f# - 'indeterminate type' 在哪里？

Question

我正在定义以下类型扩展:

type System.Reflection.MemberInfo with
    member x.GetAttribute<'T when 'T :> Attribute>(required, inherit') =
        match required, Attribute.GetCustomAttribute(x, typeof<'T>, inherit') with
        | true, null -> invalidOp (sprintf "Missing required attribute: %s" typeof<'T>.FullName)
        | _, attr -> attr :> 'T

最后一个匹配表达式 (attr :> 'T) 给出错误:

从 Attribute 到 'T 的静态强制转换涉及基于此程序点之前的信息的不确定类型。某些类型不允许静态强制转换。需要进一步的类型注释。

我尝试注释函数返回类型，但得到了相同的结果。我不想将其更改为动态 Actor 阵容。有没有办法让静态 Actor 工作？

Answer 1

这一定是一个动态的 Actor 阵容，对吧？您有一个静态类型为 System.Attribute 的对象，并且您希望将其向下转换为其实际的具体类型。

open System
type System.Reflection.MemberInfo with 
  member x.GetAttribute<'T when 'T :> Attribute>(required, inherit') =  // '
    match required, Attribute.GetCustomAttribute(x, typeof<'T>, inherit') with 
    | true, null -> invalidOp (
        sprintf "Missing required attribute: %s" typeof<'T>.FullName)  // '
    | _, attr -> attr :?> 'T