.net - F#方法返回null而不是Option

Question

我在F#上开发.net 4.6.1应用程序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，我收到的是 null而不是 option。

我做错了什么？

Answer 1

在F#中，可以在运行时用null常数表示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。有时出于优化目的是很好的:我不是每次都分配实例，而是使用一个常量。如果该值使用率很高，则有帮助。

因此， Option类型在 None情况下使用了相同的技术，这就是为什么 None在调试器中显示为 null的原因。

有一天，调试器将具有适当的扩展点，以实现此功能和其他F#功能。在此之前，调试器会讲C#，然后您就可以进行翻译了。