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.

You did not define a default constructor (one that gets no arguments), so new Student() cannot work. What do you expect the Student fields to hold in this case ? — wohlstad
– wohlstad, Commented May 12, 2024 at 18:31
In C#, classes do have an implicit default constructor, unless you define a constructor yourself (which you did), then you also have to define the default (parameterless) constructor explicitly. — marsze
– marsze, Commented May 12, 2024 at 18:32
You mean that I have to add another constructor to my Student class ? — Fatemeh Rostamipour
– Fatemeh Rostamipour, Commented May 12, 2024 at 18:32
Yes, a default one (without parameters). And you should probably set the fields to some meaninful default value in it. — wohlstad
– wohlstad, Commented May 12, 2024 at 18:32

wohlstad · Accepted Answer · 2024-05-12 19:13:50Z

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);

Agota Kristof · Accepted Answer · 2024-05-12 18:34:40Z

1

As soon as you wrote some kind of constructor - in your case the copy constructor, then the default constructor is deleted, And so when you do: Student student = new Student(); It has no empty constructor to refer to. Your solution is to simply create a default constructor that initializes the parameters with the default you choose

public Student()
    {
        StudentId = 0;//for example
        FullName = "Jhon Doe";//for example
    }

answered May 12, 2024 at 18:34

Agota Kristof

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Comments

marsze · Accepted Answer · 2024-05-12 18:49:45Z

1

If you need a copy-constructor, use a record type, which already has one by default:

public record class Student(int StudentId, string FullName);

Usage example:

var student2 = student1 with { StudentId = 42 };

answered May 12, 2024 at 18:49

marsze

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5 Comments

wohlstad Over a year ago

As far as I know a proper copy-constructor is one that accepts an exitsing instance to copy from (not the value for the fields). See here.

marsze Over a year ago

@wohlstad Yes, and records have an implicit one, so you could basically just do var student2 = student1 with {} to create a 1:1 copy (also there is no real benefit in doing that, if the type is immutable)

wohlstad Over a year ago

I see. Good to know. As for the benefit - the OP asked for it specifically, I assumed for educational purposes, and so I answered accordingly.

marsze Over a year ago

@wohlstad I figured the point that confused them was that they couldn't use the default constructor anymore, that's what I addressed in my comment. The info about records is just in addition to the other answers.

wohlstad Over a year ago

You might be right. I added an explicit note about that to my answer to clear up any confusion of the sort you mentioned.

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