Extension method syntax in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extension method call

What is the output of this C# program using an extension method?

C Sharp (C#)

using System;

public static class StringExtensions
{
    public static string AddExclamation(this string s)
    {
        return s + "!";
    }
}

class Program
{
    static void Main()
    {
        string greeting = "Hello";
        Console.WriteLine(greeting.AddExclamation());
    }
}

A Hello!

B Hello

C Hello!!

D Compilation error

💡 Hint

Look at what the extension method does to the string.

The extension method AddExclamation adds an exclamation mark to the string. So calling greeting.AddExclamation() returns "Hello!".

❓ Predict Output

intermediate

Extension method with value type

What is the output of this C# code using an extension method on an int?

C Sharp (C#)

using System;

public static class IntExtensions
{
    public static int Square(this int x)
    {
        return x * x;
    }
}

class Program
{
    static void Main()
    {
        int number = 4;
        Console.WriteLine(number.Square());
    }
}

A Compilation error

B 8

C 4

D 16

💡 Hint

The extension method returns the number multiplied by itself.

The Square extension method returns the input integer multiplied by itself, so 4 * 4 = 16.

🔧 Debug

advanced

Identify the error in extension method syntax

Which option contains the syntax error in defining an extension method?

C Sharp (C#)

public static class Extensions
{
    public static int Double(int x)
    {
        return x * 2;
    }
}

A The method cannot be static inside a static class.

B The method must be static and have 'this' keyword before the first parameter type.

C The method must return void for extension methods.

D The method name must start with 'Ext' for extension methods.

💡 Hint

Check the method parameter for extension method syntax.

Extension methods must be static and the first parameter must have the 'this' keyword to specify the type being extended.

🧠 Conceptual

advanced

Extension method resolution priority

Given a class with an instance method and an extension method with the same name and signature, which method is called when invoked on an instance?

A Both methods are called sequentially.

B The extension method is called, instance method is ignored.

C The instance method is called, extension method is ignored.

D Compilation error due to ambiguity.

💡 Hint

Instance methods have higher priority than extension methods.

If a class has an instance method with the same signature as an extension method, the instance method is called. Extension methods are only used if no instance method matches.

❓ Predict Output

expert

Output of chained extension methods

What is the output of this C# program using chained extension methods on a string?

C Sharp (C#)

using System;

public static class StringExtensions
{
    public static string AddPeriod(this string s)
    {
        return s + ".";
    }

    public static string ToUpperCase(this string s)
    {
        return s.ToUpper();
    }
}

class Program
{
    static void Main()
    {
        string text = "hello";
        string result = text.AddPeriod().ToUpperCase();
        Console.WriteLine(result);
    }
}

A HELLO.

B hello.

C HELLO

D hello

💡 Hint

Check the order of method calls and their effects on the string.

The string "hello" first gets a period added, becoming "hello.", then it is converted to uppercase, resulting in "HELLO.".