Not use "override" keyword in abstract method

Let's illustrate the difference with an example:

using System;

abstract class Animal
{
    public abstract void MakeSound();
}

class Dog : Animal
{
    // This method is specific to the Dog class and does not override MakeSound from Animal
    public void MakeSound()
    {
        Console.WriteLine("Woof");
    }
}

class Program
{
    static void Main(string[] args)
    {
        Animal dog = new Dog();
        dog.MakeSound(); // This won't compile because MakeSound in Dog is not considered an override
    }
}

In this example, the MakeSound method in the Dog class does not override the abstract method MakeSound from the Animal class because it doesn't use the override keyword. As a result, when you try to call MakeSound on a reference of type Animal that points to a Dog object, it won't compile because MakeSound is not defined in the Animal class.

Now, let's fix this by properly overriding the method in the Dog class:

using System;

abstract class Animal
{
    public abstract void MakeSound();
}

class Dog : Animal
{
    // This method overrides MakeSound from Animal
    public override void MakeSound()
    {
        Console.WriteLine("Woof");
    }
}

class Program
{
    static void Main(string[] args)
    {
        Animal dog = new Dog();
        dog.MakeSound(); // This will compile and print "Woof"
    }
}

Now, the MakeSound method in the Dog class correctly overrides the abstract method MakeSound from the Animal class. When you call MakeSound on an Animal reference pointing to a Dog object, it will execute the implementation provided in the Dog class, printing "Woof".

So, when you try to compile the code dog.MakeSound();, it will indeed throw a compilation error, indicating that there is no suitable method MakeSound available for the Animal reference.