Question

让我们看一下下面这段代码，并假设MyAttribute和test函数都无法更改。

type MyAttribute() =
    inherit Attribute()

    let mutable prop = null

    member this.Prop
        with get(): obj = prop
        and  set(value) = prop <- value


type MyEnum = 
    | A = 1
    | B = 2

[<My(Prop = MyEnum.B)>]
type MyClass = class
    end

let test () =
   let t = typeof<MyClass>
   let a = t.GetCustomAttributes(false).[0] :?> MyAttribute

   let e = a.Prop
   Convert.ToString(e, Globalization.CultureInfo.CurrentCulture)

我希望test返回B但返回2.生成的IL代码显示有关枚举类型的信息丢失，传递给属性的值仅为2.

是否有任何方法（我想应该是某些属性）来保留属性值中的类型？什么更有趣的C＃等效代码按预期工作

等价C＃：

class MyAttribute : Attribute
{
    public object A { get; set; }
}

enum T { A,B,C }

[My(A = T.A)]
class MyClass
{ }

var a = typeof(MyClass).GetCustomAttributes(false)[0] as MyAttribute;

Convert.ToString(a.A, System.Globalization.CultureInfo.CurrentCulture)

Answer 1

我猜测，我对编译器内部的知识实际上是安静的，但我想这与类型推断有关。 int＆lt; - ＆gt;之间存在等价关系。枚举，我怀疑类型推断是将其减少到可能的最低类型，在本例中为int。您可以通过执行以下操作来解决此问题

open System

type MyAttribute() =
    inherit Attribute()

    let mutable prop = null

    member this.Prop
        with get(): obj = prop
        and  set(value) = prop <- value


type MyEnum = 
    | A = 1
    | B = 2

[<My(Prop = MyEnum.B)>]
type MyClass = class
    end

let test () =
   let t = typeof<MyClass>
   let a = t.GetCustomAttributes(false).[0] :?> MyAttribute

   let e = a.Prop :?> MyEnum //Note
   Convert.ToString(e, Globalization.CultureInfo.CurrentCulture)

如何在枚举中保留值类型

1 个答案: