Multiple interface implementation in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class that implements two interfaces.

C Sharp (C#)

interface IFirst { void MethodA(); }
interface ISecond { void MethodB(); }

class MyClass : [1] {
    public void MethodA() { }
    public void MethodB() { }
}

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

AIFirst, ISecond

BIFirst ISecond

CIFirst & ISecond

DIFirst | ISecond

Attempts:

3 left

💡 Hint

Common Mistakes

Using spaces or symbols instead of commas between interface names.

Trying to use '&' or '|' which are not valid here.

In C#, multiple interfaces are implemented by listing them separated by commas.

2fill in blank

medium

Complete the code to explicitly implement the MethodA from IFirst interface.

C Sharp (C#)

interface IFirst { void MethodA(); }
class MyClass : IFirst {
    void [1].MethodA() {
        Console.WriteLine("First");
    }
}

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

AMethodA

BMyClass

CISecond

DIFirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of the interface name.

Omitting the interface name prefix.

Explicit interface implementation requires prefixing the method with the interface name.

3fill in blank

hard

Fix the error in the code by completing the interface implementation syntax.

C Sharp (C#)

interface IFirst { void MethodA(); }
interface ISecond { void MethodB(); }

class MyClass : IFirst [1] ISecond {
    public void MethodA() { }
    public void MethodB() { }
}

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

A,

B;

C&

D|

Attempts:

3 left

💡 Hint

Common Mistakes

Using semicolon or other symbols instead of comma.

Leaving out the separator entirely.

Interfaces must be separated by commas in the class declaration.

4fill in blank

hard

Fill both blanks to complete the explicit implementation of two interfaces in the class.

C Sharp (C#)

interface IFirst { void MethodA(); }
interface ISecond { void MethodB(); }

class MyClass : IFirst, ISecond {
    void [1].MethodA() {
        Console.WriteLine("First");
    }
    void [2].MethodB() {
        Console.WriteLine("Second");
    }
}

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

AIFirst

BMyClass

CISecond

DMethodA

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of interface names.

Mixing up interface names between methods.

Explicit implementation requires prefixing each method with its interface name.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps interface names to method names if method name length is greater than 5.

C Sharp (C#)

var methods = new Dictionary<string, string[]> {
    { "IFirst", new string[] { "MethodA", "Init" } },
    { "ISecond", new string[] { "MethodB", "Start" } }
};

var filtered = methods.ToDictionary(
    [1] => [2],
    [3] => [3].Value.Where(m => m.Length > 5).ToArray()
);

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

Akvp

Bkvp.Key

Ckvp.Value

Dm

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable names in lambdas.

Mixing keys and values in the wrong places.

In ToDictionary, the first lambda gets the key (kvp.Key), the second gets the value (kvp.Value).

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