Multiple interface implementation in C Sharp (C#) - Mini Project: Build & Apply

Multiple interface implementation

📖 Scenario: Imagine you are building a simple system where a device can perform multiple roles. For example, a device can act as a printer and a scanner. Each role has its own set of actions defined by interfaces.

🎯 Goal: You will create two interfaces IPrinter and IScanner, then create a class MultiFunctionDevice that implements both interfaces. Finally, you will create an object of this class and call both printing and scanning methods.

📋 What You'll Learn

Create an interface called IPrinter with a method Print() that returns void.

Create an interface called IScanner with a method Scan() that returns void.

Create a class called MultiFunctionDevice that implements both IPrinter and IScanner.

Implement the Print() method in MultiFunctionDevice to print the message "Printing document...".

Implement the Scan() method in MultiFunctionDevice to print the message "Scanning document...".

Create an object of MultiFunctionDevice and call both Print() and Scan() methods.

Print the output messages exactly as specified.

💡 Why This Matters

🌍 Real World

Many devices like printers, scanners, and fax machines have multiple functions. Using multiple interfaces models this real-world scenario in code.

💼 Career

Understanding multiple interface implementation is important for designing clean, maintainable, and flexible software in professional C# development.

1

Create interfaces IPrinter and IScanner

Create an interface called IPrinter with a method Print() that returns void. Also create an interface called IScanner with a method Scan() that returns void.

C Sharp (C#)

// Your code here

Hint

Use the interface keyword to create interfaces. Define methods without bodies.

2

Create class MultiFunctionDevice implementing both interfaces

Create a class called MultiFunctionDevice that implements both IPrinter and IScanner. Do not add method bodies yet.

C Sharp (C#)

public interface IPrinter
{
    void Print();
}

public interface IScanner
{
    void Scan();
}

// Your code here

Hint

Use class ClassName : Interface1, Interface2 syntax to implement multiple interfaces.

3

Implement Print() and Scan() methods

In the MultiFunctionDevice class, implement the Print() method to print "Printing document..." and the Scan() method to print "Scanning document..." using Console.WriteLine.

C Sharp (C#)

public interface IPrinter
{
    void Print();
}

public interface IScanner
{
    void Scan();
}

public class MultiFunctionDevice : IPrinter, IScanner
{
    // Your code here
}

Hint

Use Console.WriteLine("message") inside each method to print the messages.

4

Create object and call methods to display output

Create an object called device of type MultiFunctionDevice. Call the Print() method on device. Then call the Scan() method on device. This will print the messages to the console.

C Sharp (C#)

public interface IPrinter
{
    void Print();
}

public interface IScanner
{
    void Scan();
}

public class MultiFunctionDevice : IPrinter, IScanner
{
    public void Print()
    {
        Console.WriteLine("Printing document...");
    }

    public void Scan()
    {
        Console.WriteLine("Scanning document...");
    }
}

// Your code here

Hint

Create the object and call the methods inside a Main method to see the output.

Practice

1.

What does it mean when a C# class implements multiple interfaces?

easy

A. The class inherits code from multiple classes.

B. The class agrees to provide code for all methods defined in those interfaces.

C. The class can only use one interface at a time.

D. The class automatically gets all properties from the interfaces without coding.

Solution

1. Step 1: Understand interface implementation

   Interfaces define method signatures but no code. A class implementing them must provide the code.

2. Step 2: Multiple interfaces require all methods

   When a class implements several interfaces, it must write code for every method in all interfaces.

3. Final Answer:

   The class agrees to provide code for all methods defined in those interfaces. -> Option B

4. Quick Check:

   Multiple interface implementation = implement all methods [OK]

Hint: Interfaces are contracts; class must fulfill all method contracts [OK]

Common Mistakes:

• Thinking interfaces provide code to inherit
• Believing class can skip some interface methods
• Confusing interfaces with classes

2.

Which of the following is the correct syntax to declare a class Car implementing interfaces IMovable and IEngine?

?

easy

A. public class Car : IMovable, IEngine { }

B. public class Car implements IMovable, IEngine { }

C. public class Car inherits IMovable, IEngine { }

D. public class Car : IMovable & IEngine { }

Solution

1. Step 1: Recall C# interface syntax

   In C#, a class uses a colon ':' followed by interface names separated by commas.

2. Step 2: Check each option

   public class Car : IMovable, IEngine { } uses ':' and commas correctly. Options B and C use wrong keywords. public class Car : IMovable & IEngine { } uses '&' which is invalid.

3. Final Answer:

   public class Car : IMovable, IEngine { } -> Option A

4. Quick Check:

   Interfaces listed after ':' separated by commas [OK]

Hint: Use ':' and commas to list interfaces after class name [OK]

Common Mistakes:

• Using 'implements' keyword (Java style)
• Using '&' instead of commas
• Using 'inherits' keyword incorrectly

3.

What will be the output of the following C# code?

interface IA { void Show(); }
interface IB { void Show(); }
class Demo : IA, IB {
    public void Show() { Console.WriteLine("Hello"); }
}
class Program {
    static void Main() {
        Demo d = new Demo();
        d.Show();
    }
}

medium

A. Hello

B. Compilation error due to ambiguous Show method

C. Runtime error

D. No output

Solution

1. Step 1: Understand method implementation for multiple interfaces

   Both interfaces have Show method. The class Demo implements one Show method that satisfies both.

2. Step 2: Check program output

   Main creates Demo and calls Show, which prints "Hello".

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Single method implements both interfaces' Show [OK]

Hint: One method can implement same method from multiple interfaces [OK]

Common Mistakes:

• Expecting compile error for same method name
• Thinking separate methods needed for each interface
• Confusing interface method calls

4.

Identify the error in this code snippet:

interface IA { void Run(); }
interface IB { void Jump(); }
class Player : IA, IB {
    public void Run() { Console.WriteLine("Running"); }
}

medium

A. Run method should be private.

B. Class Player cannot implement two interfaces.

C. Interfaces cannot have methods.

D. Class Player must implement Jump method from IB interface.

Solution

1. Step 1: Check interface methods

   IA requires Run(), IB requires Jump().

2. Step 2: Verify class implementation

   Player implements Run() but misses Jump(), so it is incomplete.

3. Final Answer:

   Class Player must implement Jump method from IB interface. -> Option D

4. Quick Check:

   All interface methods must be implemented [OK]

Hint: Implement all interface methods to avoid errors [OK]

Common Mistakes:

• Forgetting to implement all interface methods
• Thinking interfaces can't have methods
• Assuming methods can be private

5.

Given these interfaces and class:

interface IAlpha { void Action(); }
interface IBeta { void Action(); }
class Combined : IAlpha, IBeta {
    void IAlpha.Action() { Console.WriteLine("Alpha Action"); }
    void IBeta.Action() { Console.WriteLine("Beta Action"); }
}
class Program {
    static void Main() {
        Combined c = new Combined();
        // Which calls are valid?
    }
}

Which of the following calls will compile and print output?

hard

A. ((IBeta)c).Action(); // prints 'Beta Action'

B. c.Action(); // prints 'Alpha Action'

C. ((IAlpha)c).Action(); // prints 'Alpha Action'

D. c.Action(); // prints 'Beta Action'

Solution

1. Step 1: Understand explicit interface implementation

   Combined class implements IAlpha.Action and IBeta.Action explicitly, so these methods are not accessible via class instance directly.

2. Step 2: Check method calls

   Only calls through interface references like (IAlpha)c or (IBeta)c are valid. c.Action() is invalid and causes compile error.

3. Final Answer:

   ((IAlpha)c).Action(); // prints 'Alpha Action' -> Option C

4. Quick Check:

   Explicit interface methods need interface cast to call [OK]

Hint: Explicit interface methods require casting to interface type [OK]

Common Mistakes:

• Calling explicit interface methods directly on class instance
• Confusing explicit and implicit implementation
• Assuming c.Action() works without cast