Question

I have created a program for my ASP.NET class to create a class name and number and then enter in ten student names with student ID. So my web page will prompt the user to enter these 4 information pieces separating them into the MyClass ( the course name and number) and the Student class ( where it holds the student name and ID). I am terribly stuck on the NullReferenceException error and don’t know what to do. If anyone here could give me some pointers and direction so I can pick myself up from this assignment. One thing is certain, I am not allowed to enter any new methods and variables. I am only able to use the current ones I have initialized.

• The Student[] array is the array that holds my Student class with the name and id of students.
• The user first will create the class number and name in which to add students(with id and name) to it afterwards.
• Only 1 class name and number(they go together) is allowed and only 10 students is the maximum.

I am getting the null exception on my ToString() method in MyClass where classNumberAndName += ((Student)students[i]).ToString();

Here’s my code with no syntax errors.

public class MyClass
{
    private string courseNumber;
    private string courseName;
    private int numberOfStudents;
    private Student[] students;       
    public MyClass(string CourseNumber , string CourseName, Student[] Student)
    {
        students = new Student[Student.Length];
        courseNumber = CourseNumber;
        courseName = CourseName;
        for (int i = 0; i < Student.Length; i++)
        {
            Student tmpArrayStudent = new Student(((Student)students[i]).ToString(),((Student)students[i]).ToString());
            tmpArrayStudent = students[i];
        }
    }   
    public MyClass(string CourseNumber, string CourseName)
    {
        courseNumber = CourseNumber;
        courseName = CourseName;
        students = new Student[10];
    }
    public void addAStudent(Student student)
    {//possible to create temp array?
        Student[] students = new Student[10];
        if(numberOfStudents < students.Length)
        {
            students[numberOfStudents] = student;
            numberOfStudents++;
        }
        else
        {
            throw new Exception("Amount of Students Exceeded, no more than 10");
        }
    }
    public string getClassNumberAndName()
    {
        return "Course Number: " + courseNumber + "   " + " Course Name: " + courseName;
    }
    public override string ToString()
    {
        string classNumberAndName = getClassNumberAndName();
        for(int i = 0; i < students.Length; i++)
        {
            classNumberAndName += ((Student)students[i]).ToString();
        }
        classNumberAndName += "Students Registered: " + numberOfStudents.ToString();
        return classNumberAndName;
    }
}


public class Student
{
    private string idNumber;
    private string name;
    public Student(string ID, string Name)
    {
        idNumber = ID;
        name = Name;
    }
    public override string ToString()
    {
        return "Student ID: " + idNumber + " " + " Student Name " + name;
    }       
}

And here is the .aspx.cs file for the web code.

public partial class WebForm1 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if(!IsPostBack)
        {
            pnlStudent.Visible = false;//use panels to disable the ability to enter new course information
        }
    }
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        MyClass course = new MyClass(txtCnum.Text, txtCname.Text);
        Session.Add("Course", course);
        ((MyClass)Session["Course"]).addAStudent(new Student(txtCnum.Text, txtCname.Text));//since we entered to Student, course information or student information?

        txtAnswers.Text += course.ToString();

        pnlStudent.Visible = true;
    }
    protected void btnAdd_Click(object sender, EventArgs e)
    {
        Student student = new Student(txtID.Text, txtName.Text);
        pnlCourse.Visible = false;
        try
        {
            ((MyClass)Session["Course"]).addAStudent(new Student(txtID.Text, txtName.Text));
            txtAnswers.Text += Session["Course"].ToString();
        }
        catch(Exception ex)
        {
            txtAnswers.Text += "Amount of students are at maximum." + ex.Message;
        }
    }
}

Answer 1

The problem is in this loop:

for(int i = 0; i < students.Length; i++)
{
    classNumberAndName += ((Student)students[i]).ToString();
}

You are initialising students with students = new Student[10] so students.Length will be 10 even if you have inserted less than 10 students. The values you haven’t filled in are null, so you end up doing null.ToString, hence the NullReferenceException.

I believe you should be using numberOfStudents instead:

for(int i = 0; i < numberOfStudents; i++)
{
    classNumberAndName += ((Student)students[i]).ToString();
}