Polymorphism means that a operation can also be applied to values of some other types.

There are multiple types of Polymorphism:

• Ad hoc polymorphism:
  contains function overloading. The target is that a Method can be used with different types without the need of being generic.
• Parametric polymorphism:
  is the use of generic types. See Generics
• Subtyping:
  has the target inherit of a class to generalize a similar functionality

---

Ad hoc polymorphism

The target of Ad hoc polymorphism is to create a method, that can be called by different datatypes without a need of type-conversion in the function call or generics. The following method(s) sumInt(par1, par2) can be called with different datatypes and has for each combination of types a own implementation:

public static int sumInt( int a, int b)
{
    return a + b;    
}

public static int sumInt( string a, string b)
{
    int _a, _b;
    
    if(!Int32.TryParse(a, out _a))
        _a = 0;
    
    if(!Int32.TryParse(b, out _b))
        _b = 0;
    
    return _a + _b;
}

public static int sumInt(string a, int b)
{
    int _a;
    
    if(!Int32.TryParse(a, out _a))
        _a = 0;    
    
    return _a + b;
}

public static int sumInt(int a, string b)
{        
    return sumInt(b,a);
}

Here's a example call:

public static void Main()
{
    Console.WriteLine(sumInt( 1 , 2 ));  //  3
    Console.WriteLine(sumInt("3","4"));  //  7
    Console.WriteLine(sumInt("5", 6 ));  // 11
    Console.WriteLine(sumInt( 7 ,"8"));  // 15
}

---

Subtyping

Subtyping is the use of inherit from a base class to generalize a similar behavior:

public interface Car{
    void refuel();
}

public class NormalCar : Car
{
    public void refuel()
    {
        Console.WriteLine("Refueling with petrol");    
    }
}

public class ElectricCar : Car
{
    public void refuel()
    {
        Console.WriteLine("Charging battery");    
    }
}

Both classes NormalCar and ElectricCar now have a method to refuel, but their own implementation. Here's a Example:

public static void Main()
{
    List<Car> cars = new List<Car>(){
        new NormalCar(),
        new ElectricCar()
    };
    
    cars.ForEach(x => x.refuel());
}

The output will be was following:

Refueling with petrol
Charging battery

Polymorphism is one of the pillar of OOP. Poly derives from a Greek term which means 'multiple forms'.

Below is an example which exhibits Polymorphism. The class Vehicle takes multiple forms as a base class.

The Derived classes Ducati and Lamborghini inherits from Vehicle and overrides the base class's Display() method, to display its own NumberOfWheels.

public class Vehicle
{
    protected int NumberOfWheels { get; set; } = 0;
    public Vehicle()
    {
    }

    public virtual void Display()
    {
        Console.WriteLine($"The number of wheels for the {nameof(Vehicle)} is {NumberOfWheels}");
    }
}

public class Ducati : Vehicle
{
    public Ducati()
    {
        NoOfWheels = 2;
    }

    public override void Display()
    {
        Console.WriteLine($"The number of wheels for the {nameof(Ducati)} is {NumberOfWheels}");
    }
}

public class Lamborghini : Vehicle
{
    public Lamborghini()
    {
        NoOfWheels = 4;
    }

    public override void Display()
    {
        Console.WriteLine($"The number of wheels for the {nameof(Lamborghini)} is {NumberOfWheels}");
    }
}

Below is the code snippet where Polymorphism is exhibited. The object is created for the base type Vehicle using a variable vehicle at Line 1. It calls the base class method Display() at Line 2 and display the output as shown.

 static void Main(string[] args)
 {
    Vehicle vehicle = new Vehicle();    //Line 1
    vehicle.Display();                  //Line 2  
    vehicle = new Ducati();             //Line 3
    vehicle.Display();                  //Line 4
    vehicle = new Lamborghini();        //Line 5
    vehicle.Display();                  //Line 6
 }

At Line 3, the vehicle object is pointed to the derived class Ducati and calls its Display() method, which displays the output as shown. Here comes the polymorphic behavior, even though the object vehicle is of type Vehicle, it calls the derived class method Display() as the type Ducati overrides the base class Display() method, since the vehicle object is pointed towards Ducati.

The same explanation is applicable when it invokes the Lamborghini type's Display() method.

The Output is shown below

The number of wheels for the Vehicle is 0        // Line 2 
The number of wheels for the Ducati is 2         // Line 4
The number of wheels for the Lamborghini is 4    // Line 6