Reading object value from other class in C#

Question

I always have coded in C++ but I am learning C# now. I got a trouble with accessing the object value.

Here is what I tried.

class Program
{         
    static void Main(string[] args)
    {
        StudentInfo[] student = new StudentInfo[2];

        student[0] = new StudentInfo(100, 4);
        student[1] = new StudentInfo(101, 3);

        Trying ty = new Trying();
        ty.trial(); /// I got trouble with this line
    }
}
class Trying
{
    StudentInfo[] student = new StudentInfo[2]; // I added this line since it prevents compiler error
    public void trial()
    {   
        Console.WriteLine(student[1].Gpa); // I got trouble with this line
    }
}
class StudentInfo
{        
    public int studentNo {get; set;}
    public int Gpa {get; set;}
    public StudentInfo(int cc, int ct)
    {
        studentNo = cc;
        Gpa = ct;
    }
}

The error says

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.

How can I access the value student[1].Gpa correctly in the class Trying?

Answer 1

In your Trying class you just created an array named student of type StudentInfo then with trial() method you tried to access element at index 1 of the student array which is null by default because you have not added any element to your array.

here is my solution: in your trial method you can get two arguments like this:

public void trial(StudentInfo std1, StudentInfo std2)
{
    student[0] = std1;
    student[1] = std2;
    Console.WriteLine(student[1].Gpa); 
}

and then in your Main method: you can pass two object from your StudentInfo class to the trial() method:

static void Main(string[] args)
{
    StudentInfo[] student = new StudentInfo[2];
    
    StudentInfo std1 = new StudentInfo(100, 4);
    StudentInfo std2 = new StudentInfo(101, 3);

    Trying ty = new Trying();
    ty.trial(std1,std2); 
}

This way you can achieve the result you want.

Answer 2

The student in your main is NOT connected to the student in your Trying. You can disregard student code in your main. In Trying You did initialize an array but not added any objects. Array of nulls. See in comments how to fix it

class Trying
{
    StudentInfo[] students = new StudentInfo[2]; // <-- init array of nulls 

    // option 1 - init in constructor
    public Trying()
    {
        // option 1
        students[0] = new StudentInfo(1, 1); 
        students[1] = new StudentInfo(2, 2);
        students[2] = new StudentInfo(3, 3);
    }

    

    // option 2 - init in class declatrations
    StudentInfo[] students = new StudentInfo[] 
    {
        new StudentInfo(1, 1),
        new StudentInfo(2, 2),
        new StudentInfo(3, 3)
    };
  
    public void trial()
    {   
        Console.WriteLine(student[1].Gpa); // I got trouble with this line
    }
}

Ideal code

class Trying
{
    private StudentInfo[] students = new StudentInfo[] 
    {
        new StudentInfo(1, 1),
        new StudentInfo(2, 2),
        new StudentInfo(3, 3)
    };
  
    public void trial()
    {   
        Console.WriteLine(students[1].Gpa); // I got trouble with this line
    }
}

Answer 3

The problem here is that you are indeed creating a StudentInfo array, but you are not passing it into the Trying class. The StudentInfo array you create inside the Trying class does solve the compiler error, but it is composed of object with no initialized values. I think a good approach would be to create a constructor for the Trying class into which you can pass your StudentInfo array, so that it contains the values you set in the Program Main method. Here you have an example:

class Program
{
    static void Main(string[] args)
    {
        StudentInfo[] students = new StudentInfo[2]; //<-- changed student to students

        students[0] = new StudentInfo(100, 4);
        students[1] = new StudentInfo(101, 3);

        Trying ty = new Trying(students); //<-- Here you pass the array from above
        ty.trial(); /// I got trouble with this line
    }
}
class Trying
{
    StudentInfo[] students; //<-- Also added the plural, the "s", since it is a colletion of objects
    public Trying(StudentInfo[] students)
    {
        this.students = students; //<-- Here the passed array is used to initialize the students field of the trying class
    }

    public void trial()
    {
        Console.WriteLine(students[1].Gpa); //<-- Now the second student is no longer null, same with its Gpa field
    }
}
class StudentInfo
{
    public int studentNo { get; set; }
    public int Gpa { get; set; }
    public StudentInfo(int cc, int ct)
    {
        studentNo = cc;
        Gpa = ct;
    }
}