[英]F# Generic Map.count using Reflection
This is a follow-up on this previous question , but with a different twist.这是对上一个问题的跟进,但有不同的转折。
I would like to write a function which, given an object oMap, returns its count if oMap happens to be of type Map<'k,'v>
, and -1 otherwise.我想编写一个函数,给定一个对象 oMap,如果 oMap 恰好是
Map<'k,'v>
类型,则返回其计数,否则返回 -1。 My constraint : oMap type can only be 'discovered' at runtime.我的约束:oMap 类型只能在运行时“发现”。
As apparently "there is no built-in way to pattern match on a generic Map."显然“没有内置的方法可以在通用 Map 上进行模式匹配。” (see link to previous question), I am using reflection for this.
(请参阅上一个问题的链接),我为此使用了反射。
namespace genericDco
module Test1 =
let gencount (oMap : obj) : int =
let otype = oMap.GetType()
let otypenm = otype.Name
if otypenm = "FSharpMap`2" then
// should work, as oMap of type Map<'a,'b>, but does not. *How to fix this?*
Map.count (unbox<Map<_,_>> oMap)
else
// fails, as oMap is not of any type Map<'a,'b>.
-1
let testfailObj : int = gencount ("foo")
// FAILS
let testsuccessObj : int =
let oMap = [| ("k1", "v1"); ("k1", "v1") |] |> Map.ofArray
gencount (box oMap)
The error being :错误是:
System.InvalidCastException: Unable to cast object of type 'Microsoft.FSharp.Collections.FSharpMap`2[System.String,System.String]' to type 'Microsoft.FSharp.Collections.FSharpMap`2[System.IComparable,System.Object]'. at Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicFunctions.UnboxGeneric[T](Object source)
My question: How should I rewrite the above to get this to work?我的问题:我应该如何重写上述内容以使其正常工作?
PS : I am not looking for solutions where we know at compile time that oMap is of type Map<'k,'v>
, eg : PS:我不是在寻找我们在编译时知道 oMap 是
Map<'k,'v>
类型的解决方案,例如:
module Test2 =
let gencount2<'k,'v when 'k : comparison> (gMap : Map<'k,'v>) : int =
Map.count gMap
let testsuccessStr : int =
let gMap = [| ("k1", "v1"); ("k2", "v2") |] |> Map.ofArray
gencount2<string,string> gMap
let testsuccessDbl : int =
let gMap = [| ("k1", 1.0); ("k2", 2.0); ("k3", 3.0) |] |> Map.ofArray
gencount2<string,double> gMap
== EDIT == == 编辑 ==
Thanks to Asti's suggestion, that's the solution that worked for me :感谢 Asti 的建议,这是对我有用的解决方案:
let gencount (oMap : obj) : int =
let otype = oMap.GetType()
let propt = otype.GetProperty("Count")
try
propt.GetValue(oMap) :?> int
with
| _ -> -1
Since Map.count
is just defined as let count m = m.Count
, we can just go for the Count
property.由于
Map.count
刚刚定义为let count m = m.Count
,我们可以直接使用Count
属性。
let gencount<'k,'v when 'k : comparison> map =
let mtype = typeof<Map<'k, 'v>>
let propt = mtype.GetProperty("Count")
if map.GetType() = mtype then
propt.GetValue(map) :?> int
else
-1
Test:测试:
[<EntryPoint>]
let main argv =
let m = Map.ofSeq [ ("a", 1); ("b", 2)]
printfn "%d" (gencount<string, int> m)
printfn "%d" (gencount<string, string> m)
Console.ReadKey() |> ignore
0 // return exit code 0
Using _
in place of a type will simply end up as object if no additional constraint information is available.如果没有额外的约束信息可用,使用
_
代替类型将简单地作为对象结束。 You use unbox
when you strongly know what type your value is, except that the value is boxed in.当您非常清楚您的值是什么类型时,您可以使用
unbox
装箱,除非该值被装箱。
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