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POBeginner headaches: NullReferenceException from array on ASP.NET webfrom
 

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  1. POBeginner headaches: NullReferenceException from array on ASP.NET webfrom
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    <p>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 <strong>MyClass ( the course name and number)</strong> and the <strong>Student class ( where it holds the student name and ID)</strong>. 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 <strong>not allowed</strong> to enter any new methods and variables. I am only able to use the current ones I have initialized.</p> <ul> <li>The Student[] array is the array that holds my Student class with the name and id of students. </li> <li>The user first will create the class number and name in which to add students(with id and name) to it afterwards.</li> <li>Only 1 class name and number(they go together) is allowed and only 10 students is the maximum.</li> </ul> <p><em><strong>I am getting the null exception on my ToString() method in MyClass where classNumberAndName += ((Student)students[i]).ToString();</em></strong></p> <p>Here's my code with no syntax errors. </p> <pre><code>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 &lt; 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 &lt; 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 &lt; 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; } } </code></pre> <p>And here is the .aspx.cs file for the web code. </p> <pre><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; } } } </code></pre>
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    1. USDavid Robinson
    1. USGivenPie
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    1. COPlease make it clear where you are getting the exception (and preferably make sample much smaller 5-7lines is usually enough). Also "thank you notes" are not needed as they add no information to the question(upvote/comment/accept instead).
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      1. POBeginner headaches: NullReferenceException from array on ASP.NET webfrom
      1. USAlexei Levenkov
    2. COOk, I just added the "I am getting the null exception on my ToString() method in MyClass where classNumberAndName += ((Student)students[i]).ToString();" I'm guessing all you need to see is the MyClass and the aspx.cs file. I was afraid I did not construct the array correctly.
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      1. POBeginner headaches: NullReferenceException from array on ASP.NET webfrom
      1. USGivenPie
 

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