Runtime polymorphism execution in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is runtime polymorphism in C#?

Runtime polymorphism means that the method to be called is determined during program execution, not at compile time. It allows a base class reference to call derived class methods using method overriding.

beginner

How do you enable runtime polymorphism in C#?

By declaring a method in the base class as <code>virtual</code> and overriding it in the derived class using <code>override</code>. Then, calling the method through a base class reference triggers the derived class version at runtime.

beginner

What keyword is used in the base class method to allow overriding?

The <code>virtual</code> keyword marks a method in the base class as overridable, enabling runtime polymorphism.

intermediate

What happens if you call a non-virtual method through a base class reference?

The base class method is called, even if the derived class has a method with the same name. This is because non-virtual methods are resolved at compile time, not runtime.

beginner

Explain the role of the override keyword in runtime polymorphism.

The <code>override</code> keyword tells the compiler that the derived class method replaces the base class virtual method. This enables the runtime to call the correct method version based on the actual object type.

Which keyword in C# allows a method to be overridden for runtime polymorphism?

A virtual

B static

C sealed

D const

The virtual keyword marks a method as overridable, enabling runtime polymorphism.

What keyword must a derived class method use to override a base class virtual method?

A new

B override

C virtual

D abstract

The override keyword is used in the derived class to replace the base class virtual method.

If a method is not marked virtual, what happens when called through a base class reference?

A Derived class method is called

B Compile error occurs

C Runtime error occurs

D Base class method is called

Non-virtual methods are resolved at compile time, so the base class method is called.

What determines which method is called in runtime polymorphism?

A Runtime type of the object

B Compile-time type of the reference

C Method name length

D Number of parameters

The actual object type at runtime decides which overridden method is executed.

Which of these is NOT required for runtime polymorphism in C#?

A Base class method marked virtual

B Calling method through base class reference

C Method marked as static

D Derived class method marked override

Static methods cannot be overridden and do not support runtime polymorphism.

Describe how runtime polymorphism works in C# with an example.

Think about how a base class pointer can call a method that behaves differently depending on the actual object.

Explain the difference between virtual and non-virtual methods in the context of runtime polymorphism.

Consider what happens when you call a method through a base class reference.

Practice

1. What does runtime polymorphism in C# allow you to do?

easy

A. Create multiple instances of the same class

B. Change variable types at runtime

C. Call derived class methods through a base class reference

D. Use static methods without creating objects

Solution

1. Step 1: Understand runtime polymorphism concept

   Runtime polymorphism allows a base class reference to call methods overridden in derived classes.

2. Step 2: Identify correct behavior

   This means the actual method called depends on the object's real type, not the reference type.

3. Final Answer:

   Call derived class methods through a base class reference -> Option C

4. Quick Check:

   Runtime polymorphism = base ref calls derived method [OK]

Hint: Think: base class ref calls derived method at runtime [OK]

Common Mistakes:

• Confusing polymorphism with changing variable types
• Thinking static methods are polymorphic
• Believing polymorphism creates multiple instances

2. Which keyword is used in C# to allow a method to be overridden in a derived class?

easy

A. virtual

B. override

C. new

D. abstract

Solution

1. Step 1: Identify keyword to enable overriding

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

2. Step 2: Understand roles of keywords

   override is used in derived classes, virtual in base classes.

3. Final Answer:

   virtual -> Option A

4. Quick Check:

   Base method uses virtual to allow override [OK]

Hint: Base method uses virtual; derived uses override [OK]

Common Mistakes:

• Using override in base class instead of virtual
• Confusing new keyword with override
• Thinking abstract is required for all overrides

3. What is the output of this C# code?

class Animal {
  public virtual string Speak() => "Animal sound";
}
class Dog : Animal {
  public override string Speak() => "Bark";
}
class Cat : Animal {
  public override string Speak() => "Meow";
}

Animal a = new Dog();
Console.WriteLine(a.Speak());

medium

A. Animal sound

B. Bark

C. Meow

D. Compile error

Solution

1. Step 1: Identify object type and method called

   Variable a is of type Animal but references a Dog object.

2. Step 2: Apply runtime polymorphism

   Since Speak is virtual and overridden in Dog, the Dog version runs, printing "Bark".

3. Final Answer:

   Bark -> Option B

4. Quick Check:

   Base ref calls Dog's Speak() = Bark [OK]

Hint: Base ref calls derived override method at runtime [OK]

Common Mistakes:

• Expecting base class method output
• Confusing object type with reference type
• Thinking compile error due to override

4. Identify the error in this C# code related to runtime polymorphism:

class Base {
  public override void Show() {
    Console.WriteLine("Base Show");
  }
}
class Derived : Base {
  public override void Show() {
    Console.WriteLine("Derived Show");
  }
}

medium

A. Base class method must be virtual, not override

B. Derived class method cannot override base method

C. Missing semicolon after method declaration

D. No error, code is correct

Solution

1. Step 1: Check base class method declaration

   Base class method incorrectly uses override instead of virtual.

2. Step 2: Understand override rules

   Only derived classes use override; base class must use virtual to allow overriding.

3. Final Answer:

   Base class method must be virtual, not override -> Option A

4. Quick Check:

   Base method needs virtual keyword [OK]

Hint: Base method uses virtual, not override [OK]

Common Mistakes:

• Using override in base class method
• Thinking override is allowed without virtual
• Ignoring method signature correctness

5. Given these classes:

class Vehicle {
  public virtual string Describe() => "Vehicle";
}
class Car : Vehicle {
  public override string Describe() => "Car";
}
class SportsCar : Car {
  public override string Describe() => "SportsCar";
}

Vehicle v = new SportsCar();
Car c = new SportsCar();
SportsCar s = new SportsCar();

Console.WriteLine(v.Describe());
Console.WriteLine(c.Describe());
Console.WriteLine(s.Describe());

What is the output?

hard

A. Vehicle\nCar\nSportsCar

B. Car\nCar\nCar

C. Vehicle\nVehicle\nVehicle

D. SportsCar\nSportsCar\nSportsCar

Solution

1. Step 1: Identify actual object type for all references

   All variables v, c, and s reference a SportsCar object.

2. Step 2: Apply runtime polymorphism for Describe()

   Since Describe is overridden in SportsCar, all calls print "SportsCar" regardless of reference type.

3. Final Answer:

   SportsCar\nSportsCar\nSportsCar -> Option D

4. Quick Check:

   All calls use SportsCar override [OK]

Hint: Actual object type decides method, not reference type [OK]

Common Mistakes:

• Assuming base class method runs for base type variable
• Confusing reference type with object type
• Ignoring override in most derived class