F# 为什么会有空名单

我正在编写一个读取xml文件并添加到列表中的应用程序。代码如下所示：

open System.Net
open System.Collections.Generic
open System.Xml.Linq
open System

type IpRanges = {ipStart: IPAddress; ipEnd: IPAddress; subnet: IPAddress; mask: IPAddress}

type System.String with
    member s1.isEqual(s2: string) =
        System.String.Equals(s1, s2, System.StringComparison.CurrentCultureIgnoreCase)


let xname name = XName.Get name
let xattr (elem: XElement) (name:string) = elem.Attribute(xname name).Value

let loc (filename:string) (location:string) = 
    query {
        for doc in XDocument.Load(filename).Descendants(xname "location") do
        where ((xattr doc "name").isEqual(location))
        select doc
    }

let extractAddr (doc:seq<XElement>) = 
    let mutable startAddr:IPAddress = null
    let mutable endAddr:IPAddress = null
    let mutable subnet:IPAddress = null

    let list:List<IpRanges> = new List<IpRanges>()

    //let mutable mask = "0.0.0.0"

    let parseIP (addr:string) = IPAddress.Parse addr 

    for e in doc do
        match e.Name.LocalName with
         | "start" -> startAddr <- parseIP e.Value
         | "end" -> endAddr <- parseIP e.Value
         | "subnet" -> subnet <- parseIP e.Value
         | "mask" -> list.Add({ipStart = startAddr; ipEnd = endAddr; subnet = subnet; mask = parseIP(e.Value)}) |> ignore
         | _ -> ()

    // Return list
    list


[<EntryPoint>]
let main argv = 

    let location = (loc ("C:\Temp\file.xml") "location").Descendants(xname "range").Descendants()
                    |> extractAddr

    //let list = extractAddr location

    Console.ReadLine() |> ignore
    0 // return an integer exit code

不会添加到列表中。 Xml结构

192.20.10.1
192.20.10.255
255.255.255.0
192.20.16.1
附属的
192.20.12.1
192.20.12.255
255.255.255.0
192.20.16.1

XML中没有“mask”元素，因此此模式永远不会匹配。 另外，我会将您的

extractAddr

函数更改为类似这样的“XML结构证明”（如果没有名为

elmname

的元素，则不会引发异常）：

让parseRangeElement（范围：XElement）elmname=
设e=range.Element（xname-elmname）
如果e为空，则
将IPAddress.TryParse（e.Value）与匹配
|正确，ip->ip
|->null
否则无效
let extractAddr（文件编号：seq）=
医生
|>Seq.map（有趣的rngelm->
{
ipStart=parseRangeElement rngelm“开始”
ipEnd=parseRangeElement rngelm“结束”
子网=parseRangeElement rngelm“子网”
掩码=parseRangeElement rngelm“网关”
})
|>List.ofSeq//如果您确实需要列表
[]
让主argv=
let location=（loc（xml）“test”）.子体（xname“range”）
|>提取器地址
Console.ReadLine（）|>忽略
0//返回整数退出代码

您能给出一个正在使用的xml文件示例吗？也许只是片段XML中没有“mask”元素

let list:List<IpRanges> = new List<IpRanges>()

| "mask" -> list.Add({ipStart = startAddr; ipEnd = endAddr; subnet = subnet; mask = parseIP(e.Value)}) |> ignore

let parseRangeElement (range: XElement) elmname = 
    let e = range.Element(xname elmname)
    if  e <> null then 
        match IPAddress.TryParse(e.Value) with
        | true, ip -> ip
        | _ -> null
    else null

let extractAddr (doc:seq<XElement>) =
    doc
    |> Seq.map (fun rngelm -> 
                    {
                    ipStart = parseRangeElement rngelm "start"
                    ipEnd = parseRangeElement rngelm "end" 
                    subnet = parseRangeElement rngelm "subnet" 
                    mask = parseRangeElement rngelm "gateway"
                    })
    |> List.ofSeq // if you really need list

[<EntryPoint>]
let main argv = 

    let location = (loc (xml) "test").Descendants(xname "range") 
                    |> extractAddr

    Console.ReadLine() |> ignore
    0 // return an integer exit code