F#可变列表为null [英] F# mutable list is null

问题描述

当我尝试运行以下代码时，属性为null.为什么会这样??我为属性分配了一个空列表，但是第一次遍历循环时，它为null.

When I try to run the code below, properties is null. Why is that? I assign an empty list to properties, but the first time through the loop, it is null.

这将导致它删除我附加到它的第一个值.我也不明白这一点.似乎一个与null串联的值应该是该值的列表.

This causes it to drop the first value that I append to it. I do not understand this either. It seems that a value concatenated with a null should be list of the value.

[<TechTalk.SpecFlow.Binding>]
module FSharpLeftistHeapStepDefinition

open TechTalk.SpecFlow

let mutable properties = [0]
let [<Given>] ``I have a list with the following numbers``(values:Table) = 
    let rec insertItems i =
        if i < 0 then ()
        else 
            let value = int ((values.Rows.Item i).Item 0)            
            properties <- value::properties
            insertItems (i-1)

    insertItems (values.RowCount - 1)

这很奇怪.

基于Tarmil答案的解决方案

The solution based on Tarmil's answer

 [<Binding>]
 type Example ()= 
     let mutable properties = []
     let [<Given>] ``I have a list with the following numbers``(values:Table) = 
         let rec insertItems i =
             if i < 0 then ()
             else 
                 let value = int ((values.Rows.Item i).Item 0)            
                 properties <- value::properties
                 insertItems (i-1)

         insertItems (values.RowCount - 1)

推荐答案

我假定此[<Given>]属性来自测试框架，这使我怀疑该程序集是否以非常规方式加载了程序集.初始值[0]由模块的静态构造函数设置，因此如果该构造函数未由测试框架运行，则可以解释为什么它保持等于null的原因.

I assume this [<Given>] attribute comes from a testing framework, which makes me suspect that the assembly is loaded in a non-conventional way by it. The initial value [0] is set by the module's static constructor, so if this constructor is not run by the testing framework, then that would explain why it stays equal to null.

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