F＃方法返回null而不是Option

Question

我在F#上开发了.net 4.6.1申请VS2015。我有方法：

type CommonHelper = 
    static member SideEffectOnNull act x = if x = null then act(); x else x
    static member SideEffectOnNotNull act x = if x <> null then act(); x else x

...

    static member GetPerformanceCounter ( fname: CounterFullName ) = 

        let getCounterInternal ( counterFullName: CounterFullName ) =
            Log.Information("Getting counter: {category}\\{name}\\{instance} ",  counterFullName.Category, counterFullName.Name, counterFullName.Instance)
            let receivedCategory = PerformanceCounterCategory.GetCategories().FirstOrDefault( fun x -> String.Equals( x.CategoryName, counterFullName.Category.Category, StringComparison.InvariantCultureIgnoreCase ) )
            if receivedCategory = null  then
                Serilog.Log.Warning ( "Category not found: {category}", counterFullName.Category ); null
            else
                let receivedCounters = PerforrmanceCounterProxy.GetPerformanceCountersFromCategoryOrNull counterFullName.Instance receivedCategory
                if receivedCounters = null then 
                    Log.Warning ("Instance not found {name}(instance: {instance}) in category {category}", counterFullName.Name, counterFullName.Instance, counterFullName.Category); null
                else
                    receivedCounters.FirstOrDefault( fun y -> String.Equals( y.CounterName, counterFullName.Name.Name, StringComparison.InvariantCultureIgnoreCase ) ) 
                    |> CommonHelper.SideEffectOnNull ( fun unit -> Log.Warning ("Name {name}(instance: {instance}) not found for category {category}", counterFullName.Name, counterFullName.Instance, counterFullName.Category) )

        getCounterInternal fname
        |> CommonHelper.SideEffectOnNull (fun unit ->Log.Warning( "Getting counter failed: {category}\\{name}\\{instance}", fname.Category, fname.Name, fname.Instance )) 
        |> CommonHelper.SideEffectOnNotNull (fun unit ->Log.Information( "Getting Counter secceed: {category}\\{name}\\{instance}", fname.Category, fname.Name, fname.Instance ))
        |> (fun x -> if x = null then None else Option.Some(x)) 

但是当我调用此方法时，我会收到null而不是option。  我做错了什么？

Answer 1

在F＃中，可以在运行时表示具有null常量的DU的一个无数据值。您可以使用CompilationRepresentationFlags.UseNullAsTrueValue指示编译器执行此操作：

[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
type A = B | C of int

printfn "%A" (obj.ReferenceEquals( B, null ))  // will print "true"

在上面的代码中，DU值B被编译为null。这对于优化目的来说有时很好：不是每次都分配一个实例，而是使用常量。如果价值被大量使用，则有助于此。

因此，Option类型对None案例使用了相同的技术，这就是None在调试器中显示为null的原因。

有一天，调试器将有适当的扩展点来实现这个和其他F＃功能。在那之前，调试器会说C＃，你可以进行翻译。