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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of explicit interface implementation call

What is the output of this C# program when calling obj.Print() and ((IPrinter)obj).Print()?

C Sharp (C#)

using System;
interface IPrinter {
    void Print();
}
class Document : IPrinter {
    void IPrinter.Print() {
        Console.WriteLine("Printing from interface method");
    }
    public void Print() {
        Console.WriteLine("Printing from class method");
    }
}
class Program {
    static void Main() {
        Document obj = new Document();
        obj.Print();
        ((IPrinter)obj).Print();
    }
}

A

Printing from class method
Printing from class method

B

Printing from class method
Printing from interface method

C

Printing from interface method
Printing from class method

D

Printing from interface method
Printing from interface method

💡 Hint

Remember explicit interface methods can only be called through the interface reference.

The class Document has two Print methods: one public and one explicit interface implementation. Calling obj.Print() calls the public method. Casting to IPrinter calls the explicit interface method.

❓ Predict Output

intermediate

Which method is called with explicit interface implementation?

Given the code below, what will be printed when obj.Print() is called?

C Sharp (C#)

using System;
interface IPrinter {
    void Print();
}
class Document : IPrinter {
    void IPrinter.Print() {
        Console.WriteLine("Interface Print");
    }
}
class Program {
    static void Main() {
        Document obj = new Document();
        obj.Print();
    }
}

A Interface Print

B Runtime error

C No output

D Compilation error: 'Document' does not contain a definition for 'Print'

💡 Hint

Explicit interface methods are not accessible through the class instance directly.

The explicit interface method Print is not visible on the class instance. Calling obj.Print() causes a compile error because Document does not have a public Print method.

🔧 Debug

advanced

Why does this explicit interface implementation cause a runtime error?

Examine the code below. Why does calling printer.Print() cause a runtime error?

C Sharp (C#)

using System;
interface IPrinter {
    void Print();
}
class Document : IPrinter {
    void IPrinter.Print() {
        Console.WriteLine("Printing document");
    }
}
class Program {
    static void Main() {
        Document doc = new Document();
        IPrinter printer = null;
        printer.Print();
    }
}

A Explicit interface implementation is invalid and causes runtime error

B Interface method cannot be called through interface reference

C printer is null, so calling Print() causes a NullReferenceException

D Document does not implement Print method properly

💡 Hint

Check the value of the variable used to call the method.

The variable printer is null. Calling any method on a null reference causes a NullReferenceException at runtime.

📝 Syntax

advanced

Identify the syntax error in explicit interface implementation

Which option contains a syntax error in explicit interface implementation?

A public void IPrinter.Print() { Console.WriteLine("Print"); }

B void IPrinter.Print() { Console.WriteLine("Print"); }

C void IPrinter.Print() => Console.WriteLine("Print");

D } ;)"tnirP"(eniLetirW.elosnoC { )(tnirP.retnirPI diov

💡 Hint

Explicit interface implementations cannot have access modifiers.

Explicit interface implementations must not have access modifiers like public or private. Option A incorrectly uses 'public'.

🚀 Application

expert

How many methods are accessible from Document instance?

Given the code below, how many Print methods can be called directly from a Document instance without casting?

C Sharp (C#)

interface IPrinter {
    void Print();
}
interface IScanner {
    void Print();
}
class Document : IPrinter, IScanner {
    void IPrinter.Print() { }
    void IScanner.Print() { }
    public void Print() { }
}

A 1

B 2

C 3

D 0

💡 Hint

Explicit interface methods are not accessible without casting.

Only the public Print method is accessible directly from Document instance. The two explicit interface implementations require casting.

Practice

1. What is the main purpose of explicit interface implementation in C#?

easy

A. To separate methods with the same name from different interfaces

B. To make interface methods public by default

C. To allow interface methods to be called without casting

D. To override base class methods automatically

Solution

1. Step 1: Understand interface method conflicts

   When a class implements multiple interfaces with methods of the same name, explicit implementation helps separate them.

2. Step 2: Recognize explicit implementation behavior

   Explicit interface methods are only accessible through the interface, avoiding confusion and conflicts.

3. Final Answer:

   To separate methods with the same name from different interfaces -> Option A

4. Quick Check:

   Explicit interface implementation = separate same-named methods [OK]

Hint: Explicit means access only via interface, not class object [OK]

Common Mistakes:

• Thinking explicit makes methods public on the class
• Assuming explicit allows calling without casting
• Confusing explicit with overriding base methods

2. Which of the following is the correct syntax for explicit interface implementation of method Show() from interface IExample in class Demo?

easy

A. public void Show() { }

B. public void IExample.Show() { }

C. void IExample.Show() { }

D. void Show() IExample { }

Solution

1. Step 1: Recall explicit implementation syntax

   Explicit interface methods are implemented with the interface name before the method: void InterfaceName.MethodName().

2. Step 2: Match correct syntax

   void IExample.Show() { } matches this pattern exactly: void IExample.Show() { }.

3. Final Answer:

   void IExample.Show() { } -> Option C

4. Quick Check:

   Explicit syntax = void Interface.Method() [OK]

Hint: Explicit methods use interface name before method [OK]

Common Mistakes:

• Adding public modifier to explicit method
• Placing interface name after method name
• Using invalid syntax like 'void Show() IExample'

3. What will be the output of the following code?

interface IA { void Display(); }
interface IB { void Display(); }
class Test : IA, IB {
    void IA.Display() { Console.WriteLine("IA Display"); }
    void IB.Display() { Console.WriteLine("IB Display"); }
}

var obj = new Test();
// obj.Display(); // Line A
((IA)obj).Display();
((IB)obj).Display();

medium

A. Compilation error at Line A

B. IA Display\nIB Display

C. IB Display\nIA Display

D. Runtime error

Solution

1. Step 1: Understand explicit interface calls

   Explicit interface methods cannot be called directly on the class object, so obj.Display() causes error if uncommented.

2. Step 2: Check interface casts and calls

   Casting to IA calls IA.Display() printing "IA Display"; casting to IB calls IB.Display() printing "IB Display".

3. Final Answer:

   IA Display IB Display -> Option B

4. Quick Check:

   Explicit calls via interface = correct output [OK]

Hint: Call explicit methods only via interface cast [OK]

Common Mistakes:

• Trying to call explicit method directly on class object
• Mixing output order
• Expecting runtime errors instead of compile errors

4. Identify the error in the following code snippet implementing explicit interface method:

interface IAlpha { void Run(); }
class Beta : IAlpha {
    public void IAlpha.Run() {
        Console.WriteLine("Running");
    }
}

medium

A. Explicit interface method cannot have public modifier

B. Method name must be different from interface

C. Interface name should not be used in method implementation

D. Missing override keyword

Solution

1. Step 1: Check explicit implementation rules

   Explicit interface methods must not have access modifiers like public; they are implicitly private.

2. Step 2: Identify error in code

   The code uses public void IAlpha.Run(), which is invalid syntax for explicit implementation.

3. Final Answer:

   Explicit interface method cannot have public modifier -> Option A

4. Quick Check:

   Explicit methods = no public keyword [OK]

Hint: Remove public from explicit interface methods [OK]

Common Mistakes:

• Adding public modifier to explicit methods
• Confusing explicit with normal method override
• Forgetting interface name in method signature

5. Given two interfaces IX and IY both having method Process(), how can a class Worker implement both explicitly and allow calling Process() without casting?

hard

A. Use inheritance instead of interfaces

B. Implement explicit methods only, no public method needed

C. Implement only one interface explicitly and the other implicitly

D. Implement explicit methods and add a public method calling one interface method

Solution

1. Step 1: Understand explicit implementation limits

   Explicit interface methods are accessible only via interface references, not directly on class objects.

2. Step 2: Provide public method to call explicit method

   To call Process() without casting, class must have a public method that internally calls one explicit interface method.

3. Final Answer:

   Implement explicit methods and add a public method calling one interface method -> Option D

4. Quick Check:

   Public wrapper calls explicit method = direct access [OK]

Hint: Add public method calling explicit interface method [OK]

Common Mistakes:

• Expecting explicit methods callable without casting
• Implementing only one interface explicitly
• Ignoring need for public wrapper method