Method overriding with virtual and override in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a method that can be overridden in a derived class.

C Sharp (C#)

public class Animal {
    public [1] void Speak() {
        Console.WriteLine("Animal sound");
    }
}

Drag options to blanks, or click blank then click option'

Avirtual

Boverride

Cstatic

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'override' in the base class method declaration.

Using 'static' which does not allow overriding.

Using 'sealed' which prevents overriding.

The virtual keyword allows a method to be overridden in a derived class.

2fill in blank

medium

Complete the code to override the Speak method in the Dog class.

C Sharp (C#)

public class Dog : Animal {
    public [1] void Speak() {
        Console.WriteLine("Bark");
    }
}

Drag options to blanks, or click blank then click option'

Anew

Bsealed

Coverride

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' instead of 'override' in the derived class.

Using 'new' which hides the base method but does not override.

Using 'sealed' which is not valid here.

The override keyword is used in the derived class to provide a new implementation of a virtual method.

3fill in blank

hard

Fix the error in the derived class method declaration to correctly override the base method.

C Sharp (C#)

public class Cat : Animal {
    public [1] void Speak() {
        Console.WriteLine("Meow");
    }
}

Drag options to blanks, or click blank then click option'

Avirtual

Bsealed

Cstatic

Doverride

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' in the derived class method.

Using 'static' which is not allowed for overriding.

Omitting the override keyword.

To override a virtual method from the base class, the derived method must use the override keyword.

4fill in blank

hard

Fill both blanks to declare a virtual method in the base class and override it in the derived class.

C Sharp (C#)

public class Bird {
    public [1] void Fly() {
        Console.WriteLine("Flying");
    }
}

public class Sparrow : Bird {
    public [2] void Fly() {
        Console.WriteLine("Sparrow flying fast");
    }
}

Drag options to blanks, or click blank then click option'

Avirtual

Bstatic

Coverride

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' in either method which prevents overriding.

Using 'sealed' incorrectly.

Omitting the override keyword in the derived class.

The base class method must be virtual to allow overriding, and the derived class method must use override to replace it.

5fill in blank

hard

Fill all three blanks to override a virtual method and call the base method inside the override.

C Sharp (C#)

public class Vehicle {
    public [1] void Start() {
        Console.WriteLine("Vehicle starting");
    }
}

public class Car : Vehicle {
    public [2] void Start() {
        [3].Start();
        Console.WriteLine("Car starting with extra steps");
    }
}

Drag options to blanks, or click blank then click option'

Avirtual

Boverride

Cbase

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling the base method inside the override.

Using 'sealed' incorrectly.

Omitting the override keyword in the derived method.

The base method is declared virtual. The derived method uses override and calls the base method with base.

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