The C# language is designed so that versioning between base and derived classes in different libraries can evolve and maintain backwards compatibility. This means, for example, that the introduction of a new member in a base class with the same name as a member in a derived class is completely supported by C# and does not lead to unexpected behavior. It also means that a class must explicitly state whether a method is intended to override an inherited method, or whether a method is a new method that simply hides a similarly named inherited method.

C# allows derived classes to contain methods with the same name as base class methods.

The base class method must be defined virtual.

If the method in the derived class is not preceded by new or override keywords, the compiler will issue a warning and the method will behave as if the new keyword were present.

If the method in the derived class is preceded with the new keyword, the method is defined as being independent of the method in the base class.

If the method in the derived class is preceded with the override keyword, objects of the derived class will call that method rather than the base class method.

The base class method can be called from within the derived class using the base keyword.

The override, virtual, and new keywords can also be applied to properties, indexers, and events.

By default, C# methods are not virtual ¡X if a method is declared as virtual, any class inheriting the method can implement its own version. To make a method virtual, the virtual modifier is used in the method declaration of the base class. The derived class can then override the base virtual method by using the override keyword or hide the virtual method in the base class by using the new keyword. If neither the override keyword nor the new keyword is specified, the compiler will issue a warning and the method in the derived class will hide the method in the base class. For more information, see Compiler Warning CS0108.



// Define the base class
class Car
{
    public virtual void DescribeCar()
    {
        System.Console.WriteLine("Four wheels and an engine.");
    }
}

// Define the derived classes
class ConvertibleCar : Car
{
    public new virtual void DescribeCar()
    {
        base.DescribeCar();
        System.Console.WriteLine("A roof that opens up.");
    }
}

class Minivan : Car
{
    public override void DescribeCar()
    {
        base.DescribeCar();
        System.Console.WriteLine("Carries seven people.");
    }
}

public static void TestCars1()
{
    Car car1 = new Car();
    car1.DescribeCar();
    System.Console.WriteLine("----------");

    ConvertibleCar car2 = new ConvertibleCar();
    car2.DescribeCar();
    System.Console.WriteLine("----------");

    Minivan car3 = new Minivan();
    car3.DescribeCar();
    System.Console.WriteLine("----------");
}

output:
Four wheels and an engine. 

---------- 

Four wheels and an engine. 

A roof that opens up. 

---------- 

Four wheels and an engine. 

Carries seven people. 

----------