0

タブを使用しているフォームに、XML ファイルからいくつかの要素を表示しようとしています。

ただし、次のコード行で「NullReferenceException」というエラーが発生します。

var assessment1 = from d in document.Descendants("moduleTitle")
                            where d.Value == (comboBoxModuleSelect[i] as ComboBox).SelectedItem.ToString() //error here on this line
                            select d.Parent.Element("assessmentOne").Value;

このコードは、クリックすると新しいコンボ ボックスを生成するボタン クリック内にあります。私が行った方法は次のとおりです。

private void buttonLevel4Add_Click(object sender, EventArgs e)
        {
            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);
        }

integer i はクラス変数で、新しいコンボ ボックスを作成するためにボタンがクリックされるたびに増加します。

上記のスニペットは順序が正しくないため混乱を招く可能性があるため、クラス全体を次に示します。

public partial class Form1 : Form
    {
        String workingDir = Directory.GetCurrentDirectory();
        ArrayList comboBoxModuleSelect = new ArrayList();
        ArrayList labelAssessments = new ArrayList();
        int i = 0; //for combo boxes

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            tabControl.SelectTab(1);
        }

        private void buttonLevel4Add_Click(object sender, EventArgs e)
        {
            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);

            this.labelAssessments.Add(new Label());
            System.Drawing.Point pLabel = new System.Drawing.Point(200 + i * 25, 81);
            (labelAssessments[i] as Label).Location = pLabel;
            (labelAssessments[i] as Label).Size = new System.Drawing.Size(80, 13);
            tabPageLevel4.Controls.Add(labelAssessments[i] as Label);


            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);
            }

            var assessment1 = from d in document.Descendants("moduleTitle")
                            where d.Value == (comboBoxModuleSelect[i] as ComboBox).SelectedItem.ToString()
                            select d.Parent.Element("assessmentOne").Value;
            foreach (var item in assessment1)
            {

                (labelAssessments[i] as Label).Text = item.ToString();

            }


            i++;//for drawing

        }

XML は次のとおりです。

<Course>
  <CourseName>BEng Software Engineering</CourseName>
  <Modules>
    <Module>
      <moduleCode>ECSE401</moduleCode>
      <moduleTitle>Programming Methodology</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Coursework</assessmentOne>
      <assessmentOneWeight>40</assessmentOneWeight>
      <assessmentTwo>Coursework</assessmentTwo>
      <assessmentTwoWeight>40</assessmentTwoWeight>
      <assessmentThree>Test</assessmentThree>
      <assessmentThreeWeight>20</assessmentThreeWeight>
    </Module>
    <Module>
      <moduleCode>ECSC404</moduleCode>
      <moduleTitle>Computer Systems Fundamentals</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Test1</assessmentOne>
      <assessmentOneWeight>30</assessmentOneWeight>
      <assessmentTwo>Test2</assessmentTwo>
      <assessmentTwoWeight>30</assessmentTwoWeight>
      <assessmentThree>Test3</assessmentThree>
      <assessmentThreeWeight>40</assessmentThreeWeight>
    </Module>
    <Module>
      <moduleCode>EBSY401</moduleCode>
      <moduleTitle>Information and Data Modelling</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Test</assessmentOne>
      <assessmentOneWeight>25</assessmentOneWeight>
      <assessmentTwo>Coursework1</assessmentTwo>
      <assessmentTwoWeight>10</assessmentTwoWeight>
      <assessmentThree>Coursework2</assessmentThree>
      <assessmentThreeWeight>35</assessmentThreeWeight>
      <assessmentFour>Coursework3</assessmentFour>
      <assessmentFourWeight>30</assessmentFourWeight> 
    </Module>
    <Module>
      <moduleCode>ECSC405</moduleCode>
      <moduleTitle>Software Development Principles</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Test1</assessmentOne>
      <assessmentOneWeight>30</assessmentOneWeight>
      <assessmentTwo>Coursework</assessmentTwo>
      <assessmentTwoWeight>40</assessmentTwoWeight>
      <assessmentThree>Test2</assessmentThree>
      <assessmentThreeWeight>30</assessmentThreeWeight>
    </Module>
    <Module>
      <moduleCode>ECSC407</moduleCode>
      <moduleTitle>Web Technology</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Tutorials</assessmentOne>
      <assessmentOneWeight>20</assessmentOneWeight>
      <assessmentTwo>Coursework</assessmentTwo>
      <assessmentTwoWeight>20</assessmentTwoWeight>
      <assessmentThree>Exam</assessmentThree>
      <assessmentThreeWeight>60</assessmentThreeWeight>
    </Module>
    <Module>
      <moduleCode>ECSC409</moduleCode>
      <moduleTitle>Software Engineering Principles</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Coursework1</assessmentOne>
      <assessmentOneWeight>40</assessmentOneWeight>
      <assessmentTwo>Coursework2</assessmentTwo>
      <assessmentTwoWeight>30</assessmentTwoWeight>
      <assessmentThree>Coursework3</assessmentThree>
      <assessmentThreeWeight>30</assessmentThreeWeight>
    </Module>
    <Module>
      <moduleCode>ECSC408</moduleCode>
      <moduleTitle>Mathematics for Computing</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Coursework</assessmentOne>
      <assessmentOneWeight>50</assessmentOneWeight>
      <assessmentTwo>Exam</assessmentTwo>
      <assessmentTwoWeight>50</assessmentTwoWeight>
    </Module>
    <Module>
      <moduleCode>EBSY400</moduleCode>
      <moduleTitle>Communication and Learning Skills</moduleTitle>
      <credits>15</credits>
      <assessmentOne>Coursework</assessmentOne>
      <assessmentOneWeight>30</assessmentOneWeight>
      <assessmentTwo>Coursework</assessmentTwo>
      <assessmentTwoWeight>70</assessmentTwoWeight>
    </Module>
  </Modules>
</Course>

nullreference 例外が発生する理由がわかりません。誰か助けてください

4

2 に答える 2

2

index のコンボボックスiが選択されていない ( SelectedItemnull を返す) か、dnull である可能性があります

上記で問題なく使用されているのを見てdocument.Descendants、ご迷惑をおかけしているのは選択だと思います

試す：

ComboBox cb = comboBoxModuleSelect[i] as ComboBox;
if (cb.SelectedItem != null ) {
  var assessment1 = from d in document.Descendants("moduleTitle") 
    where (d.Value == cb.SelectedItem.ToString())
    select d.Parent.Element("assessmentOne").Value;
}
于 2012-04-11T19:08:47.860 に答える
0

コンボボックスにたくさんのアイテムを追加したと思いますが、どれも選択されていません。そのため、SelectedItemが返されますnull。

于 2012-04-11T19:14:19.000 に答える