F#和MEF:导出函数 [英] F# and MEF: Exporting Functions

问题描述

因此，我试图在F#控制台应用程序中进行以下简单测试:

So, I was trying to get this simple test working in an F# console app:

open System.Reflection
open System.ComponentModel.Composition
open System.ComponentModel.Composition.Hosting

[<Export(typeof<int -> string>)>]
let toString(i: int) = i.ToString()

[<EntryPoint>]
let main argv = 
    use catalog = new AssemblyCatalog(Assembly.GetEntryAssembly())
    use container = new CompositionContainer(catalog)

    let myFunction = container.GetExportedValue<int -> string>()
    let result = myFunction(5)
    0

我希望MEF能够正确解析该功能，但事实并非如此. 相反，我得到了:

I expected MEF to get the function properly resolved, but it doesn't. Instead, I get this:

类型的未处理异常 'System.ComponentModel.Composition.CompositionContractMismatchException' 发生在System.ComponentModel.Composition.dll

An unhandled exception of type 'System.ComponentModel.Composition.CompositionContractMismatchException' occurred in System.ComponentModel.Composition.dll

其他信息:

Cannot cast the underlying exported value of type 'Program.toString (ContractName="Microsoft.FSharp.Core.FSharpFunc(System.Int32,System.String)")' to type 'Microsoft.FSharp.Core.FSharpFunc``2[System.Int32,System.String]'.

• 我在这里想念什么?
• FSharpFunc(System.Int32, System.String)和FSharpFunc``2[System.Int32, System.String]有什么区别?
• 通过MEF导入/导出F#函数的正确方法是什么?
• What am I missing here?
• What is the difference between FSharpFunc(System.Int32, System.String) and FSharpFunc``2[System.Int32, System.String]?
• What is the correct way to import/export F# functions via MEF?

推荐答案

编译器将顶级F#函数转换为方法，因此您的示例将编译为:

The compiler turns top-level F# functions into methods, so your example will be compiled as:

[Export(FSharpFunc<int,string>)]
public string toString(int i) { return i.ToString(); }

这可能是导致错误的原因.您可以通过调用一些返回函数的操作来强制编译器生成FSharpFunc类型的属性获取器-即使是简单的标识函数也可以:

This is probably causing the error. You can force the compiler to produce a property getter of FSharpFunc type by calling some operation that returns a function - even a simple identity function will don:

let makeFunc f = f 

[<Export(typeof<int -> string>)>]
let toString = makeFunc <| fun (i:int) -> 
  i.ToString()

我还没有测试过，但是我认为它可以工作.也就是说，在这种情况下，使用简单的单方法接口可能更安全.

I have not tested this, but I think it could work. That said, it is probably safer to go with a simple single-method interface in this case.

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