C# NullReferenceException Thrown: My StudentList static class array is null

Question

I have been trying to figure out what is happening with my code. I wrote an application where the user is able to enter marks of the student through a GUI application. The first form shows options, the next form is to enter the student information (name, number, mark), and the last form is to display a summary of the student information (total number of students, highest mark, lowest mark, name of student with highest mark, list of students).

To store all the student entries, I had made a student class. I created a static Student array and placed it in my ProgramFunctions class file (which is all static methods).

When I try to run the form which displays the student summary, thats where it crashes - If I look at the Auto's tab, it tells me the value of student[a] is null (I made use of a for loop to go through each student object in the array). I did trace through the adding of students and it does show that I have added new entries to the Student array.

Exceptions would be thrown at my calculation methods (highestMark, lowestMark, average). Here is my code:

class ProgramFunctions
{
    private static Student[] studentList = new Student[25];
    private static int counter = 0;

    public static void addNewStudent(Student newStudent)
    {
        if (studentList.Count() == counter)
        {
            MessageBox.Show("The Student List is Full", "List is Full");
        }
        else
        {
            studentList[counter] = newStudent;
            counter++;
        }

    }

    public static void displayErrorMessage(String message, String title)
    {
        MessageBox.Show(message, title);
    }

    public static TextBox validateTextBox(int textboxNumber, TextBox thisTextBox)
    {
        if (textboxNumber.Equals(1)) //Student Name textbox
        {
            if (thisTextBox.Text.Length < 0 || thisTextBox.Text.Length > 100)
            {
                displayErrorMessage("The Student Name specified is out of allowed region (greater than 100 or less than 0 characters. Please fix this", "Student Name Error");
            }
            else
            {
                thisTextBox.Text = thisTextBox.Text.Trim();
                return thisTextBox;
            }
        }
        else if (textboxNumber.Equals(2)) //Student number text box (only 10 characters allowed)
        {
            if (thisTextBox.Text.Length < 0 || thisTextBox.Text.Length > 10)
            {
                displayErrorMessage("The student number you specified is greater than 10 characters or less than 0. Please fix this", "Student Number Error");
            }
            else
            {
                thisTextBox.Text = thisTextBox.Text.Trim();
                return thisTextBox;
            }
        }
        else
        {
            if (thisTextBox.Text.Length > 2 || thisTextBox.Text.Length < 0)
            {
                displayErrorMessage("Invalid Length for exam mark", "Invalid Exam Mark");
            }
            else
            {
                thisTextBox.Text = thisTextBox.Text.Trim();
                return thisTextBox;
            }
        }
        return null;
    }

    public static int getMaximumMarkPosition()
    {
        int highestMark = -999;
        int positionFound = -1; 

        for (int a = 0; a < counter; a++)
        {
            if (studentList[a].getExamMark > highestMark)
            {
                highestMark = studentList[a].getExamMark; //This is where the error would occur
                positionFound = a;
            }
        }

        return positionFound;
    }

    public static int getHighestMark(int position)
    {
        return studentList[position].getExamMark;
    }

    public static int getLowestMark(int position)
    {
        return studentList[position].getExamMark;
    }

    public static string getHighestStudentMarkName(int position)
    {
        return studentList[position].getStudentName;
    }

    public static int getLowestMarkPosition()
    {
        int lowestMark = 999;
        int positionFound = -1;
        int studentMark = 0;

        for (int a = 0; a < studentList.Count(); a++)
        {


            studentMark = studentList[a].getExamMark; //This is where the error would occur
            if (studentMark < lowestMark)
            {
                lowestMark = studentMark;
                positionFound = a;
            }
        }

        return positionFound;
    }

    public static double calculateClassAverage()
    {
        double sum = 0;
        double average = 0;

        for (int a = 0; a < studentList.Count(); a++)
        {
            sum = sum + studentList[a].getExamMark;
        }

        average = sum / studentList.Count();

        return average;
    }

    public static int getTotalNumberOfStudents()
    {
        return counter;
    }

    public static RichTextBox getTextBoxData(RichTextBox thisTextBox)
    {
        thisTextBox.Text = "STUDENT MARK DATA: \n\n";

        for (int a = 1; a < studentList.Count(); a++)
        {
            Student currentStudent = returnStudentInformation(a);
            thisTextBox.Text = thisTextBox.Text + "\n" + currentStudent.getStudentName + "\t\t" + currentStudent.getExamMark + "\t" + currentStudent.getStudentNumber;
        }

        return thisTextBox;
    }

    public static Student returnStudentInformation(int index)
    {
        return studentList[index];
    }

}

}

Answer 1

It seems to me that if the student list isn't full accessing any index at counter or higher will result in a null object. I would suggest only looping up to counter:

for (int a = 1; a < counter; a++)
{
    Student currentStudent = returnStudentInformation(a);
    thisTextBox.Text = thisTextBox.Text + "\n" + currentStudent.getStudentName + "\t\t" + currentStudent.getExamMark + "\t" + currentStudent.getStudentNumber;
}

All this begs the question why use a static array for dynamic data when List<T> is available and is made for dynamic data.