Interface as contract mental model in C Sharp (C#) - Mini Project: Build & Apply

Interface as contract mental model

📖 Scenario: Imagine you are building a simple system for different types of vehicles. Each vehicle must be able to start and stop. We will use an interface to make sure every vehicle follows this rule.

🎯 Goal: You will create an interface called IVehicle that acts like a contract. Then you will create classes that follow this contract by implementing the interface. Finally, you will show how to use these classes through the interface.

📋 What You'll Learn

Create an interface called IVehicle with two methods: Start() and Stop().

Create a class called Car that implements the IVehicle interface.

Create a class called Bike that implements the IVehicle interface.

Create a variable of type IVehicle and assign it to instances of Car and Bike.

Call the Start() and Stop() methods on the IVehicle variable and print messages.

💡 Why This Matters

🌍 Real World

Interfaces are used in software to ensure different parts follow the same rules, like how different vehicles must be able to start and stop.

💼 Career

Understanding interfaces is important for writing clean, maintainable code and working with many programming frameworks and libraries.

1

Create the IVehicle interface

Create an interface called IVehicle with two methods: void Start() and void Stop().

C Sharp (C#)

// Your code here

Hint

An interface is like a promise that classes will have certain methods.

2

Create the Car class implementing IVehicle

Create a class called Car that implements the IVehicle interface. Implement the Start() method to print "Car started" and the Stop() method to print "Car stopped".

C Sharp (C#)

public interface IVehicle
{
    void Start();
    void Stop();
}

// Your code here

Hint

Use : IVehicle after the class name to implement the interface.

3

Create the Bike class implementing IVehicle

Create a class called Bike that implements the IVehicle interface. Implement the Start() method to print "Bike started" and the Stop() method to print "Bike stopped".

C Sharp (C#)

public interface IVehicle
{
    void Start();
    void Stop();
}

public class Car : IVehicle
{
    public void Start()
    {
        System.Console.WriteLine("Car started");
    }

    public void Stop()
    {
        System.Console.WriteLine("Car stopped");
    }
}

// Your code here

Hint

Implement the interface methods just like in the Car class.

4

Use the IVehicle interface to control vehicles

Create a variable of type IVehicle called vehicle. Assign it to a new Car() instance. Call vehicle.Start() and vehicle.Stop(). Then assign vehicle to a new Bike() instance. Call vehicle.Start() and vehicle.Stop() again.

C Sharp (C#)

public interface IVehicle
{
    void Start();
    void Stop();
}

public class Car : IVehicle
{
    public void Start()
    {
        System.Console.WriteLine("Car started");
    }

    public void Stop()
    {
        System.Console.WriteLine("Car stopped");
    }
}

public class Bike : IVehicle
{
    public void Start()
    {
        System.Console.WriteLine("Bike started");
    }

    public void Stop()
    {
        System.Console.WriteLine("Bike stopped");
    }
}

// Your code here

Hint

Use the interface type IVehicle to hold different vehicle objects and call their methods.

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