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.

Practice

1. What does it mean to implement an interface in C#?

easy

A. A class provides code for all methods declared in the interface.

B. An interface inherits from a class.

C. A class hides all methods of the interface.

D. An interface creates objects directly.

Solution

1. Step 1: Understand interface purpose

   An interface declares methods without code, setting a contract.

2. Step 2: Implementing means coding methods

   A class that implements the interface must write the code for all those methods.

3. Final Answer:

   A class provides code for all methods declared in the interface. -> Option A

4. Quick Check:

   Implementing interface = writing required methods [OK]

Hint: Implementing means writing all interface methods in the class [OK]

Common Mistakes:

• Thinking interfaces can create objects
• Believing interfaces inherit from classes
• Assuming methods are hidden, not implemented

2. Which of the following is the correct syntax to implement an interface IMyInterface in a class MyClass?

easy

A. interface MyClass : IMyInterface { }

B. class MyClass : IMyInterface { }

C. class MyClass implements IMyInterface { }

D. class MyClass inherits IMyInterface { }

Solution

1. Step 1: Recall C# interface syntax

   In C#, a class implements an interface using a colon (:), not 'implements' or 'inherits'.

2. Step 2: Check each option

   class MyClass : IMyInterface { } uses correct syntax: class MyClass : IMyInterface { }. Others use wrong keywords or declare interface as class.

3. Final Answer:

   class MyClass : IMyInterface { } -> Option B

4. Quick Check:

   Use colon (:) to implement interface [OK]

Hint: Use ':' to implement interface, not 'implements' or 'inherits' [OK]

Common Mistakes:

• Using 'implements' keyword like Java
• Trying to inherit interface with 'inherits'
• Declaring interface as a class

3. What will be the output of this code?

interface IGreet { void SayHello(); }
class Person : IGreet {
  public void SayHello() { Console.WriteLine("Hi!"); }
}
var p = new Person();
p.SayHello();

medium

A. No output

B. SayHello

C. Hi!

D. Compile error

Solution

1. Step 1: Understand interface and class

   The interface IGreet requires SayHello method. Person implements it by printing "Hi!".

2. Step 2: Trace code execution

   Creating Person object and calling SayHello prints "Hi!" to console.

3. Final Answer:

   Hi! -> Option C

4. Quick Check:

   Implemented method runs and prints output [OK]

Hint: Implemented method runs exactly as coded in class [OK]

Common Mistakes:

• Expecting method name printed instead of output
• Thinking interface prints output
• Assuming compile error without method body

4. Identify the error in this code:

interface IRun { void Run(); }
class Animal : IRun { }

medium

A. Class Animal must implement Run method.

B. Interface IRun cannot be empty.

C. Class Animal cannot implement interface.

D. No error, code is correct.

Solution

1. Step 1: Check interface requirements

   IRun interface requires a method Run() to be implemented.

2. Step 2: Verify class implementation

   Animal class implements IRun but does not provide Run() method, causing error.

3. Final Answer:

   Class Animal must implement Run method. -> Option A

4. Quick Check:

   Implement all interface methods [OK]

Hint: All interface methods must be in the class [OK]

Common Mistakes:

• Forgetting to implement interface methods
• Thinking interfaces must have code
• Assuming empty class is allowed

5. You have two interfaces:

interface IWalk { void Walk(); }
interface ITalk { void Talk(); }

How can a class Human implement both interfaces correctly?

hard

A. interface Human : IWalk, ITalk { }

B. class Human : IWalk { public void Walk() { } } class Human : ITalk { public void Talk() { } }

C. class Human implements IWalk, ITalk { }

D. class Human : IWalk, ITalk { public void Walk() { Console.WriteLine("Walking"); } public void Talk() { Console.WriteLine("Talking"); } }

Solution

1. Step 1: Understand multiple interface implementation

   A class can implement multiple interfaces by listing them separated by commas.

2. Step 2: Provide all required methods

   Human class must provide Walk() and Talk() methods with code.

3. Final Answer:

   class Human : IWalk, ITalk { public void Walk() { Console.WriteLine("Walking"); } public void Talk() { Console.WriteLine("Talking"); } } -> Option D

4. Quick Check:

   Multiple interfaces implemented with all methods [OK]

Hint: Separate interfaces with commas and implement all methods [OK]

Common Mistakes:

• Trying to declare multiple classes with same name
• Using 'implements' keyword (Java style)
• Declaring interface instead of class