办法是有正常的F＃（或正常.NET）API，然后在其他样式自然​​的使用提供包装。包装器可以在不同的命名空间（如 MyLibrary.FSharp 和在MyLibrary ）。

在你的榜样，你可以离开 MyLibrary.FSharp F＃的实现，然后添加.NET（C＃ - 型）包装（在在MyLibrary 命名空间的某个类的静态方法类似的代码，丹尼尔帐）。但同样，.NET库可能会比函数组合更具体的API。

I am trying to design a library in F#. The library should be friendly for use from both F# and C#.

And this is where I'm stuck a little bit. I can make it F# friendly, or I can make it C# friendly, but the problem is how to make it friendly for both.

Here is an example. Imagine I have the following function in F#:

let compose (f: 'T -> 'TResult) (a : 'TResult -> unit) = f >> a

This is perfectly usable from F#:

let useComposeInFsharp() =
    let composite = compose (fun item -> item.ToString) (fun item -> printfn "%A" item)
    composite "foo"
    composite "bar"

In C#, the compose function has the following signature:

FSharpFunc<T, Unit> compose<T, TResult>(FSharpFunc<T, TResult> f, FSharpFunc<TResult, Unit> a);

But of course I don't want FSharpFunc in the signature, what I want is Func and Action instead, like this:

Action<T> compose2<T, TResult>(Func<T, TResult> f, Action<TResult> a);

To achieve this, I can create compose2 function like this:

let compose2 (f: Func<'T, 'TResult>) (a : Action<'TResult> ) = 
    new Action<'T>(f.Invoke >> a.Invoke)

Now this is perfectly usable in C#:

void UseCompose2FromCs()
{
    compose2((string s) => s.ToUpper(), Console.WriteLine);
}

But now we have problem using compose2 from F#! Now I have to wrap all standard F# funs into Func and Action, like this:

let useCompose2InFsharp() =
    let f = Func<_,_>(fun item -> item.ToString())
    let a = Action<_>(fun item -> printfn "%A" item)
    let composite2 = compose2 f a

    composite2.Invoke "foo"
    composite2.Invoke "bar"

The question: How can we achieve first-class experience for the library written in F# for both F# and C# users?

So far, I couldn't come up with anything better than these two approaches:

1. Two separate assemblies: one targeted to F# users, and the second to C# users.
2. One assembly but different namespaces: one for F# users, and the second for C# users.

For the first approach, I would do something like this:

1. Create F# project, call it FooBarFs and compile it into FooBarFs.dll.

   • Target the library purely to F# users.
   • Hide everything unnecessary from the .fsi files.

2. Create another F# project, call if FooBarCs and compile it into FooFar.dll

   • Reuse the first F# project at the source level.
   • Create .fsi file which hides everything from that project.
   • Create .fsi file which exposes the library in C# way, using C# idioms for name, namespaces etc.
   • Create wrappers that delegate to the core library, doing the conversion where necessary.

I think the second approach with the namespaces can be confusing to the users, but then you have one assembly.

The question: none of these are ideal, perhaps I am missing some kind of compiler flag/switch/attribte or some kind of trick and there is a better way of doing this?

The question: has anyone else tried to achieve something similar and if so how did you do it?

EDIT: to clarify, the question is not only about functions and delegates but the overall experience of a C# user with an F# library. This includes namespaces, naming conventions, idioms and suchlike that are native to C#. Basically, a C# user shouldn't be able to detect that the library was authored in F#. And vice versa, an F# user should feel like dealing with a C# library.

---

EDIT 2:

I can see from the answers and comments so far that my question lacks the necessary depth, perhaps mostly due to use of only one example where interoperability issues between F# and C# arise, the issue of functions a values. I think this is the most obvious example and so this led me to use it to ask the question, but by the same token gave the impression that this is the only issue I am concerned with.

Let me provide more concrete examples. I have read through the most excellent F# Component Design Guidelines document (many thanks @gradbot for this!). The guidelines in the document, if used, do address some of the issues but not all.

The document is split it two main parts: 1) guidelines for targeting F# users; and 2) guidelines for targeting C# users. Nowhere does it even attempt to pretend that it is possible to have a uniform approach, which exactly echoes my question: we can target F#, we can target C#, but what is the practical solution for targeting both?

To remind, the goal is to have a library authored in F#, and which can be used idiomatically from both F# and C# languages.

The keyword here is idiomatic. The issue is not the general interoperability where it is just possible to use libraries in different languages.

Now to the examples, which I take straight from F# Component Design Guidelines.

1. Modules+functions (F#) vs Namespaces+Types+functions

   • F#: Do use namespaces or modules to contain your types and modules. The idiomatic use is to place functions in modules, e.g.:

     // library
     module Foo
     let bar() = ...
     let zoo() = ...
     
     
     // Use from F#
     open Foo
     bar()
     zoo()
     

   • C#: Do use namespaces, types and members as the primary organizational structure for your components (as opposed to modules), for vanilla .NET APIs.

     This is incompatible with the F# guideline, and the example would need to be re-written to fit the C# users:

     [<AbstractClass; Sealed>]
     type Foo =
         static member bar() = ...
         static member zoo() = ...
     

     By doing so though, we break the idiomatic use from F# because we can no longer use bar and zoo without prefixing it with Foo.

2. Use of tuples

   • F#: Do use tuples when appropriate for return values.

   • C#: Avoid using tuples as return values in vanilla .NET APIs.

3. Async

   • F#: Do use Async for async programming at F# API boundaries.

   • C#: Do expose asynchronous operations using either the .NET asynchronous programming model (BeginFoo, EndFoo), or as methods returning .NET tasks (Task), rather than as F# Async objects.

4. Use of Option

   • F#: Consider using option values for return types instead of raising exceptions (for F#-facing code).

   • Consider use the TryGetValue pattern instead of returning F# option values (option) in vanilla .NET APIs, and prefer method overloading to taking F# option values as arguments.

5. Discriminated unions

   • F#: Do use discriminated unions as an alternative to class hierarchies for creating tree-structured data

   • C#: no specific guidelines for this, but the concept of discriminated unions is foreign to C#

6. Curried functions

   • F#: curried functions are idiomatic for F#

   • C#: Do not use currying of parameters in vanilla .NET APIs.

7. Checking for null values

   • F#: this is not idiomatic for F#

   • C#: Consider checking for null values on vanilla .NET API boundaries.

8. Use of F# types list, map, set, etc

   • F#: it is idiomatic to use these in F#

   • C#: Consider using the .NET collection interface types IEnumerable and IDictionary for parameters and return values in vanilla .NET APIs. (i.e. do not use F# list, map, set)

9. Function types (the obvious one)

   • F#: use of F# functions as values is idiomatic for F#, obiously

   • C#: Do use .NET delegate types in preference to F# function types in vanilla .NET APIs.

I think these should be sufficient to demonstrate the nature of my question.

Incidentally, the guidelines also have partial answer:

... a common implementation strategy when developing higher-order methods for vanilla .NET libraries is to author all the implementation using F# function types, and then create the public API using delegates as a thin façade atop the actual F# implementation.

To summarise.

There is one definite answer: there are no compiler tricks that I missed.

As per the guidelines doc, it seems that authoring for F# first and then creating a facade wrapper for .NET is the reasonable strategy.

The question then remains regarding the practical implementation of this:

• Separate assemblies? or

• Different namespaces?

If my interpretation is correct, Tomas suggests that using separate namespaces should be sufficient, and should be an acceptable solution.

I think I will agree with that given that the choice of namespaces is such that it does not surprise or confuse the .NET/C# users, which means that the namespace for them should probably look like it is the primary namespace for them. The F# users will have to take the burden of choosing F#-specific namespace. For example:

• FSharp.Foo.Bar -> namespace for F# facing library

• Foo.Bar -> namespace for .NET wrapper, idiomatic for C#

Daniel already explained how to define a C#-friendly version of the F# function that you wrote, so I'll add some higher-level comments. First of all, you should read the F# Component Design Guidelines (referenced already by gradbot). This is a document that explains how to design F# and .NET libraries using F# and it should answer many of your questions.

When using F#, there are basically two kinds of libraries you can write:

• F# library is designed to be used only from F#, so it's public interface is written in a functional style (using F# function types, tuples, discriminated unions etc.)

• .NET library is designed to be used from any .NET language (including C# and F#) and it typically follows .NET object-oriented style. This means that you'll expose most of the functionality as classes with method (and sometimes extension methods or static methods, but mostly the code should be written in the OO design).

In your question, you're asking how to expose function composition as a .NET library, but I think that functions like your compose are too low level concepts from the .NET library point of view. You can expose them as methods working with Func and Action, but that probably isn't how you would design a normal .NET library in the first place (perhaps you'd use the Builder pattern instead or something like that).

In some cases (i.e. when designing numerical libraries that do not really fit well with the .NET library style), it makes a good sense to design a library that mixes both F# and .NET styles in a single library. The best way to do this is to have normal F# (or normal .NET) API and then provide wrappers for natural use in the other style. The wrappers can be in a separate namespace (like MyLibrary.FSharp and MyLibrary).

In your example, you could leave the F# implementation in MyLibrary.FSharp and then add .NET (C#-friendly) wrappers (similar to code that Daniel posted) in the MyLibrary namespace as static method of some class. But again, .NET library would probably have more specific API than function composition.