C# foreach 循环外部字符串未正常更新答案

【问题标题】：C# foreach loop outer string not updating as normalC# foreach 循环外部字符串未正常更新
【发布时间】：2015-07-13 22:04:29
【问题描述】：

我知道最好用适当的类编写 Xml 文档，但我时间紧迫，想要快速获得结果。

我尝试将 xml 文件写入字符串，然后将其保存为 xml 文件。结果不正确，因为字符串每次都只获取foreach循环的最后一个值。

我搜了一下问题，发现需要在foreach循环里面初始化字符串，但是没看懂概念，也不知道具体在哪里。

我应该在我的代码中修改什么？

string siteMap = "<? xml version = \"1.0\" encoding = \"UTF-8\" ?>";

List<string> Niveau1Titles = getNiveau1Titles(aux1);
List<List<Couple>> Niveau2Titles = getNiveau2Titles(aux1);

int i = 0;
foreach (var n2 in Niveau2Titles)
{
    siteMap += "<Niveau_1 Title = \"" + Niveau1Titles[i] + "\" >";
    foreach (var n22 in n2)
    {
        siteMap += "<Niveau_2 Title = \"" + n22.Title + "\" Link = \"" + n22.Link + "\" >";
        List<Couple> Niveau3Titles = new List<Couple>();
        Niveau3Titles = getNiveau3Titles(n22.Link);
        foreach(var n3 in Niveau3Titles)
        {
            siteMap += "<Niveau_3 Title = \"" + n3.Title + "\" Link = \"" + n3.Link + "\" />";
        }
        siteMap += "</Niveau_2>";

    }
    siteMap += "</Niveau_1>";
    i++;
}


Console.WriteLine(siteMap);

【问题讨论】：

The result is not correct, because the string is getting only the last value of the foreach loop every time 您的代码中有 3 个foreach，是哪一个导致了问题？
乍一看代码没有任何问题。它应该按预期工作。你能给我们展示一个给你意想不到的结果的例子吗？另外，您是否验证过 getNiveauXTitles 函数是否都为您提供了正确的数据？
我没有看到您的代码在哪里创建了单个根元素。附言我真的不明白，不使用正确的 XML API 怎么能节省你的时间。
内部的（第三个 foreach）正在制造问题。当我使用 Console.WriteLine() 时，结果是正确的。
@DeepBlue，更新您当前和所需的输出？并且还包括所有 getNiveauTitles 函数 + NiveauXTitles 类。我们总是需要所有相关信息。

标签： c# xml foreach

【解决方案1】：

花 5 分钟时间来确保这是使用 LINQ to XML 生成的似乎并不难：

var result =
    new XDocument(
        new XDeclaration("1.0", "UTF-8", "yes"),
        new XElement(
            "sitemap",
            Niveau2Titles.Zip(Niveau1Titles, (n2, n1) =>
                new XElement(
                    "Niveau_1",
                    new XAttribute("Title", n1),
                    n2.Select(n22 =>
                        new XElement(
                            "Niveau_2",
                            new XAttribute("Title", n22.Title),
                            new XAttribute("Link", n22.Link),
                            getNiveau3Titles(n22.Link).Select(n3 =>
                                new XElement(
                                    "Niveau_3",
                                    new XAttribute("Title", n3.Title),
                                    new XAttribute("Link", n3.Link)))))))));

【讨论】：

【解决方案2】：

我认为代码运行正常。您的 xml 中有多个根元素。您需要添加一个根。此外，您在 xml 标识行中有一个额外的空间。试试下面的代码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string aux1 = "";
            //remove extra space
            string siteMap = "<?xml version = \"1.0\" encoding = \"UTF-8\" ?>";
            siteMap += "<Root>";

            List<string> Niveau1Titles = getNiveau1Titles(aux1);
            List<List<Couple>> Niveau2Titles = getNiveau2Titles(aux1);

            int i = 0;
            foreach (var n2 in Niveau2Titles)
            {
                siteMap += "<Niveau_1 Title = \"" + Niveau1Titles[i] + "\" >";
                foreach (var n22 in n2)
                {
                    siteMap += "<Niveau_2 Title = \"" + n22.Title + "\" Link = \"" + n22.Link + "\" >";
                    List<Couple> Niveau3Titles = new List<Couple>();
                    Niveau3Titles = getNiveau3Titles(n22.Link);
                    foreach (var n3 in Niveau3Titles)
                    {
                        siteMap += "<Niveau_3 Title = \"" + n3.Title + "\" Link = \"" + n3.Link + "\" />";
                    }
                    siteMap += "</Niveau_2>";

                }
                siteMap += "</Niveau_1>";
                i++;
            }
            siteMap += "</Root>";


            Console.WriteLine(siteMap);
            Console.ReadLine();
        }
        static List<string> getNiveau1Titles(string aux1)
        {
            return new List<string> { "aaa", "aab", "aac", "aad" };
        }
        static List<List<Couple>> getNiveau2Titles(string aux1)
        {
            return new List<List<Couple>>() { 
                  new List<Couple>() {new Couple() { Title = "T11", Link =  "L11"}, new Couple() { Title = "T12", Link =  "L12"}, new Couple() { Title = "T13", Link =  "L13"} },
                  new List<Couple>() {new Couple() { Title = "T21", Link =  "L21"}, new Couple() { Title = "T22", Link =  "L22"}, new Couple() { Title = "T23", Link =  "L23"} },
                  new List<Couple>() {new Couple() { Title = "T31", Link =  "L31"}, new Couple() { Title = "T32", Link =  "L32"}, new Couple() { Title = "T33", Link =  "L33"} },
                  new List<Couple>() {new Couple() { Title = "T41", Link =  "L41"}, new Couple() { Title = "T42", Link =  "L42"}, new Couple() { Title = "T43", Link =  "L43"} }
            };
        }
        static List<Couple> getNiveau3Titles(string aux1)
        {
            return new List<Couple>() { new Couple() { Title = "T100", Link = "L100" }, new Couple() { Title = "T101", Link = "L101" }, new Couple() { Title = "T102", Link = "L102" } };
        }

    }
    public class Couple
    {
        public string Title { get; set; }
        public string Link { get; set; }
    }

}

这是xml结果

<?xml version = "1.0" encoding = "UTF-8" ?>
<Root>
  <Niveau_1 Title = "aaa" >
    <Niveau_2 Title = "T11" Link = "L11" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T12" Link = "L12" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T13" Link = "L13" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
  </Niveau_1>
  <Niveau_1 Title = "aab" >
    <Niveau_2 Title = "T21" Link = "L21" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T22" Link = "L22" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T23" Link = "L23" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
  </Niveau_1>
  <Niveau_1 Title = "aac" >
    <Niveau_2 Title = "T31" Link = "L31" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T32" Link = "L32" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T33" Link = "L33" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
  </Niveau_1>
  <Niveau_1 Title = "aad" >
    <Niveau_2 Title = "T41" Link = "L41" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T42" Link = "L42" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
    <Niveau_2 Title = "T43" Link = "L43" >
      <Niveau_3 Title = "T100" Link = "L100" />
      <Niveau_3 Title = "T101" Link = "L101" />
      <Niveau_3 Title = "T102" Link = "L102" />
    </Niveau_2>
  </Niveau_1>
</Root>

【讨论】：

他们说一张图片值 1000 字。

【解决方案3】：

在 C# 中使用 lambda 表达式或匿名方法时，我们必须警惕 访问修改闭包 的陷阱。例如：

foreach (var s in strings)
{
   query = query.Where(i => i.Prop == s); // access to modified closure
   ...
}

由于修改了闭包，上面的代码将导致查询中的所有Where子句都基于s的最终值。

正如here 解释的那样，发生这种情况是因为在上面的foreach 循环中声明的s 变量在编译器中是这样翻译的：

string s;
while (enumerator.MoveNext())
{
   s = enumerator.Current;
   ...
}

而不是这样：

while (enumerator.MoveNext())
{
   string s;
   s = enumerator.Current;
   ...
}

正如here 指出的那样，在循环外声明变量并没有性能优势，在正常情况下，我能想到这样做的唯一原因是，如果您打算在循环：

string s;
while (enumerator.MoveNext())
{
   s = enumerator.Current;
   ...
}
var finalString = s;

但是，foreach 循环中定义的变量不能在循环外使用：

foreach(string s in strings)
{
}
var finalString = s; // won't work: you're outside the scope.

因此，编译器声明变量的方式使其极易出现通常难以发现和调试的错误，同时不会产生明显的好处。

【讨论】：

问题解决了吗？不确定上面的答案试图告诉我们什么。这只是对 Enigmativity 的回应吗？