Default interface methods in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a default method in the interface.

C Sharp (C#)

public interface IExample {
    void Show();
    [1] DefaultMethod() {
        Console.WriteLine("Default implementation");
    }
}

Drag options to blanks, or click blank then click option'

Adefault

Bpublic

Cstatic

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' keyword before method name.

Trying to make the method static inside interface.

The method in an interface with a default implementation must have an access modifier like public. The keyword default is not used here.

2fill in blank

medium

Complete the code to call the default interface method from a class implementing the interface.

C Sharp (C#)

public class MyClass : IExample {
    public void Show() {
        Console.WriteLine("Show method");
    }
    public void CallDefault() {
        [1].DefaultMethod();
    }
}

Drag options to blanks, or click blank then click option'

AIExample

Bbase

Cthis

Ddefault

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' or 'base' to call the interface method.

Trying to call the method without specifying the interface.

To call a default interface method explicitly, you use the interface name followed by the method.

3fill in blank

hard

Fix the error in the interface default method declaration.

C Sharp (C#)

public interface ICalc {
    [1] int Add(int a, int b) {
        return a + b;
    }
}

Drag options to blanks, or click blank then click option'

Apublic

Bvoid

Cstatic

Ddefault

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' keyword incorrectly.

Declaring the method as 'static' inside interface.

Interface methods with default implementations must be declared with an access modifier like public. The default keyword is not used here.

4fill in blank

hard

Fill both blanks to create a default method with a return value and call it from the class.

C Sharp (C#)

public interface IPrinter {
    [1] string GetMessage() {
        return "Hello from interface";
    }
}

public class Printer : IPrinter {
    public string GetMessage() {
        return [2].GetMessage();
    }
}

Drag options to blanks, or click blank then click option'

Apublic

BIPrinter

Cdefault

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' keyword in method declaration.

Using 'this' instead of interface name to call the method.

The default method must be declared public. To call it from the class, use the interface name.

5fill in blank

hard

Fill all three blanks to override a default interface method in a class and call both versions.

C Sharp (C#)

public interface ILogger {
    [1] void Log(string message) {
        Console.WriteLine($"Interface log: {message}");
    }
}

public class Logger : ILogger {
    public void Log(string message) {
        Console.WriteLine($"Class log: {message}");
        [2].[3](message);
    }
}

Drag options to blanks, or click blank then click option'

Apublic

BILogger

CLog

Ddefault

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' keyword in method declaration.

Trying to call the default method without interface name.

The interface method must be declared public. To call the default method from the class, use the interface name and method name.

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