C#
Summary - Polymorphism
Combine virtual, override and pattern matching to write an employee hierarchy.
Summary: the polymorphism toolkit
In this group you met four key polymorphism tools: virtual/override for runtime dispatch, new for method hiding, is/as for type testing and safe casts, and switch expressions with type patterns.
Now you will combine them in a real scenario: an employee hierarchy. First you will compute salary with virtual dispatch, then distribute bonuses by position with a switch expression. The same code will behave differently for different subtypes; that is the power of polymorphism.
Exercise Compute salary with virtual dispatch
Define Employee with public virtual int Pay() that does return 1000;. Define Manager : Employee with override int Pay() that does return 2000;. In Main build a List<Employee> with new Manager() and new Employee(). In a foreach print each Pay() via Console.WriteLine.
Show example solution
Employee { public override int Pay() { return 2000; } }. In Main build List<Employee> { new Manager(), new Employee() } and foreach (var e in emps) Console.WriteLine(e.Pay());
Example solution:
using System;
using System.Collections.Generic;
class Employee
{
public virtual int Pay()
{
return 1000;
}
}
class Manager : Employee
{
public override int Pay()
{
return 2000;
}
}
class Program
{
static void Main()
{
var emps = new List<Employee> { new Manager(), new Employee() };
foreach (var e in emps)
{
Console.WriteLine(e.Pay());
}
}
}
Exercise Give bonuses with a switch expression
Reuse the Employee and Manager classes above. In Program write static int Bonus(Employee e) using a switch expression: return 500 for Manager, 100 for Employee. In Main call Bonus with a Manager and an Employee, printing each result with Console.WriteLine.
Show example solution
Write static int Bonus(Employee e) { return e switch { Manager m => 500, _ => 100 }; }. In Main add Console.WriteLine(Bonus(new Manager())); Console.WriteLine(Bonus(new Employee()));
Example solution:
using System;
class Employee { }
class Manager : Employee { }
class Program
{
static int Bonus(Employee e)
{
return e switch
{
Manager m => 500,
_ => 100
};
}
static void Main()
{
Console.WriteLine(Bonus(new Manager()));
Console.WriteLine(Bonus(new Employee()));
}
}
Polymorphism is yours
Congratulations! You now know the four polymorphism tools: virtual/override, new, is/as and switch expressions. You can manage many different subtypes through a single base reference and let runtime select the right behavior.
In the next groups you will combine these tools with collections, generic classes and LINQ. Polymorphism is a load-bearing pillar of object-oriented C#; most modern libraries are built on these mechanisms.
Test yourself
-
Which performs runtime dispatch?
Employee e = new Manager();
e.CalculatePay();
- Hiding with new
- A virtual + override call
- A static method call
- A sealed class call
Show answer
A virtual + override call — Correct. The virtual + override pair calls Manager.CalculatePay at runtime.
-
The operator for a type pattern
if (e ____ Manager m) { m.Bonus = 1000; }
Show answer
if (e is Manager m) { m.Bonus = 1000; }
Next lesson: Model design
