Multiple interface implementation in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of multiple interface implementation with explicit methods

What is the output of this C# program that implements two interfaces with explicit method implementations?

C Sharp (C#)

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

A IA Show\nIB Show

B Compilation error due to ambiguous Show method

C Show\nShow

D IB Show\nIA Show

💡 Hint

Explicit interface implementations require casting to the interface to call the method.

The class Demo implements IA.Show and IB.Show explicitly. To call these methods, you must cast the object to the interface type. The calls ((IA)d).Show() and ((IB)d).Show() invoke the respective interface implementations, printing 'IA Show' and 'IB Show'.

❓ Predict Output

intermediate

Output when calling method without explicit interface cast

Given this code, what will be the output when calling Show() directly on the object?

C Sharp (C#)

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

A Compilation error due to ambiguous Show method

B IB Show

C Demo Show

D IA Show

💡 Hint

Public method Show() in the class is called directly without casting.

The class Demo has a public Show() method. This method is called directly on the object d, so it prints 'Demo Show'. The explicit interface implementations are not called unless the object is cast to the interface.

🔧 Debug

advanced

Identify the error in multiple interface implementation

What error will this code produce when compiled?

C Sharp (C#)

interface IA { void Display(); }
interface IB { void Display(); }
class Test : IA, IB {
    public void Display() { }
    public void Display(int x) { }
}

A Compilation error: 'Test' does not implement interface member 'IB.Display()'

B Compilation error: Method 'Display(int)' hides inherited member

C Runtime error: Ambiguous method call

D No compilation error, code compiles successfully

💡 Hint

Check if all interface members are implemented explicitly or implicitly.

The class Test has a public Display() method that matches IA.Display() and IB.Display(). Since both interfaces have the same method signature, the single public method satisfies both. The overload Display(int x) does not affect interface implementation. Therefore, the code compiles successfully.

📝 Syntax

advanced

Which option correctly implements two interfaces with same method name?

Select the option that correctly implements interfaces IA and IB, both having void Run(), in one class without ambiguity.

A class C : IA, IB { void IA.Run() { } void IB.Run() { } }

B class C : IA, IB { public void Run() { } }

C class C : IA, IB { public void Run() { } void IB.Run() { } }

D class C : IA, IB { void Run() { } }

💡 Hint

Explicit interface implementation avoids ambiguity when methods have same signature.

Option A uses explicit interface implementation for both IA.Run() and IB.Run(), so the class implements both interfaces without ambiguity. Option A compiles but the single Run() method implements both interfaces implicitly. Option A is invalid because IB.Run() is explicit but IA.Run() is implicit, causing ambiguity. Option A is invalid because void Run() without access modifier is private and does not implement interfaces.

🚀 Application

expert

Determine the output of interface method calls with inheritance and multiple interfaces

Consider these interfaces and classes. What is the output when running the Main method?

C Sharp (C#)

using System;
interface IA { void Action(); }
interface IB : IA { void Action(); }
class Base : IA {
    public virtual void Action() { Console.WriteLine("Base Action"); }
}
class Derived : Base, IB {
    public override void Action() { Console.WriteLine("Derived Action"); }
}
class Program {
    static void Main() {
        Derived d = new Derived();
        IA ia = d;
        IB ib = d;
        Base b = d;
        d.Action();
        ia.Action();
        ib.Action();
        b.Action();
    }
}

A Base Action\nBase Action\nBase Action\nBase Action

B Derived Action\nDerived Action\nDerived Action\nDerived Action

C Derived Action\nBase Action\nDerived Action\nBase Action

D Compilation error due to interface inheritance conflict

💡 Hint

Virtual and override methods are called based on the actual object type, not the reference type.

The Derived class overrides the virtual method Action from Base. All references (IA, IB, Base, Derived) point to the same Derived object. Calling Action() on any of these calls the overridden Derived.Action(), printing 'Derived Action' four times.

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