Default interface methods in C Sharp (C#) - Mini Project: Build & Apply

Using Default Interface Methods in C#

📖 Scenario: Imagine you are building a simple system for different types of devices that can perform actions. You want to define a common interface for devices with a default way to describe themselves.

🎯 Goal: Create an interface with a default method, implement it in a class, and see how the default method works without needing to be overridden.

📋 What You'll Learn

Create an interface called IDevice with a method Start() and a default method Describe() that returns a string.

Create a class called Printer that implements IDevice and provides its own Start() method.

Use the default Describe() method from the interface without overriding it in the class.

Create an instance of Printer and call both Start() and Describe() methods, then print their outputs.

💡 Why This Matters

🌍 Real World

Default interface methods help add new features to interfaces in large software projects without forcing all existing classes to change.

💼 Career

Understanding default interface methods is important for maintaining and evolving C# codebases in professional software development.

1

Create the IDevice interface with Start() method

Create an interface called IDevice with a method signature void Start().

C Sharp (C#)

// Your code here

Hint

Use the interface keyword and declare void Start(); inside it.

2

Add a default method Describe() to IDevice

Add a default method called Describe() to the IDevice interface that returns the string "This is a device.".

C Sharp (C#)

public interface IDevice
{
    void Start();
    // Your code here
}

Hint

Use a method with a body inside the interface to provide a default implementation.

3

Create Printer class implementing IDevice with Start()

Create a class called Printer that implements IDevice and provide an implementation for the Start() method that returns no value but prints "Printer started." to the console.

C Sharp (C#)

public interface IDevice
{
    void Start();
    
    string Describe()
    {
        return "This is a device.";
    }
}

// Your code here

Hint

Use public class Printer : IDevice and implement the Start() method with a console print.

4

Create Printer instance and call Start() and Describe()

Create an instance of Printer called printer. Call printer.Start() and then print the result of printer.Describe() to the console.

C Sharp (C#)

public interface IDevice
{
    void Start();
    
    string Describe()
    {
        return "This is a device.";
    }
}

public class Printer : IDevice
{
    public void Start()
    {
        System.Console.WriteLine("Printer started.");
    }
}

// Your code here

Hint

Create the Printer object, call Start(), then print Describe() result.

Practice

1. What is the main purpose of default interface methods in C#?

easy

A. Allow interfaces to have method bodies with default behavior

B. Force all implementing classes to override every method

C. Prevent interfaces from having any methods

D. Make interfaces behave like abstract classes

Solution

1. Step 1: Understand interface limitations before default methods

   Interfaces could only declare methods without bodies, forcing all implementations to define them.

2. Step 2: Recognize the role of default interface methods

   Default interface methods allow interfaces to provide a method body, so implementing classes can use or override it.

3. Final Answer:

   Allow interfaces to have method bodies with default behavior -> Option A

4. Quick Check:

   Default interface methods = method bodies in interfaces [OK]

Hint: Default interface methods add bodies to interfaces [OK]

Common Mistakes:

• Thinking interfaces cannot have any method bodies
• Confusing default methods with abstract methods
• Believing all methods must be overridden

2. Which of the following is the correct syntax to declare a default interface method in C#?

easy

A. abstract void Show() { Console.WriteLine("Hello"); }

B. void Show();

C. default void Show() { Console.WriteLine("Hello"); }

D. void Show() => Console.WriteLine("Hello");

Solution

1. Step 1: Recall default method syntax in interfaces

   Default interface methods can have bodies using either block or expression-bodied syntax.

2. Step 2: Identify correct syntax among options

   void Show() => Console.WriteLine("Hello"); uses expression-bodied syntax correctly inside interface method declaration.

3. Final Answer:

   void Show() => Console.WriteLine("Hello"); -> Option D

4. Quick Check:

   Default method syntax = method with body in interface [OK]

Hint: Default methods have bodies, unlike abstract declarations [OK]

Common Mistakes:

• Using 'default' keyword before method
• Omitting method body
• Writing method body without braces or expression syntax

3. What will be the output of this code?

interface IExample { void Show() => Console.WriteLine("Default"); }
class Test : IExample { }
var t = new Test();
t.Show();

medium

A. Runtime error

B. Compilation error: Show() not implemented

C. Default

D. No output

Solution

1. Step 1: Check if class implements Show()

   Class Test does not implement Show(), but interface provides default implementation.

2. Step 2: Understand default method usage

   Since Test inherits IExample, it uses the default Show() method from interface.

3. Final Answer:

   Default -> Option C

4. Quick Check:

   Default method runs if not overridden [OK]

Hint: If not overridden, default interface method runs [OK]

Common Mistakes:

• Assuming missing implementation causes error
• Expecting runtime error without override
• Thinking interface methods can't have bodies

4. Identify the error in this code snippet:

interface ICalc { int Add(int a, int b) => a + b; }
class Calc : ICalc { public int Add(int a, int b); }

medium

A. Missing method body in Calc's Add method

B. Interface cannot have method bodies

C. Calc class must be abstract

D. Add method should be static

Solution

1. Step 1: Check Calc class Add method declaration

   Calc declares Add with a semicolon but no body, which is invalid in a class.

2. Step 2: Understand method implementation requirements

   Class methods must have bodies unless abstract; here Add is not abstract, so body is required.

3. Final Answer:

   Missing method body in Calc's Add method -> Option A

4. Quick Check:

   Class methods need bodies unless abstract [OK]

Hint: Class methods need bodies unless marked abstract [OK]

Common Mistakes:

• Thinking interface can't have bodies
• Forgetting to add method body in class
• Assuming method should be static

5. Given this interface and classes:

interface ILogger { void Log(string msg) => Console.WriteLine($"Log: {msg}"); }
class FileLogger : ILogger { public void Log(string msg) => Console.WriteLine($"File: {msg}"); }
class ConsoleLogger : ILogger { }

What will be the output of:

ILogger logger1 = new FileLogger();
ILogger logger2 = new ConsoleLogger();
logger1.Log("Test1");
logger2.Log("Test2");

hard

A. File: Test1
File: Test2

B. File: Test1
Log: Test2

C. Log: Test1
Log: Test2

D. Compilation error

Solution

1. Step 1: Analyze FileLogger's Log method

   FileLogger overrides Log to print "File: {msg}".

2. Step 2: Analyze ConsoleLogger's Log method

   ConsoleLogger does not override Log, so uses interface default "Log: {msg}".

3. Step 3: Predict output for each logger

   logger1.Log("Test1") prints "File: Test1"; logger2.Log("Test2") prints "Log: Test2".

4. Final Answer:

   File: Test1 Log: Test2 -> Option B

5. Quick Check:

   Override changes output; default runs if no override [OK]

Hint: Override changes output; default runs if no override [OK]

Common Mistakes:

• Assuming both use default method
• Expecting compilation error due to default method
• Confusing which method runs