Base keyword behavior in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the base class method from the derived class.

C Sharp (C#)

public class Animal {
    public virtual string Speak() {
        return "Animal sound";
    }
}

public class Dog : Animal {
    public override string Speak() {
        return [1].Speak();
    }
}

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

Abase

Bthis

Csuper

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' instead of 'base' to call the base class method.

Using 'super' which is not a C# keyword.

The 'base' keyword in C# is used to access members of the base class from a derived class.

2fill in blank

medium

Complete the constructor to call the base class constructor with a parameter.

C Sharp (C#)

public class Vehicle {
    public string Brand;
    public Vehicle(string brand) {
        Brand = brand;
    }
}

public class Car : Vehicle {
    public Car(string brand) : [1](brand) {
    }
}

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

Athis

Bbase

Csuper

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' instead of 'base' to call the base constructor.

Using 'super' which is not valid in C#.

In C#, the 'base' keyword is used to call the base class constructor from a derived class constructor.

3fill in blank

hard

Fix the error by correctly calling the base class method inside the overridden method.

C Sharp (C#)

public class Printer {
    public virtual void Print() {
        Console.WriteLine("Printing from Printer");
    }
}

public class ColorPrinter : Printer {
    public override void Print() {
        [1].Print();
        Console.WriteLine("Printing in color");
    }
}

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

Abase

Bthis

Csuper

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' which calls the current class method causing recursion.

Using 'super' which is not a C# keyword.

The 'base' keyword is used to call the base class method from an overridden method in C#.

4fill in blank

hard

Fill both blanks to correctly override a method and call the base method inside it.

C Sharp (C#)

public class Logger {
    public virtual void Log(string message) {
        Console.WriteLine("Log: " + message);
    }
}

public class FileLogger : Logger {
    public override void Log(string message) {
        [1].Log(message);
        Console.WriteLine("Writing to file");
    }
}

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

Abase

Bthis

Csuper

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'super' or 'parent' which are invalid in C#.

Using 'this' to call the base method causing recursion.

Use 'base' to call the base class method and 'this' to refer to the current instance if needed.

5fill in blank

hard

Fill all three blanks to override a method, call the base method, and add extra behavior.

C Sharp (C#)

public class Shape {
    public virtual double Area() {
        return 0;
    }
}

public class Circle : Shape {
    private double radius;
    public Circle(double radius) {
        this.radius = radius;
    }
    public override double Area() {
        double baseArea = [1].Area();
        double circleArea = Math.PI * radius * radius;
        return baseArea + [2];
    }
}

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

Athis

BcircleArea

Cbase

Dradius

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' instead of 'base' to call the base method.

Returning only circleArea without adding baseArea.

Use 'base' to call the base method, add 'circleArea' to it, and 'this' refers to the current instance.

Practice

1. What does the base keyword do in C#?

easy

A. It creates a new instance of a class.

B. It allows a child class to access members of its parent class.

C. It defines a new class.

D. It deletes an object from memory.

Solution

1. Step 1: Understand the role of base

   The base keyword is used in a child class to refer to its parent class members.

2. Step 2: Identify what base allows

   It allows access to parent class methods, properties, or constructors from the child class.

3. Final Answer:

   It allows a child class to access members of its parent class. -> Option B

4. Quick Check:

   base keyword = access parent members [OK]

Hint: Remember: base = parent class access [OK]

Common Mistakes:

• Confusing base with new instance creation
• Thinking base deletes objects
• Assuming base defines a class

2. Which of the following is the correct way to call a parent class constructor using base in C#?

easy

A. public Child() : base() { }

B. public Child() base() { }

C. public Child() call base() { }

D. public Child() : parent() { }

Solution

1. Step 1: Recall syntax for calling parent constructor

   In C#, to call a parent constructor, use : base() after the child constructor signature.

2. Step 2: Match correct syntax

   public Child() : base() { } uses public Child() : base() { }, which is the correct syntax.

3. Final Answer:

   public Child() : base() { } -> Option A

4. Quick Check:

   Parent constructor call = : base() [OK]

Hint: Use colon and base() after constructor name [OK]

Common Mistakes:

• Omitting colon before base()
• Using base() inside constructor body incorrectly
• Using wrong keyword like parent()

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

class Parent {
    public virtual void Show() {
        Console.WriteLine("Parent Show");
    }
}
class Child : Parent {
    public override void Show() {
        base.Show();
        Console.WriteLine("Child Show");
    }
}

var obj = new Child();
obj.Show();

medium

A. Child Show

B. Parent Show

C. Compilation error

D. Parent Show\nChild Show

Solution

1. Step 1: Understand method overriding and base call

   The Child class overrides Show() and calls base.Show() which runs the Parent version first.

2. Step 2: Trace the output

   First, "Parent Show" is printed from base.Show(), then "Child Show" is printed from the child method.

3. Final Answer:

   Parent Show Child Show -> Option D

4. Quick Check:

   base.Method() runs parent method first [OK]

Hint: base.Method() runs parent method inside override [OK]

Common Mistakes:

• Ignoring base.Show() call
• Expecting only child output
• Thinking code causes error

4. Identify the error in this code snippet:

class Parent {
    public void Display() {
        Console.WriteLine("Parent Display");
    }
}
class Child : Parent {
    public override void Display() {
        base.Display();
        Console.WriteLine("Child Display");
    }
}

medium

A. Cannot override non-virtual method Display() in Parent.

B. base.Display() is invalid syntax.

C. Child class must not call base.Display().

D. No error, code runs fine.

Solution

1. Step 1: Check method overriding rules

   In C#, only methods marked virtual or abstract in the parent can be overridden.

2. Step 2: Analyze the Parent class method

   The Display() method in Parent is not virtual, so override in Child causes a compile error.

3. Final Answer:

   Cannot override non-virtual method Display() in Parent. -> Option A

4. Quick Check:

   Override requires virtual method [OK]

Hint: Only virtual methods can be overridden [OK]

Common Mistakes:

• Trying to override non-virtual method
• Confusing base call syntax
• Assuming override works without virtual

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

class A {
    public virtual string GetName() => "A";
}
class B : A {
    public override string GetName() => "B";
}
class C : B {
    public override string GetName() => base.GetName() + "C";
}

var obj = new C();
Console.WriteLine(obj.GetName());

hard

A. AC

B. Compilation error

C. BC

D. C

Solution

1. Step 1: Trace method calls through inheritance

   Class C overrides GetName() and calls base.GetName(), which refers to B's override returning "B".

2. Step 2: Combine returned strings

   C appends "C" to the result from B, so the final string is "B" + "C" = "BC".

3. Final Answer:

   BC -> Option C

4. Quick Check:

   base.GetName() calls immediate parent method [OK]

Hint: base calls immediate parent method, not grandparent [OK]

Common Mistakes:

• Assuming base calls grandparent method
• Ignoring string concatenation
• Expecting only "C" output