NullReferenceException on combo box object - C#

Question

I'm trying to display some elements from my XML file to my form which is using tabs.

However I'm getting the error "NullReferenceException" on the following line of code:

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;

This code is inside a button click which generates new combo boxes once clicked. The way I did that is as follows:

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 is a class variable, which I increment every time the button is clicked in order to create a new combo box.

The above snippets might be confusing because they're not in order, so here is the whole class:

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

        }

and here is the 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>

I don't understand why I'm getting a nullreference exception, please could someone help

On a side note, don't do this: (comboBoxModuleSelect[i] as ComboBox).Location. If you are assuming that the safe cast won't return null then there is no reason to perform a safe cast to begin with, just use a C style cast (and for that matter, perform one cast and store the reference in a variable). Also, there is no reason to use ArrayList here (there almost never is since the introduction of generics in 1.1). Your code is making a lot of assumptions and apparently at least one of them is wrong. — Ed Swangren
– Ed Swangren, Commented Apr 11, 2012 at 19:13
Actually I think the problem is in my logic, the whole operation is in a button click. When I run the program there are no combo boxes on the screen, so the operation compare d.Value with comboBox.SelectedItem is always going to be null because I haven't selected anything, hence the program closes in the first click. Am I correct in assuming this? — a7omiton
– a7omiton, Commented Apr 11, 2012 at 19:28

Attila · Accepted Answer · 2012-04-11 21:19:38Z

2

The combobox at index i might not be selected (so SelectedItem returns null), or d might be null

Seeing that you use document.Descendants above with no problem, I think it is the selection that causing you the trouble

Try:

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;
}

edited Apr 11, 2012 at 21:19

answered Apr 11, 2012 at 19:08

Attila

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4 Comments

a7omiton Over a year ago

ya, the thing is that once I click the button, it pops up the error, I don't get to enter any value in the combo box

Attila Over a year ago

Select an element before clicking the button?

a7omiton Over a year ago

that solves the error problem, but I don't get any value outputted onto my label, I think there's something wrong with getting the XML elements

Attila Over a year ago

You could output the values of item in assessment1 for each (incremental) part of the from/where/select construct and see it agrees with what you would expect

the_joric · Accepted Answer · 2012-04-11 19:14:19Z

0

I guess you've added a bunch of items to combobox but made none of them selected. That's why SelectedItem returns null.

answered Apr 11, 2012 at 19:14

the_joric

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