Whether it is possible

Question

http://www.c-sharpcorner.com/Forums/Thread/277334/constructor-body.aspx

I wish to know whether it is possible to do in the following program the way code is highlighted in the above web page. Problem is coloured.

using System;
public class CreateEmployee
{
public static void Main()
{
int id = 345;

Employee myAssistant = new Employee(id);

Console.WriteLine("ID # is {0}", myAssistant.GetId());

Console.ReadKey();
}
}
internal class Employee
{
private int idNumber;

public Employee(Employee myA)
{
idNumber = myA.idNumber;
}
public int GetId()
{
return idNumber;
}
}
// ID # is 345

Vulpes · Answer

It's incorrect because as things stand the Employee class does not define an explicit conversion to int. However it will work if you add the highlighted code: using System; public class CreateEmployee { public static void Main() { int id = 345; Employee myAssistant = new Employee(id); Console.WriteLine(ID # is {0} myAssistant.GetId()); Employee myAssistant2 = new Employee(myAssistant); Console.WriteLine(ID # is {0} myAssistant2.GetId()); Console.ReadKey(); } } internal class Employee { private int idNumber; public Employee(int id) { idNumber = id; } public Employee(Employee myA) { idNumber = (int)myA; } public int GetId() { return idNumber; } public static explicit operator int(Employee myA) { return myA.idNumber; } } // ID # is 345

Vulpes · Answer

I've also just found an MSDN link on writing 'copy' constructors in C# which is worth reading: http://msdn.microsoft.com/en-us/library/ms173116.aspx

Maha · Answer

Thank you for your explanation.

Maha · Answer

This code is taken from the above program. public Employee( Employee myA) { idNumber = myA.idNumber; } Why it is incorrect to write following way public Employee( Employee myA) { idNumber = ( int ) myA; }

Maha · Answer

Thank you for your help.

Michal Habalcik · Answer

Yep I know but you have to have at least one instance constructor available. I see you have added it so it runs just fine.

Vulpes · Answer

I think what you have in mind Michal is where you introduce a field (as opposed to a constructor parameter) of the same type as the enclosing class. That certainly does create an infinite sequence of constructor calls leading to an out of memory exception. However copy constructors are OK - they're used in C++ all the time - and the code I posted above compiles and runs fine.

Michal Habalcik · Answer

it won't even compile. You have just one constuctor with the Employee parameter so giving it just pure int number won't work. Also the whole principle is just wrong. You can't use the class in its own constructor like this. The constructor awaits the Employee and that Employee again awaits Employee in the constructor. Something like: Employee myAssistant = new Employee ( new Employee ( new Employee (and so on a on)))

Vulpes · Answer

Yes it's perfectly possible to create one object from another of the same type using a 'copy' constructor. I've altered your code to achieve it here: using System; public class CreateEmployee { public static void Main() { int id = 345; Employee myAssistant = new Employee(id); Console.WriteLine(ID # is {0} myAssistant.GetId()); Employee myAssistant2 = new Employee(myAssistant); Console.WriteLine(ID # is {0} myAssistant2.GetId()); Console.ReadKey(); } } internal class Employee { private int idNumber; public Employee(int id) { idNumber = id; } public Employee(Employee myA) { idNumber = myA.idNumber; } public int GetId() { return idNumber; } } // ID # is 345