method with new keyword

Question

In this program new keyword is used in a SetCredits() method to hide similar method in the base class.

If you change ScholarshipStudent freeStudent = new ScholarshipStudent();
into
Student freeStudent = new ScholarshipStudent(); the output is

/*
Megan's tuition is $836.25
Luke's tuition is $836.25
*/

How can this be explained?

using System;
class DemoStudents
{
public static void Main()
{
Student payingStudent = new Student();
ScholarshipStudent freeStudent = new ScholarshipStudent();

payingStudent.SetName("Megan");
payingStudent.SetCredits(15);

freeStudent.SetName("Luke");
freeStudent.SetCredits(15);

Console.WriteLine("{0}'s tuition is {1}", payingStudent.GetName(), payingStudent.GetTuition().ToString("C"));

Console.WriteLine("{0}'s tuition is {1}",
freeStudent.GetName(),
freeStudent.GetTuition().ToString("C"));

Console.ReadKey();
}
}

class Student
{
private const double RATE = 55.75;
private string name;
protected int credits;
protected double tuition;

public string GetName()
{
return name;
}
public void SetName(string name)
{
this.name = name;
}

public void SetCredits(int creditHours)//SetCredits method in the child class as well
{
credits = creditHours;
tuition = credits * RATE;
}

public double GetTuition()//no SetTuition() method
{
return tuition;
}
}

class ScholarshipStudent : Student
{
public new void SetCredits(int creditHours) //polymorphism
{
credits = creditHours;
tuition = 0;
}
}
/*
Megan's tuition is $836.25
Luke's tuition is $0.00
*/

Vulpes · Answer

That's right, Maha. Luke's tuition will then be zero in both cases.

Maha · Answer

Thank you

Maha · Answer

That means if we use virtual keyword in the base class and override keyword in the derived class then only runtime type of freeStudent ( ScholarshipStudent ) will become active.

Maha · Answer

Thank you for your explanation.

Vulpes · Answer

As the code currently stands, freeStudent is a ScholarshipStudent variable and the latter class hides Student's version of SetCredits with its own version. Consequently, Luke's tuition depends on ScholarshipStudent's version of SetCredits which sets it to zero. However, when you change the line to this: Student freeStudent = new ScholarshipStudent(); freeStudent is now a Student variable, even though a reference to a ScholarshipStudent object has been assigned to it. Consequently, Luke's tuition now depends on Student's version of SetCredits which sets it to 15 x 55.75 = $836.25 which is the same as Megan. Notice that there is no polymorphism here. SetCredits is not a virtual method and so it's merely hidden by the child class method, not overridden by it. This means that the runtime type of freeStudent is immaterial - the version of SetCredits which gets called simply depends on its compile time type which is Student.

method with new keyword

Suthish Nair

Naresh Babu Gopavaram

Suthish Nair