I can not instantiate my Student class in Program.cs

Question

I have a Student class in my console application like this :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    public class Student
    {
        public Student(Student student)
        {
            StudentId = student.StudentId;
            FullName = student.FullName;
        }

        public int StudentId { get; set; }
        public string FullName { get; set; }
        
    }
}

It has a copy constructor in its body and when I want to instantiate it in the Program class like this:

Student student = new Student();

I will have an error that

"There is no argument given that corresponds to the required formal parameter 'student' of 'Student.Student(Student)'"

When I can not instantiate my Student class how could I pass the argument of Student to its constructor ?

I want to instantiate my Student class but I've got the error on it that I described.

Answer 1

You mention that you would like to use a copy constructor to create a new Student.

This requires you to have an existing Student instance to copy from.

Moreover - as long as you only have a copy constructor you will never be able to create such an instance (because the implicit default constructor will no longer be available).

The solution is to add a constructor - either a default one (without parameters), or one that requires e.g. an int and a String and initializes the fields of the new instance:

// Default constructor:
public Student()
{
   StudentId = -1;        // some default ID
   FullName = "SomeName"; // some default name
}

Or:

// Constructor with parameters:
public Student(int id, String name)
{
   StudentId = id;
   FullName = name;
}

(or even both).
Note that if you use the default constructor you should probably set the fields to proper values afterwards.

Only after creating an instance with such a constructor you can use your copy constructor:

// Create a default instance and then set the fields:
Student studentToCopyFrom = new Student();
studentToCopyFrom.StudentId  = 111;
studentToCopyFrom.FullName = "Joe";
// Or create an instance with arguments for the fields:
Student studentToCopyFrom = new Student(111, "Joe");

// Now create a new instance with the copy constructor:
Student student = new Student(studentToCopyFrom);