Interface as contract mental model in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface named IAnimal.

C Sharp (C#)

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

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

AIAnimal

BAnimal

CInterfaceAnimal

DAnimalInterface

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name instead of an interface name.

Not starting the interface name with 'I'.

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

2fill in blank

medium

Complete the code to make the Dog class implement the IAnimal interface.

C Sharp (C#)

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

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

AIDog

BDogInterface

CIAnimal

DAnimal

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name instead of the interface name after the colon.

Forgetting to implement the interface methods.

The class Dog must implement the interface IAnimal to fulfill the contract.

3fill in blank

hard

Fix the error in the code by completing the interface method declaration.

C Sharp (C#)

public interface IAnimal {
    [1] Speak();
}

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

Apublic

Bint

Cstring

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Adding 'public' keyword inside interface method declarations.

Using a return type that does not match the implementation.

Interface methods do not have access modifiers and must specify the return type. Here, void means the method returns nothing.

4fill in blank

hard

Fill both blanks to create a dictionary that maps animal names to their sounds using an interface and class.

C Sharp (C#)

Dictionary<string, string> animalSounds = new Dictionary<string, string> {
    {"Dog", [1],
    {"Cat", [2]
};

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

A"Woof"

B"Meow"

C"Bark"

D"Purr"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect animal sounds.

Forgetting to put quotes around string values.

The dog sound is "Woof" and the cat sound is "Meow". These strings represent the sounds animals make.

5fill in blank

hard

Fill all three blanks to implement the IAnimal interface in the Cat class with a Speak method.

C Sharp (C#)

public class [1] : [2] {
    public [3] Speak() {
        Console.WriteLine("Meow");
    }
}

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

ACat

BIAnimal

Cvoid

DAnimal

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong class or interface names.

Incorrect return type in the method declaration.

The class name is Cat, it implements the interface IAnimal, and the Speak method returns void.

Practice

1.

What does an interface in C# represent?

easy

A. A contract that defines methods a class must implement

B. A class that contains method implementations

C. A variable type that stores data

D. A method that runs automatically

Solution

1. Step 1: Understand the role of an interface

   An interface defines a set of method signatures without implementations.

2. Step 2: Compare with classes

   Classes implement interfaces by providing method bodies, fulfilling the contract.

3. Final Answer:

   A contract that defines methods a class must implement -> Option A

4. Quick Check:

   Interface = contract for methods [OK]

Hint: Interfaces define method rules, not code bodies [OK]

Common Mistakes:

• Thinking interfaces contain method code
• Confusing interfaces with classes
• Believing interfaces store data

2.

Which of the following is the correct way to declare an interface in C#?

?

easy

A. interface IAnimal { void Speak(); }

B. class IAnimal { void Speak(); }

C. interface IAnimal() { void Speak(); }

D. interface IAnimal { void Speak() {} }

Solution

1. Step 1: Check interface declaration syntax

   Interfaces use the keyword 'interface' followed by the name and method signatures without bodies.

2. Step 2: Identify correct method signature

   Method declarations in interfaces do not have bodies, so no curly braces after method.

3. Final Answer:

   interface IAnimal { void Speak(); } -> Option A

4. Quick Check:

   Interface syntax = keyword + method signatures [OK]

Hint: Interfaces have method signatures only, no bodies [OK]

Common Mistakes:

• Adding parentheses after interface name
• Using class keyword instead of interface
• Providing method bodies inside interface

3.

What will be the output of the following code?

interface IWorker { void Work(); }
class Employee : IWorker {
  public void Work() { Console.WriteLine("Employee working"); }
}
class Robot : IWorker {
  public void Work() { Console.WriteLine("Robot working"); }
}
class Program {
  static void Main() {
    IWorker w = new Robot();
    w.Work();
  }
}

medium

A. No output

B. Employee working

C. Compilation error

D. Robot working

Solution

1. Step 1: Identify the object type assigned to interface variable

   The variable 'w' is of type IWorker but assigned a new Robot instance.

2. Step 2: Determine which Work() method runs

   Calling w.Work() runs Robot's Work method, printing "Robot working".

3. Final Answer:

   Robot working -> Option D

4. Quick Check:

   Interface variable calls actual object's method [OK]

Hint: Interface calls method of assigned object's class [OK]

Common Mistakes:

• Assuming interface variable calls Employee method
• Expecting compilation error due to interface
• Thinking no output will print

4.

Identify the error in this code snippet:

interface IShape {
  double Area();
}
class Circle : IShape {
  public double Area() {
    return 3.14 * radius * radius;
  }
}

medium

A. Interface method cannot return double

B. Missing radius field or property in Circle class

C. Circle class should not implement IShape

D. Area method should be void

Solution

1. Step 1: Check Circle class members

   The method Area uses 'radius' but no radius variable or property is declared in Circle.

2. Step 2: Understand interface method return type

   Interface method returning double is valid; no error there.

3. Final Answer:

   Missing radius field or property in Circle class -> Option B

4. Quick Check:

   Undefined variable 'radius' causes error [OK]

Hint: Check all variables used are declared [OK]

Common Mistakes:

• Thinking interface methods can't return values
• Believing class can't implement interface
• Assuming method return type must be void

5.

You want to create a system where different devices can Start() and Stop() but each device does it differently. How should you use interfaces to design this?

hard

A. Create a base class Device with Start and Stop methods and inherit it

B. Write Start and Stop methods directly in each device class without interface

C. Define an interface IDevice with Start and Stop methods, then implement it in each device class

D. Use abstract classes only, no interfaces

Solution

1. Step 1: Understand interface purpose

   Interfaces define a contract for methods without implementation, perfect for different device behaviors.

2. Step 2: Apply interface to devices

   Define IDevice with Start and Stop, then each device class implements these methods with its own details.

3. Final Answer:

   Define an interface IDevice with Start and Stop methods, then implement it in each device class -> Option C

4. Quick Check:

   Interface = shared method rules, different implementations [OK]

Hint: Use interfaces for shared method names, different code [OK]

Common Mistakes:

• Using base class limits flexibility
• Skipping interface loses contract benefits
• Confusing abstract classes with interfaces