Question

我在一个简单的XML文档中设置了我的应用程序，如下所示：

<Settings>
    <Server>
        <Id>1</Id>
        <Name>SRV123456</Name> 
        <Par Type="Desktop" Region="Western">12</Par>
        <Par Type="Laptop" Region="Western">15</Par>
        <Par Type="Desktop" Region="Eastern">22</Par>
        <Par Type="Laptop" Region="Eastern">25</Par>
        <State>WA</State>
    </Server>
</Settings>

我正在尝试使用C＃和LINQ查询它，使用以下代码：

xelement = XElement.Load(startpath + "\\Settings.xml");
var sn = from sl in xelement.Elements("Server")
       where (string)sl.Element("State") == "WA"
       where (string)sl.Element("Par").Attribute("Region") == "Western"
       where (string)sl.Element("Par").Attribute("Type") == "Desktop"
       select sl;

       foreach (XElement xele in sn)
       {
               Console.WriteLine(xele);
               Console.WriteLine(xele.Element("Par").Value);
       }

这适用于第一个“Par”值，并将返回“12”。但是如果我改变了

   where (string)sl.Element("Par").Attribute("Type") == "Desktop"

到

   where (string)sl.Element("Par").Attribute("Type") == "Laptop"

它没有返回任何结果......我错过了什么？

Answer 1

这是一个有效且看起来更漂亮的查询：

var parValue = xdoc.Descendants("Par")
    .Where(par=>par.Parent.Element("State")?.Value == "WA")
    .Where(par=>par.Attribute("Region")?.Value == "Western")
    .Where(par=>par.Attribute("Type")?.Value == "Laptop")
    .Select(par=>par.Value)
    .FirstOrDefault();

请注意？是一个CS6功能，如果使用CS5，则省略?，如果需要，检查为空

Answer 2

我绝不是LINQ的专家

然而

    var sn = from sl in xelement.Elements("Server")
           where (string)sl.Element("State") == "WA"
           where (string)sl.Element("Par").Attribute("Region") == "Western"
           where (string)sl.Element("Par").Attribute("Type") == "Desktop"
           select sl;

我相信只有返回元素属性（＆＃34;类型＆＃34;）==＆＃34;桌面＆＃34; 表示仅返回等于＆＃34;桌面＆＃34; ...所以当你循环时，结果集中没有

更改为

    var sn = from sl in xelement.Elements("Server")
             where (string)sl.Element("State") == "WA"
             where (string)sl.Element("Par").Attribute("Region") == "Western"
              select sl;

它应该返回所有类型......而不仅仅是＆＃34;桌面＆＃34;

Answer 3

也许您正在尝试返回Western / Laptop值= 15？

var state  = "WA";
var region = "Western";
var type   = "Laptop";
var xElement = XElement.Parse(@"<Settings>
    <Server>
        <Id>1</Id>
        <Name>SRV123456</Name> 
        <Par Type='Desktop' Region='Western'>12</Par>
        <Par Type='Laptop' Region='Western'>15</Par>
        <Par Type='Desktop' Region='Eastern'>22</Par>
        <Par Type='Laptop' Region='Eastern'>25</Par>
        <State>WA</State>
    </Server>
</Settings>");

foreach (XElement server in xElement.XPathSelectElements(
            String.Format("//Server[State='{0}']", state)))
{
    Console.WriteLine(server);

    // In your sample the Western/Desktop is the first element
    // If you want a specific Par element, you should query again with that filter
    foreach (XElement par in server.XPathSelectElements(
                String.Format("Par[@Region='{0}' and @Type='{1}']", region, type)))
        Console.WriteLine(par.Value); ;
}

我选择XPath over Linq来过滤XML文档，因为XPath对我来说似乎更简洁。

使用LINQ查询XML属性

3 个答案: