F#中泛型类型的运行时强制 [英] Runtime coercion of generic types in F#

问题描述

我在F#中有一个带有显式泛型参数的基类.我正在尝试检查我正在使用的给定类型是否实现了特定的接口.我以为"if ob:?ISysAware then"可以，但是抱怨总是一样的:

I have a base class with explicit generic arguments in F#. I'm trying to check if the given type I'm using implements a specific interface. I thought "if ob :? ISysAware then" would do, but the complain is always the same:

let (|SysAware|_|) t = 
    match t with
    | :? ISysAware as p -> Some(p)
    | _ -> None

错误FS0008:从类型'a到ISysAware的此运行时强制或类型测试涉及一个不确定的类型，该类型基于此程序点之前的信息.在某些类型上不允许运行时类型测试.需要进一步的类型注释.

error FS0008: This runtime coercion or type test from type 'a to ISysAware involves an indeterminate type based on information prior to this program point. Runtime type tests are not allowed on some types. Further type annotations are needed.

显然，我不想在这里使用反射. IsAssignableFrom可以解决这个问题，而且成本很高.

I'd prefer not to use reflection here, obviously. IsAssignableFrom would do the trick, at a high cost.

有想法吗?

推荐答案

某些F#类型是值类型；添加box可确保仅对引用类型执行类型测试:

Some F# types are value types; adding box ensures that type test is performed on reference types only:

let (|SysAware|_|) t = 
    match box t with
    | :? ISysAware as p -> Some(p)
    | _ -> None

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