Method overriding with virtual and override in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the virtual keyword do in C#?

The <code>virtual</code> keyword allows a method in a base class to be overridden in a derived class. It marks the method as extendable or replaceable.

beginner

What is the purpose of the override keyword in C#?

The <code>override</code> keyword tells the compiler that a method in a derived class is replacing a <code>virtual</code> method from its base class.

intermediate

Can a method without <code>virtual</code> in the base class be overridden with <code>override</code> in the derived class?

No. Only methods marked <code>virtual</code> (or <code>abstract</code>) in the base class can be overridden using <code>override</code> in the derived class.

intermediate

What happens if you call a virtual method on a base class reference that points to a derived class object?

The overridden method in the derived class is called. This is called polymorphism and allows behavior to change based on the actual object type.

beginner

Show a simple example of method overriding using virtual and override.

Example:<br><pre>class Animal {
  public virtual void Speak() {
    Console.WriteLine("Animal speaks");
  }
}
class Dog : Animal {
  public override void Speak() {
    Console.WriteLine("Dog barks");
  }
}</pre>

Which keyword marks a method in the base class to allow overriding?

A virtual

B override

C new

D abstract

The virtual keyword marks a method as overridable in the base class.

What keyword do you use in the derived class to replace a base class virtual method?

A virtual

B new

C override

D sealed

The override keyword replaces the base class virtual method in the derived class.

If a base class method is not virtual, what happens if you try to override it?

A Compiler error

B It overrides successfully

C Runs base method anyway

D Method is hidden but not overridden

You get a compiler error because only virtual or abstract methods can be overridden.

Calling a virtual method on a base class reference that points to a derived object calls:

A Base class method

B Derived class overridden method

C Compiler error

D Random method

The derived class overridden method is called due to polymorphism.

Which keyword prevents further overriding of a virtual method?

A virtual

B override

C abstract

D sealed

The sealed keyword stops further overriding of a virtual method.

Explain how method overriding works using virtual and override keywords in C#.

Think about how a base class method can be changed in a child class.

Describe what happens if you call a virtual method on a base class reference that points to a derived class object.

Focus on which method actually runs at runtime.

Practice

1. What keyword in C# allows a method in a base class to be changed by a derived class?

easy

A. new

B. override

C. virtual

D. abstract

Solution

1. Step 1: Understand base class method flexibility

   The virtual keyword marks a method in the base class as changeable by derived classes.

2. Step 2: Differentiate from other keywords

   override is used in derived classes, new hides methods, and abstract requires implementation.

3. Final Answer:

   virtual -> Option C

4. Quick Check:

   Base method change = virtual [OK]

Hint: Base class method change uses virtual keyword [OK]

Common Mistakes:

• Confusing override with virtual
• Using new instead of virtual for overriding
• Thinking abstract allows method change without implementation

2. Which of the following is the correct syntax to override a virtual method named Display in a derived class?

easy

A. public void Display() { }

B. public virtual void Display() { }

C. public new void Display() { }

D. public override void Display() { }

Solution

1. Step 1: Identify override syntax

   To change a virtual method in a derived class, use override before the method signature.

2. Step 2: Eliminate other options

   The plain public void Display() { } lacks the override keyword, public virtual void Display() { } incorrectly uses virtual in the derived class, and public new void Display() { } hides the base method but doesn't override it for polymorphism.

3. Final Answer:

   public override void Display() { } -> Option D

4. Quick Check:

   Override method uses override keyword [OK]

Hint: Override methods must use override keyword [OK]

Common Mistakes:

• Omitting override keyword in derived class
• Using virtual instead of override in derived class
• Using new keyword instead of override

3. What will be the output of the following code?

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

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

medium

A. Bark
Meow

B. Animal sound
Animal sound

C. Bark
Animal sound

D. Animal sound
Meow

Solution

1. Step 1: Understand virtual and override behavior

   Because Speak is virtual and overridden, the derived class method runs even when referenced as base type.

2. Step 2: Trace the output calls

   a is a Dog instance, so Speak() returns "Bark"; b is a Cat instance, so it returns "Meow".

3. Final Answer:

   Bark Meow -> Option A

4. Quick Check:

   Override method output = Bark, Meow [OK]

Hint: Virtual method calls use derived override at runtime [OK]

Common Mistakes:

• Expecting base class method output
• Ignoring override effect on base class reference
• Confusing new keyword behavior with override

4. Identify the error in this code snippet:

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

Base obj = new Derived();
obj.Show();

medium

A. No error; output is Base

B. No error; output is Derived

C. Compile-time error: missing override keyword

D. Runtime error: method not found

Solution

1. Step 1: Check method overriding rules

   The derived class method Show does not use override, so it hides the base method instead of overriding.

2. Step 2: Determine method called by base reference

   Because Show is virtual in base but not overridden, calling obj.Show() calls base class method, outputting "Base".

3. Final Answer:

   No error; output is Base -> Option A

4. Quick Check:

   Missing override means base method runs [OK]

Hint: Override keyword needed to replace virtual method [OK]

Common Mistakes:

• Assuming method hides override automatically
• Expecting Derived output without override
• Thinking missing override causes compile error

5. Given the classes below, what will be the output?

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

hard

A. Vehicle
Car
SportsCar

B. Car
Car
SportsCar

C. Vehicle
Vehicle
Vehicle

D. SportsCar
SportsCar
SportsCar

Solution

1. Step 1: Understand new vs override

   SportsCar uses new to hide Describe, not override it. So base class virtual dispatch applies only up to Car.

2. Step 2: Trace each call

   v.Describe() calls Vehicle reference to SportsCar instance, but virtual dispatch stops at Car override, so returns "Car".
   c.Describe() calls Car reference to SportsCar, same as above, returns "Car".
   s.Describe() calls SportsCar reference, so calls hidden method returning "SportsCar".

3. Final Answer:

   Car Car SportsCar -> Option B

4. Quick Check:

   new hides method, override dispatches virtual [OK]

Hint: new hides method; override participates in virtual dispatch [OK]

Common Mistakes:

• Expecting new method to override virtual dispatch
• Confusing new with override behavior
• Assuming base reference calls hidden method