More with override in C#

Not Using "virtual" keyword

If you don't use the virtual keyword in the base class, you won't be able to use the override keyword in the derived class to override that method, property, indexer, or event. In this case, if you want to redefine the behavior of a method in the derived class without using virtual or override, you can use the new keyword.

Here's an example illustrating the use of the new keyword instead of override when the method in the base class is not declared as virtual:

1. Method without using virtual and override:

using System;

public class Animal
{
    // Method without the virtual keyword
    public void Speak()
    {
        Console.WriteLine("Animal speaks");
    }
}

public class Dog : Animal
{
    // Use the new keyword to redefine the Speak method
    public new void Speak()
    {
        Console.WriteLine("Dog barks");
    }
}

public class Program
{
    public static void Main()
    {
        Animal myAnimal = new Animal();
        myAnimal.Speak(); // Output: Animal speaks

        Dog myDog = new Dog();
        myDog.Speak(); // Output: Dog barks

        Animal myAnimalDog = new Dog();
        myAnimalDog.Speak(); // Output: Animal speaks (base class method is called)
    }
}

In the above example:

• The Speak method in the Animal class is not declared with the virtual keyword.

• In the Dog class, the new keyword is used to redefine the Speak method.

• When calling the Speak method on a variable of type Animal but referencing a Dog object, the Speak method of the base class Animal is called, not that of the derived class Dog.

2. Property without using virtual and override:

using System;

public class Animal
{
    // Property without the virtual keyword
    public string Name { get; set; } = "Animal";
}

public class Dog : Animal
{
    // Use the new keyword to redefine the Name property
    public new string Name { get; set; } = "Dog";
}

public class Program
{
    public static void Main()
    {
        Animal myAnimal = new Animal();
        Console.WriteLine(myAnimal.Name); // Output: Animal

        Dog myDog = new Dog();
        Console.WriteLine(myDog.Name); // Output: Dog

        Animal myAnimalDog = new Dog();
        Console.WriteLine(myAnimalDog.Name); // Output: Animal (base class property is called)
    }
}

In this example:

• The Name property in the Animal class is not declared with the virtual keyword.

• In the Dog class, the new keyword is used to redefine the Name property.

• When accessing the Name property on a variable of type Animal but referencing a Dog object, the Name property of the base class Animal is called, not that of the derived class Dog.

In summary, if you don't use the virtual keyword, you won't be able to override methods, properties, indexers, or events using the override keyword. Instead, you can use the new keyword to redefine them in the derived class. However, this does not change the behavior when calling those members through a base class type variable.

Only using "override" keyword

csharpSao chép mãusing System;

public class Animal
{
    public void Speak()
    {
        Console.WriteLine("Animal speaks");
    }
}

public class Dog : Animal
{
    public override void Speak() // Error: 'Animal.Speak()' is not marked virtual, abstract, or override
    {
        Console.WriteLine("Dog barks");
    }
}

public class Program
{
    public static void Main()
    {
        Dog myDog = new Dog();
        myDog.Speak();
    }
}

Not using "new" keyword

If you don't use the new keyword and you're not using override, the behavior depends on whether the method, property, indexer, or event in the derived class has the same signature as the member in the base class.

1. Method without using override or new:

using System;

public class Animal
{
    public void Speak()
    {
        Console.WriteLine("Animal speaks");
    }
}

public class Dog : Animal
{
    public void Speak() // Without 'override' or 'new'
    {
        Console.WriteLine("Dog barks");
    }
}

public class Program
{
    public static void Main()
    {
        Animal myAnimal = new Animal();
        myAnimal.Speak(); // Output: Animal speaks

        Dog myDog = new Dog();
        myDog.Speak(); // Output: Dog barks

        Animal myAnimalDog = new Dog();
        myAnimalDog.Speak(); // Output: Dog barks (derived class method is called)
    }
}

In this example:

• The Speak method in the Dog class has the same signature as the one in the Animal class (same name and parameters).

• When calling Speak on a variable of type Animal but referencing a Dog object, the Speak method of the Dog class is called. This is because the compiler uses the most derived version of the method available at compile time, which is the one in the Dog class.

2. Property without using override or new:

csharpSao chép mãusing System;

public class Animal
{
    public string Name { get; set; } = "Animal";
}

public class Dog: Animal
{
    public string Name { get; set; } = "Dog"; // Without 'override' or 'new'
}

public class Program
{
    public static void Main()
    {
        Animal myAnimal = new Animal();
        Console.WriteLine(myAnimal.Name); // Output: Animal

        Dog myDog = new Dog();
        Console.WriteLine(myDog.Name); // Output: Dog

        Animal myAnimalDog = new Dog();
        Console.WriteLine(myAnimalDog.Name); // Output: Dog (derived class property is called)
    }
}

Similarly:

• The Name property in the Dog class has the same signature as the one in the Animal class.

• When accessing the Name property on a variable of type Animal but referencing a Dog object, the Name property of the Dog class is called. This is because the compiler uses the most derived version of the property available at compile time, which is the one in the Dog class.

So, without using override or new, if the method, property, indexer, or event in the derived class has the same signature as the member in the base class, it replaces (hides) the member in the base class.