LINQ To XML:之复杂的查询举例

XDocument xDocument = new XDocument( 
  new XElement("BookParticipants", 
    new XElement("BookParticipant", 
      new XAttribute("type", "Author"), 
      new XElement("FirstName", "Joe"), 
      new XElement("LastName", "Rattz")), 
    new XElement("BookParticipant", 
      new XAttribute("type", "Editor"), 
      new XElement("FirstName", "Ewan"), 
      new XElement("LastName", "Buckingham")))); 
 
IEnumerable<XElement> elements = xDocument 
  .Descendants("BookParticipant") 
  .Where(e => ((string)e.Element("FirstName")) == "Ewan"); 
 
foreach (XElement element in elements) 
{ 
  Console.WriteLine("Element: {0} : value = {1}", 
    element.Name, element.Value); 
} 

输入是这样的 Element: BookParticipant : value = EwanBuckingham

前面的文章都只是在简单的foreach所有元素或节点，其实我们在实际项目中是不会那么做的，多数时候会用where ...... 来跳过那个不需要的元素

在多数情况下，我们还需要排序，像下面这样

IEnumerable<XElement> elements =  
  from e in xDocument.Descendants("BookParticipant") 
  where ((string)e.Attribute("type")) != "Illustrator" 
  orderby ((string)e.Element("LastName")) 
  select e; 

输出

Element: BookParticipant : value = EwanBuckingh

Element: BookParticipant : value = JoeRattz

下面是一个更加复杂的查询，像sql里面的join语句一样，把两个XML文件按条件进行联合查询

XDocument users = XDocument.Parse( 
  @"<users> 
      <user_tuple> 
        <userid>U01</userid> 
        <name>Tom Jones</name> 
        <rating>B</rating> 
      </user_tuple> 
      <user_tuple> 
        <userid>U02</userid> 
        <name>Mary Doe</name> 
        <rating>A</rating> 
      </user_tuple> 
      <user_tuple> 
        <userid>U03</userid> 
        <name>Dee Linquent</name> 
        <rating>D</rating> 
      </user_tuple> 
      <user_tuple> 
        <userid>U04</userid> 
        <name>Roger Smith</name> 
        <rating>C</rating> 
      </user_tuple> 
      <user_tuple> 
        <userid>U05</userid> 
        <name>Jack Sprat</name> 
        <rating>B</rating> 
      </user_tuple> 
      <user_tuple> 
        <userid>U06</userid> 
        <name>Rip Van Winkle</name> 
        <rating>B</rating> 
      </user_tuple> 
    </users>"); 
XDocument items = XDocument.Parse( 
  @"<items> 
      <item_tuple> 
        <itemno>1001</itemno> 
        <description>Red Bicycle</description> 
        <offered_by>U01</offered_by> 
        <start_date>1999-01-05</start_date> 
        <end_date>1999-01-20</end_date> 
        <reserve_price>40</reserve_price> 
      </item_tuple> 
      <item_tuple> 
        <itemno>1002</itemno> 
        <description>Motorcycle</description> 
        <offered_by>U02</offered_by> 
        <start_date>1999-02-11</start_date> 
        <end_date>1999-03-15</end_date> 
        <reserve_price>500</reserve_price> 
      </item_tuple> 
<item_tuple> 
        <itemno>1003</itemno> 
        <description>Old Bicycle</description> 
        <offered_by>U02</offered_by> 
        <start_date>1999-01-10</start_date> 
        <end_date>1999-02-20</end_date> 
        <reserve_price>25</reserve_price> 
      </item_tuple> 
      <item_tuple> 
        <itemno>1004</itemno> 
        <description>Tricycle</description> 
        <offered_by>U01</offered_by> 
        <start_date>1999-02-25</start_date> 
        <end_date>1999-03-08</end_date> 
        <reserve_price>15</reserve_price> 
      </item_tuple> 
      <item_tuple> 
        <itemno>1005</itemno> 
        <description>Tennis Racket</description> 
        <offered_by>U03</offered_by> 
        <start_date>1999-03-19</start_date> 
        <end_date>1999-04-30</end_date> 
        <reserve_price>20</reserve_price> 
      </item_tuple> 
      <item_tuple> 
        <itemno>1006</itemno> 
        <description>Helicopter</description> 
        <offered_by>U03</offered_by> 
        <start_date>1999-05-05</start_date> 
        <end_date>1999-05-25</end_date> 
        <reserve_price>50000</reserve_price> 
      </item_tuple> 
      <item_tuple> 
        <itemno>1007</itemno> 
        <description>Racing Bicycle</description> 
        <offered_by>U04</offered_by> 
        <start_date>1999-01-20</start_date> 
        <end_date>1999-02-20</end_date> 
        <reserve_price>200</reserve_price> 
      </item_tuple> 
      <item_tuple> 
        <itemno>1008</itemno> 
        <description>Broken Bicycle</description> 
        <offered_by>U01</offered_by> 
        <start_date>1999-02-05</start_date> 
        <end_date>1999-03-06</end_date> 
        <reserve_price>25</reserve_price> 
      </item_tuple> 
    </items>"); 
XDocument bids = XDocument.Parse( 
  @"<bids> 
      <bid_tuple> 
        <userid>U02</userid> 
        <itemno>1001</itemno> 
        <bid>35</bid> 
        <bid_date>1999-01-07</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U04</userid> 
        <itemno>1001</itemno> 
        <bid>40</bid> 
        <bid_date>1999-01-08</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U02</userid> 
        <itemno>1001</itemno> 
        <bid>45</bid> 
        <bid_date>1999-01-11</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U04</userid> 
        <itemno>1001</itemno> 
        <bid>50</bid> 
        <bid_date>1999-01-13</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U02</userid> 
        <itemno>1001</itemno> 
        <bid>55</bid> 
        <bid_date>1999-01-15</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U01</userid> 
        <itemno>1002</itemno> 
        <bid>400</bid> 
        <bid_date>1999-02-14</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U02</userid> 
        <itemno>1002</itemno> 
        <bid>600</bid> 
        <bid_date>1999-02-16</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U03</userid> 
        <itemno>1002</itemno> 
        <bid>800</bid> 
        <bid_date>1999-02-17</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U04</userid> 
        <itemno>1002</itemno> 
        <bid>1000</bid> 
        <bid_date>1999-02-25</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U02</userid> 
        <itemno>1002</itemno> 
        <bid>1200</bid> 
        <bid_date>1999-03-02</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U04</userid> 
        <itemno>1003</itemno> 
        <bid>15</bid> 
        <bid_date>1999-01-22</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U05</userid> 
        <itemno>1003</itemno> 
        <bid>20</bid> 
        <bid_date>1999-02-03</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U01</userid> 
        <itemno>1004</itemno> 
        <bid>40</bid> 
        <bid_date>1999-03-05</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U03</userid> 
        <itemno>1007</itemno> 
        <bid>175</bid> 
        <bid_date>1999-01-25</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U05</userid> 
        <itemno>1007</itemno> 
        <bid>200</bid> 
        <bid_date>1999-02-08</bid_date> 
      </bid_tuple> 
      <bid_tuple> 
        <userid>U04</userid> 
        <itemno>1007</itemno> 
        <bid>225</bid> 
        <bid_date>1999-02-12</bid_date> 
      </bid_tuple> 
    </bids>"); 

下面是查询的方法

var biddata = from b in bids.Descendants("bid_tuple") 
              where ((double)b.Element("bid")) > 50 
              join u in users.Descendants("user_tuple") 
              on ((string)b.Element("userid")) equals 
                ((string)u.Element("userid")) 
              join i in items.Descendants("item_tuple") 
              on ((string)b.Element("itemno")) equals 
                ((string)i.Element("itemno")) 
              select new { Item = ((string)b.Element("itemno")), 
                           Description = ((string)i.Element("description")), 
                           User = ((string)u.Element("name")), 
                           Date = ((string)b.Element("bid_date")), 
                           Price = ((double)b.Element("bid"))}; 
 

打印的代码

Console.WriteLine("{0,-12} {1,-12} {2,-6} {3,-14} {4,10}", 
  "Date", 
  "User", 
  "Item", 
  "Description", 
  "Price"); 
 
Console.WriteLine("=========================================================="); 
 
There is nothing special about that. All that is left is to enumerate the sequence and display each 
bid: 
 
foreach (var bd in biddata) 
{ 
  Console.WriteLine("{0,-12} {1,-12} {2,-6} {3,-14} {4,10:C}", 
    bd.Date, 
    bd.User, 
    bd.Item, 
    bd.Description, 
    bd.Price); 
} 

输出结果

Date         User         Item   Description         Price 
========================================================== 
1999-01-15   Mary Doe     1001   Red Bicycle        $55.00 
1999-02-14   Tom Jones    1002   Motorcycle        $400.00 
1999-02-16   Mary Doe     1002   Motorcycle        $600.00 
1999-02-17   Dee Linquent 1002   Motorcycle        $800.00 
1999-02-25   Roger Smith  1002   Motorcycle      $1,000.00 
1999-03-02   Mary Doe     1002   Motorcycle      $1,200.00 
1999-01-25   Dee Linquent 1007   Racing Bicycle    $175.00 
1999-02-08   Jack Sprat   1007   Racing Bicycle    $200.00 
1999-02-12   Roger Smith  1007   Racing Bicycle    $225.00