Implementing interfaces 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# program when calling the interface method?

C Sharp (C#)

using System;
interface IGreet {
    void SayHello();
}
class Person : IGreet {
    public void SayHello() {
        Console.WriteLine("Hello from Person");
    }
}
class Program {
    static void Main() {
        IGreet greeter = new Person();
        greeter.SayHello();
    }
}

A Hello from Person

B Compile-time error: Cannot convert Person to IGreet

C Runtime error: NullReferenceException

D No output

💡 Hint

Look at how the interface method is implemented and called.

The class Person implements IGreet and its method SayHello prints "Hello from Person". The interface reference calls this method, so the output is that string.

❓ Predict Output

intermediate

Output when interface method is explicitly implemented

What will be the output of this program?

C Sharp (C#)

using System;
interface IDisplay {
    void Show();
}
class Demo : IDisplay {
    void IDisplay.Show() {
        Console.WriteLine("Explicit Show");
    }
    public void Show() {
        Console.WriteLine("Public Show");
    }
}
class Program {
    static void Main() {
        Demo d = new Demo();
        d.Show();
        ((IDisplay)d).Show();
    }
}

A

Explicit Show
Explicit Show

B

Explicit Show
Public Show

C

Public Show
Public Show

D

Public Show
Explicit Show

💡 Hint

Explicit interface implementation hides the method from the class interface.

The call d.Show() calls the public Show method. The cast to IDisplay calls the explicit interface implementation, printing "Explicit Show".

🔧 Debug

advanced

Identify the error in interface implementation

Why does this code fail to compile?

C Sharp (C#)

interface ICalc {
    int Add(int a, int b);
}
class Calculator : ICalc {
    public int Add(int x, int y) {
        return x + y;
    }
}

A No error, code compiles fine

B Interface cannot have methods with parameters

C Method signature does not match interface method

D Class must be abstract to implement interface

💡 Hint

Check parameter names and types in interface and class method.

The interface method Add expects parameters named 'a' and 'b', but the class uses 'x' and 'y'. In C#, parameter names do not affect method signature, so this is not an error. The code actually compiles fine. So the correct answer is D.

❓ Predict Output

advanced

Output of multiple interface implementations

What is the output of this program?

C Sharp (C#)

using System;
interface IFirst {
    void Print();
}
interface ISecond {
    void Print();
}
class Multi : IFirst, ISecond {
    void IFirst.Print() {
        Console.WriteLine("First");
    }
    void ISecond.Print() {
        Console.WriteLine("Second");
    }
}
class Program {
    static void Main() {
        Multi m = new Multi();
        ((IFirst)m).Print();
        ((ISecond)m).Print();
    }
}

A

First
First

B

Second
First

C

First
Second

D

Second
Second

💡 Hint

Explicit interface implementations are called via interface references.

The calls cast the object to IFirst and ISecond, calling their respective explicit implementations printing "First" and "Second".

🧠 Conceptual

expert

Interface implementation and method accessibility

Which statement about explicit interface implementation in C# is TRUE?

A Explicit interface methods are accessible like any public method of the class.

B Explicitly implemented interface methods can only be called through an interface reference.

C Explicit interface implementation allows method overloading with the same signature.

D Explicit interface methods must be declared public in the class.

💡 Hint

Think about how explicit interface methods are accessed.

Explicit interface methods are hidden from the class interface and can only be accessed via an interface reference.