Topic 7512 min readStructured Tutorial

C# Polymorphism: Method Overriding Tutorial

Quick Answer

Method overriding in C# allows a derived class to provide a specific implementation of a method already defined in its base class. It enables runtime polymorphism, letting the program decide which method to invoke based on the object type at runtime.

Learning Objectives

Explain the purpose of Method Overriding in a practical learning context.
Identify the main ideas, terms, and decisions involved in Method Overriding.
Apply Method Overriding in a simple real-world scenario or practice task.

Introduction to Method Overriding in C#

Polymorphism is a core concept in object-oriented programming that allows objects to be treated as instances of their parent class rather than their actual class.

Method overriding is a key technique to achieve polymorphism in C#. It lets a derived class change the behavior of a method defined in its base class.

This tutorial explains method overriding with simple examples and practical guidance.

Polymorphism allows methods to do different things based on the object calling them.

Understanding Method Overriding

Method overriding occurs when a derived class provides a new implementation for a method declared in its base class.

The base class method must be marked with the 'virtual' keyword to allow overriding.

The derived class method uses the 'override' keyword to indicate it replaces the base class method.

Enables runtime polymorphism.
Supports dynamic method dispatch.
Allows extending or modifying base class behavior.

Virtual and Override Keywords

The 'virtual' keyword in the base class signals that a method can be overridden.

The 'override' keyword in the derived class indicates the method replaces the base implementation.

Base class method: virtual void Display() { ... }
Derived class method: override void Display() { ... }

Rules for Method Overriding

The overriding method must have the same signature as the base method.

Access modifiers must be compatible (usually public or protected).

The base method cannot be static or private.

Signature includes method name, return type, and parameters.

Example of Method Overriding in C#

Here is a simple example demonstrating method overriding in C#.

Practical Example

The base class Animal defines a virtual method Speak. The derived class Dog overrides Speak to provide its own implementation. At runtime, the correct method is called depending on the object type.

Examples

Basic Method Overriding Example

using System;

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

class Dog : Animal
{
    public override void Speak()
    {
        Console.WriteLine("Dog barks");
    }
}

class Program
{
    static void Main()
    {
        Animal myAnimal = new Animal();
        Animal myDog = new Dog();

        myAnimal.Speak();  // Output: Animal speaks
        myDog.Speak();     // Output: Dog barks
    }
}

The base class Animal defines a virtual method Speak. The derived class Dog overrides Speak to provide its own implementation. At runtime, the correct method is called depending on the object type.

Best Practices

Always mark base class methods you expect to override with 'virtual'.
Use 'override' keyword explicitly in derived classes to avoid accidental hiding.
Call base method using 'base.MethodName()' if you want to extend base behavior.
Keep method signatures consistent between base and derived classes.
Avoid overriding methods unnecessarily to keep code maintainable.

Common Mistakes

Forgetting to mark the base method as 'virtual'.
Using 'new' keyword instead of 'override', which hides rather than overrides.
Changing method signature in derived class causing hiding instead of overriding.
Trying to override static or private methods.
Not calling base method when needed, leading to incomplete behavior.

Hands-on Exercise

Implement Method Overriding

Create a base class Shape with a virtual method Draw. Derive classes Circle and Rectangle that override Draw with their own implementations. Instantiate each and call Draw to see polymorphism in action.

Expected output: Circle and Rectangle print their specific draw messages when Draw is called.

Hint: Use 'virtual' in Shape and 'override' in derived classes.

Interview Questions

What is method overriding in C#?

Interview

Method overriding allows a derived class to provide a specific implementation of a method already defined in its base class, enabling runtime polymorphism.

What keywords are used for method overriding in C#?

Interview

The base class method is marked with 'virtual', and the derived class method uses 'override' to provide a new implementation.

Can you override a static method in C#?

Interview

No, static methods cannot be overridden because they belong to the class, not an instance.