c# - コンボボックスで選択した項目を XML 要素の値と比較する - C#

Question

コンボ ボックスに XML ファイルの値を入力し (これは実行済みです)、その値ツリーの下にいくつかの要素をラベルで表示したいと考えています。すべてがタブページにあります

ボタンクリックで次のようなコンボボックスを作成しました。

int i = 0; //global variable for keeping track of combo box objects
int j = 0; // same but for labels
//the following codes are in the button click:
var document = XDocument.Load(workingDir + @"\Level4.xml");
            comboBoxModuleSelect.Add(new ComboBox());
            System.Drawing.Point p = new System.Drawing.Point(7, 57 + i * 25);
            (comboBoxModuleSelect[i] as ComboBox).Location = p;
            (comboBoxModuleSelect[i] as ComboBox).Size = new System.Drawing.Size(183, 20);
            tabPageLevel4.Controls.Add(comboBoxModuleSelect[i] as ComboBox);

そして、次のように入力しました：

var moduleName = from d in document.Descendants("moduleTitle")
                        select d.Value;
            foreach (var item in moduleName)
            {
                //add the module names into the dropdown for students to select
                (comboBoxModuleSelect[i] as ComboBox).Items.Add(item);
            }

ここで、ユーザーが選択した現在のコンボ ボックスで選択された値を取得し、その値に関連する XML 要素を取得したいと考えています。ただし、次の行で NullReferenceException を取得しています。

var assessment = from d in document.Descendants("moduleTitle")
                            where d.Value == (comboBoxModuleSelect[i] as ComboBox).SelectedItem.ToString()
                            select d.Parent.Element("assessmentType").Value;

の上comboBoxModuleSelect[i]

必要な場合、XML は次のようになります。

<?xml version="1.0" encoding="utf-8" ?>
<Course>
  <CourseName>BEng Software Engineering</CourseName>
  <Modules>
    <Module>
      <moduleCode>ECSE401</moduleCode>
      <moduleTitle>Programming Methodology</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Coursework</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>40</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>40</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Test2</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
    <Module>
      <moduleCode>ECSC404</moduleCode>
      <moduleTitle>Computer Systems Fundamentals</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Test1</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Test2</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Test3</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>40</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
    <Module>
      <moduleCode>EBSY401</moduleCode>
      <moduleTitle>Information and Data Modelling</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Test</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>25</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework1</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>10</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework2</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>35</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework3</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
    <Module>
      <moduleCode>ECSC405</moduleCode>
      <moduleTitle>Software Development Principles</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Test1</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>40</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Test2</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
    <Module>
      <moduleCode>ECSC407</moduleCode>
      <moduleTitle>Web Technology</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Tutorials</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>20</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>20</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Exam</assessmentName>
          <assessmentType>Exam</assessmentType>
          <assessmentWeighting>60</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
    <Module>
      <moduleCode>ECSC409</moduleCode>
      <moduleTitle>Software Engineering Principles</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Test1</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>40</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework1</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Coursework2</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
    <Module>
      <moduleCode>ECSC408</moduleCode>
      <moduleTitle>Mathematics for Computing</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Coursework</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>50</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Exam</assessmentName>
          <assessmentType>Exam</assessmentType>
          <assessmentWeighting>50</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
    <Module>
      <moduleCode>EBSY400</moduleCode>
      <moduleTitle>Communication and Learning Skills</moduleTitle>
      <credits>15</credits>
      <assessmentDetails>
        <assessment>
          <assessmentName>Presentation</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>30</assessmentWeighting>
        </assessment>
        <assessment>
          <assessmentName>Portfolio</assessmentName>
          <assessmentType>Coursework</assessmentType>
          <assessmentWeighting>70</assessmentWeighting>
        </assessment>
      </assessmentDetails>
    </Module>
  </Modules>
</Course>

nullreferenceexception が発生するのはなぜですか?