Interface as contract mental model in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of interface method call

What is the output of this C# code that uses an interface as a contract?

C Sharp (C#)

interface IAnimal {
    string Speak();
}

class Dog : IAnimal {
    public string Speak() {
        return "Woof!";
    }
}

class Program {
    static void Main() {
        IAnimal pet = new Dog();
        System.Console.WriteLine(pet.Speak());
    }
}

A Woof! Woof!

B Woof!

C Compile-time error: Dog does not implement IAnimal

D Runtime error: Cannot convert Dog to IAnimal

💡 Hint

Remember, the interface defines a method contract that the class must implement.

The class Dog implements the IAnimal interface by providing the Speak method. The variable pet is of type IAnimal but holds a Dog instance. Calling Speak() on pet calls Dog's implementation, which returns "Woof!".

🧠 Conceptual

intermediate

Interface contract enforcement

Which statement best describes the role of an interface in C#?

A An interface defines a contract that classes must follow by implementing its methods and properties.

B An interface can hold state and fields to share data between classes.

C An interface provides default implementations for all its methods.

D An interface is a class that can be instantiated directly.

💡 Hint

Think about what an interface requires from classes that use it.

Interfaces in C# declare method and property signatures without implementation. Classes that implement the interface must provide those implementations, thus following the contract. Interfaces cannot hold state or provide default implementations (except default interface methods in newer versions, but that is advanced and not the main contract idea).

🔧 Debug

advanced

Why does this interface implementation fail?

Given the code below, why does it fail to compile?

C Sharp (C#)

interface IVehicle {
    void Drive();
}

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

class Program {
    static void Main() {
        IVehicle v = new Car();
        v.Drive();
    }
}

A Method name in Car is lowercase 'drive' instead of 'Drive', so it does not implement the interface method.

B Interface IVehicle cannot be implemented by a class named Car.

C The interface method Drive must return a value, but Car's method returns void.

D The class Car must be declared abstract to implement an interface.

💡 Hint

Check method names and case sensitivity carefully.

C# is case-sensitive. The interface requires a method named Drive with uppercase D. The class Car defines drive with lowercase d, which is a different method. Therefore, Car does not implement the interface method, causing a compile error.

📝 Syntax

advanced

Correct interface implementation syntax

Which option shows the correct way to implement this interface in C#?

C Sharp (C#)

interface IShape {
    double Area();
}

A

class Circle : IShape {
    public void Area() {
        return 3.14 * 2 * 2;
    }
}

B

class Circle implements IShape {
    public double Area() {
        return 3.14 * 2 * 2;
    }
}

C

class Circle : IShape {
    double Area() {
        return 3.14 * 2 * 2;
    }
}

D

class Circle : IShape {
    public double Area() {
        return 3.14 * 2 * 2;
    }
}

💡 Hint

Remember the correct keyword for implementing interfaces and method signatures.

Option D uses the correct syntax: class Circle : IShape to implement the interface and declares Area as a public method returning double. Option D uses Java syntax implements which is invalid in C#. Option D misses the public modifier, causing a compile error because interface methods must be public. Option D has wrong return type void instead of double.

🚀 Application

expert

Interface contract with multiple implementations

Consider this interface and two classes implementing it. What is the output when running the program?

C Sharp (C#)

interface ILogger {
    void Log(string message);
}

class ConsoleLogger : ILogger {
    public void Log(string message) {
        System.Console.WriteLine($"Console: {message}");
    }
}

class FileLogger : ILogger {
    public void Log(string message) {
        System.Console.WriteLine($"File: {message}");
    }
}

class Program {
    static void Main() {
        ILogger logger = new ConsoleLogger();
        logger.Log("Start");
        logger = new FileLogger();
        logger.Log("End");
    }
}

A

Start
End

B

File: Start
Console: End

C

Console: Start
File: End

D Compile-time error: Cannot assign FileLogger to ILogger

💡 Hint

Think about how the interface variable can hold different implementations.

The variable logger is of type ILogger. First, it holds a ConsoleLogger instance, so Log("Start") prints "Console: Start". Then it is assigned a FileLogger instance, so Log("End") prints "File: End". This shows how interfaces act as contracts allowing different implementations to be used interchangeably.