Default interface methods in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of default interface method call

What is the output of this C# code?

C Sharp (C#)

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

A Compile-time error: 'Person' does not implement 'SayHello()'

B Hello from interface

C Runtime error: Method not found

D No output

💡 Hint

Check if the class implements the interface method explicitly or uses the default.

The interface IGreet has a default implementation for SayHello(). The class Person does not override it, so calling SayHello() on an IGreet reference calls the default method, printing "Hello from interface".

❓ Predict Output

intermediate

Overriding default interface method

What will this program print?

C Sharp (C#)

using System;
interface IAnimal
{
    void Speak() => Console.WriteLine("Animal sound");
}
class Dog : IAnimal
{
    public void Speak() => Console.WriteLine("Bark");
}
class Program
{
    static void Main()
    {
        IAnimal a = new Dog();
        a.Speak();
    }
}

A Bark

B Runtime error

C Compile-time error: 'Dog' must implement 'Speak()'

D Animal sound

💡 Hint

Does the class Dog provide its own Speak() method?

Dog overrides the default Speak() method from IAnimal. So calling Speak() on an IAnimal reference to a Dog instance calls Dog's Speak(), printing "Bark".

🔧 Debug

advanced

Why does this code cause a compile error?

Examine the code below. Why does it cause a compile-time error?

C Sharp (C#)

interface ICalc
{
    int Add(int a, int b) => a + b;
}
class Calculator : ICalc
{
    // No Add method implemented here
}
class Program
{
    static void Main()
    {
        Calculator calc = new Calculator();
        int result = calc.Add(2, 3);
        System.Console.WriteLine(result);
    }
}

A No error, output is 5

B Compile error because default interface methods are not supported in C#

C Compile error because Calculator does not implement Add explicitly and cannot use default interface method

D Runtime error because Add method is missing

💡 Hint

Check how default interface methods are accessed through class instances.

Default interface methods can only be called through an interface reference. Calculator class instance cannot call Add() directly because it does not implement Add explicitly. So this causes a compile error.

📝 Syntax

advanced

Identify the syntax error in default interface method

Which option contains a syntax error in defining a default interface method?

A interface ITest { void Show() => Console.WriteLine("Hi"); }

B interface ITest { void Show() { Console.WriteLine("Hi"); } }

C interface ITest { void Show(); }

D interface ITest { void Show() => { Console.WriteLine("Hi"); }; }

💡 Hint

Check the syntax for expression-bodied members in interfaces.

Option D incorrectly uses braces with expression-bodied syntax. Expression-bodied members use => followed by an expression without braces. Option D is valid because default interface methods support block bodies.

🚀 Application

expert

How many methods must a class implement for this interface?

Consider this interface with default methods and one abstract method. How many methods must a class implement if it implements this interface?

C Sharp (C#)

interface IOperations
{
    void Start();
    void Stop() => Console.WriteLine("Stopping");
    int Status() => 1;
}

A One method: only Start() must be implemented

B Three methods: Start(), Stop(), and Status() must be implemented

C Two methods: Start() and Stop() must be implemented

D No methods need to be implemented

💡 Hint

Default interface methods provide implementation. Abstract methods must be implemented.

Only Start() is abstract and must be implemented by the class. Stop() and Status() have default implementations, so the class can use them as is.