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");

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💡 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.