Interface vs abstract class decision in C Sharp (C#) - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface named IShape.

C Sharp (C#)

public interface [1] { void Draw(); }

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

AShape

BIShape

CAbstractShape

DShapeInterface

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name instead of an interface name.

Not following the naming convention for interfaces.

The interface name should start with 'I' by convention, so 'IShape' is correct.

2fill in blank

medium

Complete the code to declare an abstract class named Shape.

C Sharp (C#)

public abstract class [1] { public abstract void Draw(); }

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

AShape

BIShape

CAbstractShape

DShapeInterface

Attempts:

3 left

💡 Hint

Common Mistakes

Using interface naming for abstract classes.

Confusing interface and abstract class names.

The abstract class should be named 'Shape' without the 'I' prefix.

3fill in blank

hard

Fix the error in this class declaration that tries to inherit from an interface and an abstract class.

C Sharp (C#)

public class Circle : [1], IShape { public override void Draw() { } }

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

ACircleBase

BIShape

CAbstractShape

DShape

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to inherit from multiple classes.

Using interface name where abstract class is expected.

The class should inherit from the abstract class 'Shape' and implement the interface 'IShape'.

4fill in blank

hard

Fill both blanks to complete the interface and abstract class usage correctly.

C Sharp (C#)

public interface [1] { void Draw(); } public abstract class [2] { public abstract void Draw(); }

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

AIShape

BShape

CDrawable

DAbstractDrawable

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing interface and class naming conventions.

Using unrelated names for interface or abstract class.

The interface is named 'IShape' and the abstract class is named 'Shape' to follow conventions.

5fill in blank

hard

Fill all three blanks to implement a class that inherits from an abstract class and implements an interface.

C Sharp (C#)

public interface [1] { void Draw(); } public abstract class [2] { public abstract void Draw(); } public class [3] : [2], [1] { public override void Draw() { } }

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

AIShape

BShape

CCircle

DDrawable

Attempts:

3 left

💡 Hint

Common Mistakes

Using interface name as class name.

Not overriding abstract methods properly.

The interface is 'IShape', the abstract class is 'Shape', and the class implementing both is 'Circle'.

Practice

1. Which statement best describes when to use an interface instead of an abstract class in C#?

easy

A. Use an interface when you want to provide shared code to subclasses.

B. Use an interface when unrelated classes share behavior but do not share code.

C. Use an abstract class when unrelated classes share behavior but do not share code.

D. Use an abstract class only when no methods need to be implemented.

Solution

1. Step 1: Understand interfaces

   Interfaces define method signatures without implementation, so they are ideal for unrelated classes that share behavior but not code.

2. Step 2: Understand abstract classes

   Abstract classes can provide shared code and force subclasses to implement certain methods, so they are better for related classes sharing code.

3. Final Answer:

   Use an interface when unrelated classes share behavior but do not share code. -> Option B

4. Quick Check:

   Interface = unrelated classes sharing behavior [OK]

Hint: Interfaces = behavior only, abstract classes = shared code [OK]

Common Mistakes:

• Thinking abstract classes can't have implemented methods
• Using abstract class for unrelated classes
• Confusing interfaces as code providers

2. Which of the following is the correct syntax to declare an abstract class in C#?

easy

A. abstract class Vehicle { public abstract void Move(); }

B. interface Vehicle { void Move(); }

C. class abstract Vehicle { public void Move(); }

D. abstract Vehicle class { void Move(); }

Solution

1. Step 1: Recall abstract class syntax

   In C#, the keyword abstract precedes class, followed by the class name and method declarations.

2. Step 2: Check each option

   abstract class Vehicle { public abstract void Move(); } correctly uses "abstract class Vehicle" and declares an abstract method. Options A, C, and D have incorrect keyword order or use interface syntax.

3. Final Answer:

   abstract class Vehicle { public abstract void Move(); } -> Option A

4. Quick Check:

   abstract class syntax = "abstract class" [OK]

Hint: abstract class keyword order: 'abstract class' [OK]

Common Mistakes:

• Swapping 'class abstract' instead of 'abstract class'
• Using interface syntax for abstract class
• Missing 'abstract' keyword before method

3. Consider the following code snippet:

interface IAnimal { void Speak(); }
abstract class Mammal { public void Breathe() { Console.WriteLine("Breathing"); } public abstract void Speak(); }
class Dog : Mammal, IAnimal { public override void Speak() { Console.WriteLine("Woof"); } }

var dog = new Dog();
dog.Breathe();
dog.Speak();

What will be the output when this code runs?

medium

A. Breathing
Woof

B. Woof
Breathing

C. Breathing

D. Compilation error due to multiple inheritance

Solution

1. Step 1: Understand class and interface usage

   The class Dog inherits from abstract class Mammal and implements interface IAnimal. It overrides Speak() and inherits Breathe().

2. Step 2: Trace method calls

   Calling dog.Breathe() prints "Breathing". Calling dog.Speak() prints "Woof" as overridden in Dog.

3. Final Answer:

   Breathing Woof -> Option A

4. Quick Check:

   Abstract class method + override = correct output [OK]

Hint: Abstract class methods run normally; override abstract methods [OK]

Common Mistakes:

• Thinking multiple inheritance causes error in C#
• Confusing order of output lines
• Missing override keyword causing compile error

4. The following code causes a compilation error. What is the main reason?

abstract class Shape {
  public abstract void Draw();
}

class Circle : Shape {
  public void Draw() {
    Console.WriteLine("Drawing Circle");
  }
}

medium

A. Draw() method must be static in Circle.

B. Abstract classes cannot have abstract methods.

C. Circle cannot inherit from Shape because Shape is abstract.

D. Circle must declare Draw() as override, not just public.

Solution

1. Step 1: Identify abstract method implementation rules

   When a class inherits an abstract method, it must override it using the override keyword.

2. Step 2: Check Circle class method

   The Draw() method in Circle is declared as public void Draw() but missing override, causing a compile error.

3. Final Answer:

   Circle must declare Draw() as override, not just public. -> Option D

4. Quick Check:

   Override abstract method = must use 'override' keyword [OK]

Hint: Override abstract methods with 'override' keyword [OK]

Common Mistakes:

• Omitting 'override' keyword on abstract method implementation
• Thinking abstract classes can't have abstract methods
• Assuming static needed for overridden methods

5. You need to design a system where multiple unrelated classes must implement a method Log() but also share some common logging code. Which approach is best in C#?

hard

A. Use only an interface ILogger with Log() method and no shared code.

B. Use only an abstract class with Log() as abstract method and shared code implemented.

C. Create an interface ILogger with Log() and a separate abstract class with shared code, then have classes implement both.

D. Use a concrete class with Log() and inherit it in all classes.

Solution

1. Step 1: Analyze requirements

   Multiple unrelated classes must implement Log() and share some common code.

2. Step 2: Choose interface and abstract class combination

   Interfaces allow unrelated classes to share method signatures. Abstract classes can provide shared code. Classes can implement interface and inherit abstract class to get both.

3. Step 3: Evaluate other options

   Using only abstract class limits inheritance to related classes. Using only interface lacks shared code. Concrete class inheritance limits flexibility.

4. Final Answer:

   Create an interface ILogger with Log() and a separate abstract class with shared code, then have classes implement both. -> Option C

5. Quick Check:

   Interface + abstract class = behavior + shared code [OK]

Hint: Combine interface for behavior and abstract class for shared code [OK]

Common Mistakes:

• Trying to use only abstract class for unrelated classes
• Ignoring shared code needs
• Assuming multiple inheritance of classes is allowed