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Week 8: Polymorphism in C#

⁠

1. Introduction to Polymorphism

Definition: Polymorphism allows objects of different classes to be treated as objects of a common base class.
Benefit: Enables writing flexible, reusable code by calling methods on base types that can be implemented differently in derived classes.
⁠

2. Virtual Methods and Overriding

Virtual Method: A method in a base class that can be redefined (overridden) in derived classes.
Declared with the virtual keyword in the base class and overridden in derived classes using the override keyword.
Example:
csharp
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class Animal
{
public virtual void Speak() { Console.WriteLine("Animal sound"); }
}

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

Calling the Base Method: Within an overridden method, the base class’s method can be called using base.MethodName().
⁠

3. Abstract Classes

Abstract Class: A class that cannot be instantiated and may contain abstract methods (methods without implementation).
Abstract Method: Must be implemented by any non-abstract derived class.
Syntax:
csharp
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abstract class Animal
{
public abstract void Speak();
}

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

Purpose: Provides a common template for derived classes, enforcing certain behaviors across different implementations.
⁠

4. Interfaces

Definition: Interfaces define a contract that implementing classes must follow, but they do not provide implementations.
Syntax:
csharp
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interface IAnimal
{
void Speak();
}

class Dog : IAnimal
{
public void Speak() { Console.WriteLine("Woof"); }
}

Key Points:
Interfaces contain only method signatures.
A class can implement multiple interfaces, unlike inheritance which is limited to one base class.
⁠

5. Static vs. Dynamic Types in Polymorphism

Static Type: The type of the variable as declared at compile time (e.g., Animal myAnimal).
Dynamic Type: The actual instance type that the variable refers to at runtime (e.g., Dog or Cat assigned to Animal myAnimal).
Example:
csharp
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Animal myAnimal = new Dog(); // Static type is Animal, dynamic type is Dog
myAnimal.Speak(); // Calls Dog's Speak method

⁠

6. Polymorphic Assignment

Polymorphic Behavior: Allows a variable of a base type to hold references to derived types, enabling the use of derived class methods overridden from the base.
Example:
csharp
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Animal a = new Dog();
Animal b = new Cat();
a.Speak(); // Calls Dog's Speak
b.Speak(); // Calls Cat's Speak

⁠

7. Liskov Substitution Principle (LSP)

Principle: Any instance of a derived class should be able to replace an instance of its base class without altering the correctness of the program.
Example: If Dog is derived from Animal, Dog should work wherever Animal is expected, adhering to the same interface or behavior contract as Animal​.
⁠

Quick Exam Tips for MCQs

Differentiate Abstract Classes and Interfaces: Know that abstract classes can contain implementations, but interfaces cannot.
Recognize Polymorphic Behavior: Be clear on how static and dynamic types work with overridden methods.
Understand Key Terms: Virtual methods, abstract methods, and interface requirements are often tested.
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