nullreference on a method after initializing an object

Question

I am relatively new in C# and I just want to get used in the language.

I have three classes namely Student, Course and Grade. I just can't seem to understand why it keeps getting a null value.

class Student
{
    Name name;
    Course course;

    public Student(Course course)
    {
        this.course = course;
    }

    public Course C
    {
        get { return course; }
        set { course = value; }
    }

}

class Course
{
    string name;
    string code;
    int units;
    Grade grade; // this one warns as 'Course.grade' is never assigned to, and will always have its default value null

    public Course(string name, string code)
    {
        this.name = name;
        this.code = code;
    }

    public void DisplayDetails()
    {
        Console.WriteLine("Course: {0}({1})", code, name);
    }

    public int U
    {
        get { return units; }
        set { units = value; }
    }

    public Grade G
    {
        get { return grade; }
        set { value = grade; }
    }
}

class Grade
{
    double q;
    double o;
    double fe;
    double fg;

    public Grade(double q, double o, double fe)
    {
        this.q = q;
        this.o = o;
        this.fe = fe;
    }

    public void ComputeGrade()
    {
        fg = q * .5 + o * .2 + fe * .3;
    }

    public double C
    {
        get { return fg; }
        set { value = fg; }
    }

}

And this is my main class.

class Program
{
    static void Main(string[] args)
    {
        Student s = new Student(new Course("Something", "1234"));
        s.C.U = 3;
        s.N = new Name("John", "Snow");
        Grade grade = new Grade(90.0, 90.0, 90.0); // I have initialized an object here


        s.C.G.ComputeGrade(); //but this gets a null value

        s.N.Display2();
        s.C.DisplayDetails();
        Console.WriteLine(" - {0} units", s.C.U);
        Console.WriteLine(" Grade: {0}", s.C.G.C); //as well as this
        Console.ReadKey();

    }
}

I believe to have set the values for an object of the Grade class. I have also declared it's getter/setter. Am I doing wrong with the code or lacking on the code?

Set G before use: s.C.G = new Grade(90.0, 90.0, 90.0);, then you can use it s.C.G.ComputeGrade(); — Fabio
– Fabio, Commented May 6, 2017 at 15:27
@Fabio As CodeFuller's answer rightly points out, that's not going to have any effect, because G's property setter effectively doesn't do anything. — user743382
– user743382, Commented May 6, 2017 at 15:28
Actually Fabio is also right, having correct setter is half of the job. But the setter is never called in the code. — CodeFuller
– CodeFuller, Commented May 6, 2017 at 15:30

CodeFuller · Accepted Answer · 2017-05-06 15:26:50Z

3

Correct G setter in Course class:

public Grade G
{
    get { return grade; }
    set { grade = value; }
}

and set G value before using like Fabio suggested:

s.C.G = new Grade(90.0, 90.0, 90.0);

answered May 6, 2017 at 15:26

CodeFuller

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3 Comments

programmingisfun Over a year ago

thank you! I didn't quite notice that, it removed the warning, but I am still getting the NullReferenceException on the method ComputeGrade().

CodeFuller Over a year ago

Do you set value of G property as Fabio suggested?

programmingisfun Over a year ago

Yes! Question has been answered! Thank you!

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