Linq to Xml C#在特定元素中查找特定元素

Question

我正在尝试返回具有特定 ID 号的用户的 Degree 值，该用户可以拥有多个 Degree

到目前为止我的 c# 代码（不工作） 错误：System.InvalidOperationException: 'Sequence contains no matching element'

 public static string checkLicense(string cardType)
        {
            string x="";
            var values = from e in XDocument.Load(@".\data\PersonData.Xml").Elements("User").Single(c => c.Attribute("idNumber").Value == Properties.Settings.Default.idNumber).Descendants("DrivingCardLicence")
                         select e.Element("Degree").Value;

            foreach (var e in values)
            {
                x += e;
            }
            return x;
}

我的 XML 文件：

<?xml version="1.0" encoding="utf-8"?>
<Users>
  <User idNumber="666666666">
    <IdCard>
      <FirstName>Majd</FirstName>
      <LastName>sadi</LastName>
      <FatherName>asfasf</FatherName>
      <MotherName>asf</MotherName>
      <GrandFatherName>sdfgasf</GrandFatherName>
      <Sex>זכר</Sex>
      <ImagePath></ImagePath>
      <DateOfBirth>25/06/1996 18:22:48</DateOfBirth>
      <Address>asf</Address>
    </IdCard>
  </User>
  <User idNumber="999999999">
    <IdCard>
      <FirstName>asfasf</FirstName>
      <LastName>asfasf</LastName>
      <FatherName>asfasf</FatherName>
      <MotherName>asfasf</MotherName>
      <GrandFatherName>asfasf</GrandFatherName>
      <Sex>זכר</Sex>
      <ImagePath>C:\Users\m\Desktop\pictures\me\meirl.jpg</ImagePath>
      <DateOfBirth>31/03/1996 18:27:46</DateOfBirth>
      <Address>asfdasf</Address>
    </IdCard>
    <DrivingCardLicence>
      <ImagePath>C:\Users\m\Desktop\pictures\me\IMG_20200214_015706.jpg</ImagePath>
      <imageFilePath>C:\Users\m\Desktop\pictures\2.jfif</imageFilePath>
      <Degree>D1</Degree>
      <Degree>A2</Degree>
      <ReleaseDate>31/03/2020</ReleaseDate>
      <Validation>31/03/2030</Validation>
      <CardNumber>4067510</CardNumber>
    </DrivingCardLicence>
  </User>

注意：并非所有用户都有 DrivingCardLicence 元素 正如您在 xml 代码中看到的，id=999999999 用户的学位为 d1 但是 id=666666666 用户没有，所以 x 应该是空的。

Answer 1

你在这方面做得很好。

您忘记包含根元素 Users via 。 Element("Users")

var values = 
    from e in doc.Element("Users").Elements("User")
        .Single(c => c.Attribute("idNumber").Value == Properties.Settings.Default.idNumber)
        .Elements("DrivingCardLicence")
        .Elements("Degree")
    select e.Value;