如何检查是否XML使用LINQ to XML时，包含的元素？ [英] How to check if XML contains element when using LINQ to XML?

问题描述

鉴于这种结构：

 <root>
     <user>
           <userName>user1</userName>
           <userImageLocation>/user1.png</userImageLocation>
     </user>
     <user>
           <userName>user2</userName>
     </user>
 </root>

public class User
{
    public string UserName {get; set; }
    public string UserImageLocation {get; set; }
}

我使用的LINQ to XML从XML文件中获取数据，如这样的：

I use the LINQ to XML to get data from the XML file, like this:

XDocument document = XDocument.Parse(xmlFile);
List<User> listOfUsers =  
(from user in document.Descendants("user")
 select new User {
    UserName = user.Element("userName"),
    UserImageLocation = user.Element("userImageLocation"),
 }
).ToList<User>();

我的问题是，并非所有的用户元素包含userImageLocation，并试图读取userImageLocation时，抛出一个异常。

My problem is that not all user element contains a userImageLocation, and when trying to read the userImageLocation, it throws an exception.

我如何检查是否一个XML元素存在，如果存在，读它？

How can I check if an XML element exist, and if it exists, read it?

推荐答案

您当前的代码将无法编译，因为你要分配的XElement 来的字符串属性。我的猜测是，你正在使用的 XElement.Value 属性将其转换为字符串。取而代之的是，使用显式串转换，如果你把它叫做关于空的XElement 的参考，这将返回null：

Your current code won't compile, as you're trying to assign an XElement to a string property. My guess is that you're using the XElement.Value property to convert it to a string. Instead of that, use the explicit string conversion, which will return null if you call it "on" a null XElement reference:

XDocument document = XDocument.Parse(xmlFile);
List<User> listOfUsers =  
(from user in document.Descendants("user")
 select new User {
    UserName = (string) user.Element("userName"),
    UserImageLocation = (string) user.Element("userImageLocation"),
 }
).ToList<User>();

请注意，这是这是相当更具可读性使用点符号的情况之一：

Note that this is one of those situations which is rather more readable using dot notation:

XDocument document = XDocument.Parse(xmlFile);
List<User> listOfUsers = document
    .Descendants("user")
    .Select(user => new User { 
         UserName = (string) user.Element("userName"),
         UserImageLocation = (string) user.Element("userImageLocation") })
    .ToList();

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