Question

我正在学习F＃并希望实现ThreadStatic单例。我正在使用我在类似问题中找到的内容：F# How to implement Singleton Pattern (syntax)

使用以下代码编译器抱怨The type 'MySingleton' does not have 'null' as a proper value。

type MySingleton = 
    private new () = {}
    [<ThreadStatic>] [<DefaultValue>] static val mutable private instance:MySingleton
    static member Instance =
        match MySingleton.instance with
        | null -> MySingleton.instance <- new MySingleton()
        | _ -> ()
        MySingleton.instance

如何在这种情况下初始化实例？

Answer 1

我认为[<ThreadStatic>]导致相当笨重的代码，特别是在F＃中。有更简洁的方法，例如，使用ThreadLocal：

open System.Threading

type MySingleton private () = 
  static let instance = new ThreadLocal<_>(fun () -> MySingleton())
  static member Instance = instance.Value

Answer 2

另一个F＃y解决方案是将实例存储为option

type MySingleton = 
    private new () = {}

    [<ThreadStatic>; <DefaultValue>]
    static val mutable private instance:Option<MySingleton>

    static member Instance =
        match MySingleton.instance with
        | None -> MySingleton.instance <- Some(new MySingleton())
        | _ -> ()

        MySingleton.instance.Value

Answer 3

接近Ramon所说的，将AllowNullLiteral属性应用于该类型（默认情况下，在F＃中声明的类型不允许'null'作为正确的值）：

[<AllowNullLiteral>]
type MySingleton = 
    private new () = {}
    [<ThreadStatic>] [<DefaultValue>] static val mutable private instance:MySingleton
    static member Instance =
        match MySingleton.instance with
        | null -> MySingleton.instance <- new MySingleton()
        | _ -> ()
        MySingleton.instance

在F＃中实现ThreadStatic单例

3 个答案: