组合框对象上的NullReferenceException - C＃

我试图从我的XML文件显示一些元素到我的表单使用选项卡。组合框对象上的NullReferenceException - C＃

但是我得到以下行的代码错误“的NullReferenceException”：

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;

此代码是一个按钮，点击它产生一次点击的新组合框里面。我这样做的方法如下：

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); 
     }

整数i是一个类变量，这是我每次递增单击该按钮，以创建一个新的组合框的时间。因为他们为了不是

上面的片断可能会造成混乱，所以这里是整个类：

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 

     }

这里是XML：

<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>

我不明白为什么我得到空引用异常，请有人帮忙

来源

2012-04-11 a7omiton

请注意，不要这样做：'（comboBoxModuleSelect [i]作为组合框）.Location'。如果您假设安全演员阵容不会返回null，那么就没有理由先执行安全演员，只需使用C风格演员阵容（就此而言，执行一次演员并将参考存储在变量）。另外，没有理由在这里使用'ArrayList'（自从1.1中引入泛型以来几乎没有）。你的代码做了很多假设，显然至少有一个是错误的。 – 2012-04-11 19:13:28

其实我觉得问题出在我的逻辑上，整个操作都在按钮上点击。当我运行程序时，屏幕上没有组合框，所以操作比较d.Value和comboBox.SelectedItem总是会为空，因为我没有选择任何东西，因此程序在第一次点击时关闭。我是否正确地承认这一点？ – a7omiton 2012-04-11 19:28:43

索引为i的组合框可能未被选中（所以SelectedItem返回null）或d可能为空

看到你使用document.Descendants以上没问题，我认为这是造成你麻烦的选择

尝试：

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; 
}

来源

2012-04-11 19:08:47 Attila

雅，事情是，一旦我点击按钮，它弹出错误，我不会在组合框中输入任何值 – a7omiton 2012-04-11 19:10:30

在点击按钮之前选择一个元素？ – Attila 2012-04-11 19:18:04

解决了错误问题，但是我没有将任何值输出到我的标签上，我认为获取XML元素存在问题 – a7omiton 2012-04-11 19:54:25

我想你已经添加了一堆的项目组合框但没有选择它们。这就是为什么SelectedItem返回null。

来源

2012-04-11 19:14:19

组合框对象上的NullReferenceException - C＃

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