Question

当我尝试使用以下代码扩展IDictionary时

type Generic.IDictionary<'TKey, 'TValue> with
    member this.GetOrAdd (key:'TKey, fun':'TKey -> 'TValue) =
        match this.ContainsKey(key) with
        | true -> this.Item(key)
        | false -> 
            let val_ = fun' key
            this.Add(key, val_)
            val_

let dd = dict[(1,2); (3,4)]
let f = fun x -> 2*x
let d5 = dd.GetOrAdd(5, f)

我在运行时遇到以下错误。

System.NotSupportedException：类型＆gt;＆＃39; System.NotSupportedException＆＃39;的异常被扔了。在Microsoft.FSharp.Core.ExtraTopLevelOperators.dictValueType@98.System-> Collections-Generic-IDictionary 2-Add(TKey key, T value) at FSI_0011.IDictionary 2.GetOrAdd [TKey，TValue]（IDictionary 2 this, >TKey key, FSharpFunc 2 fun＆＃39;）＆gt ; D：\ BaiduYunDownload \ DroiEtl \ Droi.MyToPG \ Util.Sync.fs：第259行 at。$ FSI_0018.main @（）in＆gt; D：\ BaiduYunDownload \ DroiEtl \ Droi.MyToPG \ Util.Sync.fs：第264行因错误而停止

但是编译器在构建时并没有抱怨...... 请帮帮我......

Answer 1

dict is documented返回只读 IDictionary<_,_> - 然后在其上调用.Add，并且支持类正确地抛出异常。

创建一个真实的Dictionary实例，您将看到它按预期工作：

open System.Collections.Generic

type IDictionary<'TKey, 'TValue> with
    member this.GetOrAdd (key:'TKey, fun':'TKey -> 'TValue) =
        match this.ContainsKey key with
          | true  -> this.Item key
          | false -> let val' = fun' key
                     this.Add (key, val')
                     val'

let dd =
    let d = Dictionary()
    d.Add (1, 2)
    d.Add (3, 4)
    d

printfn "%A" dd
dd.GetOrAdd (5, (fun x -> 2 * x)) |> printfn "%A :: %d" dd
dd.GetOrAdd (5, (fun x -> 9 * x)) |> printfn "%A :: %d" dd

输出：

seq [[1, 2]; [3, 4]]
seq [[1, 2]; [3, 4]; [5, 10]] :: 10
seq [[1, 2]; [3, 4]; [5, 10]] :: 10

Online Demo

实施方面，@ JoelMueller的建议是一个明显的改进：

type IDictionary<'TKey, 'TValue> with
    member this.GetOrAdd (key:'TKey, fun':'TKey -> 'TValue) =
        match this.TryGetValue key with
          | true, val' -> val'
          | false, _   -> let val' = fun' key
                          this.Add (key, val')
                          val'

F＃Generic IDictionary扩展失败

1 个答案: