c# 使用 Linq DefaultIfEmpty() 连接 XML 文件产生重复

Question

我正在尝试对两个 XML 文档执行 leftOuterJoin，但是我目前使用的方式似乎从 fileone 返回了一些奇怪的重复部分（在下面的代码中解释）。我尝试了很多方法，但无法弄清楚如何做到这一点。有人可以帮忙解决这个问题吗？提前谢谢了！

文件：

<fileone>

    <Book BookID="dog"> Dog </Book>

    <Book BookID="cat"> Cat </Book>

</fileone>

文件二：

<filetwo>

       <Edition BID="cat" OrderID="100"> about cat</Edition>

       <Edition BID="cat" OrderID="200">more about cat</Edition>

</RightSeq>

我正在寻找的是（bookID=BID 上的左外连接）：

<item>
   <Book BookID="cat"> Dog </Book>
</item>
<item>
    <Book BookID="cat"> Cat </Book>
    <Edition BID="cat" OrderID="100"> about cat</Edition>
</item>
<item>
    <Book BookID="cat"> Cat </Book>
    <Edition BID="cat" OrderID="200"> more cat</Edition>
</item>

我的（错误）代码：

      var result = from a in fileone.Descendants()
      join b in secondxdoc.Descendants()
     on (string)a.Attribute("BookID") equal (string)b.Attribute("BID") into inners
      from ele in inners.DefaultIfEmpty()

      select new XElement("item", new XElement(a), ele == null ? null : new XElement(ele));
        var output = new XElement("LeftOuterJoin", result);         
    }

当前错误结果：

<item>
   <fileone>  
       <Book BookID="dog"> Dog </Book> // this entire <fileone> bit is unwanted
       <Book BookID="cat"> Cat </Book>  //I don't know what I've done wrong
   </fileone> 
</item>
    <item>
   <Book BookID="cat"> Dog </Book>
</item>
<item>
    <Book BookID="cat"> Cat </Book>
    <Edition BID="cat" OrderID="100"> about cat</Edition>
</item>
<item>
    <Book BookID="cat"> Cat </Book>
    <Edition BID="cat" OrderID="200"> more cat</Edition>
</item>  

Answer 1

不需要的第一个 <item> 元素是将根元素 <fileone> 与第二个文件的后代连接的结果。如果要跳过根元素，请将 fileone.Descendants() 替换为 fileone.Root.Descendants() ：

var result = from a in fileone.Root.Descendants()
             join b in secondxdoc.Descendants()
                on (string)a.Attribute("BookID") equal (string)b.Attribute("BID") into inners
             from ele in inners.DefaultIfEmpty()
             select new XElement("item", a, ele);
var output = new XElement("LeftOuterJoin", result);

Dotnetfiddle Demo

Answer 2

我在不使用传统 linq 的情况下得到了结果。如果 fileone 在 filetwo 中没有匹配项，则连接不会给出结果。

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string input1 =
               "<fileone>" +
                   "<Book BookID=\"dog\"> Dog </Book>" +
                   "<Book BookID=\"cat\"> Cat </Book>" +
               "</fileone>";
            XDocument fileone = XDocument.Parse(input1);

            string input2 =
               "<filetwo>" +
                      "<Edition BID=\"cat\" OrderID=\"100\"> about cat</Edition>" +
                      "<Edition BID=\"cat\" OrderID=\"200\">more about cat</Edition>" +
               "</filetwo>";
            XDocument secondxdoc = XDocument.Parse(input2);

            //<item>
            //   <Book BookID="cat"> Dog </Book>
            //</item>
            //<item>
            //    <Book BookID="cat"> Cat </Book>
            //    <Edition BID="cat" OrderID="100"> about cat</Edition>
            //</item>
            //<item>
            //    <Book BookID="cat"> Cat </Book>
            //    <Edition BID="cat" OrderID="200"> more cat</Edition>
            //</item>
            XElement output = new XElement("Root");
            foreach (XElement item in fileone.Descendants("Book"))
            {
                string bookId = item.Attribute("BookID").Value;
                List<XElement> bookTwoItems = secondxdoc.Descendants("Edition").Where(x => x.Attribute("BID").Value == bookId).ToList();
                if (bookTwoItems.Count  == 0)
                {
                    XElement newItem = new XElement("item");
                    output.Add(newItem);
                    newItem.Add(item);
                }
                else
                {
                    foreach (XElement bookTwoItem in bookTwoItems)
                    {
                        XElement newItem = new XElement("item");
                        output.Add(newItem);
                        newItem.Add(item);
                        newItem.Add(bookTwoItem);
                    }
                }
                
            }
        
        }
    }
}
​

让它与左外连接一起工作

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string input1 =
               "<fileone>" +
                   "<Book BookID=\"dog\"> Dog </Book>" +
                   "<Book BookID=\"cat\"> Cat </Book>" +
               "</fileone>";
            XDocument fileone = XDocument.Parse(input1);

            string input2 =
               "<filetwo>" +
                      "<Edition BID=\"cat\" OrderID=\"100\"> about cat</Edition>" +
                      "<Edition BID=\"cat\" OrderID=\"200\">more about cat</Edition>" +
               "</filetwo>";
            XDocument secondxdoc = XDocument.Parse(input2);

            //<item>
            //   <Book BookID="cat"> Dog </Book>
            //</item>
            //<item>
            //    <Book BookID="cat"> Cat </Book>
            //    <Edition BID="cat" OrderID="100"> about cat</Edition>
            //</item>
            //<item>
            //    <Book BookID="cat"> Cat </Book>
            //    <Edition BID="cat" OrderID="200"> more cat</Edition>
            //</item>
            var results = (from one in fileone.Descendants("Book")
                           join two in secondxdoc.Descendants("Edition") on one.Attribute("BookID").Value equals two.Attribute("BID").Value into inners
                           from two in inners.DefaultIfEmpty()
                           select new { fileone = one, filetwo = two == null ? null : two }).ToList(); 
            XElement output = new XElement("Root");
            foreach (var item in results)
            {
                XElement newItem = new XElement("item");
                output.Add(newItem);
                newItem.Add(item.fileone);
                if (item.filetwo  != null)
                {
                        newItem.Add(item.filetwo);
                }

            }

        }
    }
}
​

或者这个方法

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string input1 =
               "<fileone>" +
                   "<Book BookID=\"dog\"> Dog </Book>" +
                   "<Book BookID=\"cat\"> Cat </Book>" +
               "</fileone>";
            XDocument fileone = XDocument.Parse(input1);

            string input2 =
               "<filetwo>" +
                      "<Edition BID=\"cat\" OrderID=\"100\"> about cat</Edition>" +
                      "<Edition BID=\"cat\" OrderID=\"200\">more about cat</Edition>" +
               "</filetwo>";
            XDocument secondxdoc = XDocument.Parse(input2);

            //<item>
            //   <Book BookID="cat"> Dog </Book>
            //</item>
            //<item>
            //    <Book BookID="cat"> Cat </Book>
            //    <Edition BID="cat" OrderID="100"> about cat</Edition>
            //</item>
            //<item>
            //    <Book BookID="cat"> Cat </Book>
            //    <Edition BID="cat" OrderID="200"> more cat</Edition>
            //</item>
            XElement output = new XElement("Root");
            var results = (from one in fileone.Descendants("Book")
                           join two in secondxdoc.Descendants("Edition") on one.Attribute("BookID").Value equals two.Attribute("BID").Value into inners
                           from two in inners.DefaultIfEmpty()
                           select new { fileone = one, filetwo = two == null ? null : two })
                           .Select(x => x.filetwo == null ? new XElement("item", x.fileone) : new XElement("item", new object[] { x.fileone, x.filetwo }));
            foreach (var item in results)
            {
                output.Add(item);
            }
        }
    }
}
​