Override in c#

The override modifier is required to extend or modify the abstract or virtual implementation of an inherited method, property, indexer, or event.

In this example, the Square class must provide an overridden implementation of Area because Area is inherited from the abstract ShapesClass:

abstract class ShapesClass
{
    abstract public int Area();
}
class Square : ShapesClass
{
    int side = 0;

    public Square(int n)
    {
        side = n;
    }
    // Area method is required to avoid
    // a compile-time error.
    public override int Area()
    {
        return side * side;
    }

    static void Main() 
    {
        Square sq = new Square(12);
        Console.WriteLine("Area of the square = {0}", sq.Area());
    }

    interface I
    {
        void M();
    }
    abstract class C : I
    {
        public abstract void M();
    }

}
// Output: Area of the square = 144

An override method provides a new implementation of a member that is inherited from a base class. The method that is overridden by an override declaration is known as the overridden base method. The overridden base method must have the same signature as the override method. For information about inheritance, see Inheritance (C# Programming Guide).[MSDN LINK]

You cannot override a non-virtual or static method. The overridden base method must be virtual, abstract, or override.

An override declaration cannot change the accessibility of the virtual method. Both the override method and the virtual method must have the same access level modifier.[MSDN LINK]

You cannot use the new, static, or virtual modifiers to modify an override method.

An overriding property declaration must specify exactly the same access modifier, type, and name as the inherited property, and the overridden property must be virtual, abstract, or override.

For more information about how to use the override keyword, see Versioning with the Override and New Keywords (C# Programming Guide) and Knowing when to use Override and New Keywords.[MSDN LINK]

This example defines a base class named Employee, and a derived class named SalesEmployee. The SalesEmployee class includes an extra property,salesbonus, and overrides the method CalculatePay in order to take it into account.

class TestOverride
{
    public class Employee
    {
        public string name;

        // Basepay is defined as protected, so that it may be 
        // accessed only by this class and derrived classes.
        protected decimal basepay;

        // Constructor to set the name and basepay values.
        public Employee(string name, decimal basepay)
        {
            this.name = name;
            this.basepay = basepay;
        }

        // Declared virtual so it can be overridden.
        public virtual decimal CalculatePay()
        {
            return basepay;
        }
    }

    // Derive a new class from Employee.
    public class SalesEmployee : Employee
    {
        // New field that will affect the base pay.
        private decimal salesbonus;

        // The constructor calls the base-class version, and
        // initializes the salesbonus field.
        public SalesEmployee(string name, decimal basepay, 
                  decimal salesbonus) : base(name, basepay)
        {
            this.salesbonus = salesbonus;
        }

        // Override the CalculatePay method 
        // to take bonus into account.
        public override decimal CalculatePay()
        {
            return basepay + salesbonus;
        }
    }

    static void Main()
    {
        // Create some new employees.
        SalesEmployee employee1 = new SalesEmployee("Alice", 
                      1000, 500);
        Employee employee2 = new Employee("Bob", 1200);

        Console.WriteLine("Employee4 " + employee1.name + 
                  " earned: " + employee1.CalculatePay());
        Console.WriteLine("Employee4 " + employee2.name + 
                  " earned: " + employee2.CalculatePay());
    }
}
/*
    Output:
    Employee4 Alice earned: 1500
    Employee4 Bob earned: 1200
*/