Extension methods for built-in types in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of an extension method on string

What is the output of this C# code that uses an extension method on a string?

C Sharp (C#)

public static class StringExtensions {
    public static string Repeat(this string s, int times) {
        return string.Concat(System.Linq.Enumerable.Repeat(s, times));
    }
}

class Program {
    static void Main() {
        string word = "Hi";
        System.Console.WriteLine(word.Repeat(3));
    }
}

A HiHi

B Hi 3

C Error: Extension method not found

D HiHiHi

💡 Hint

Think about what string.Concat and Enumerable.Repeat do together.

The extension method Repeat repeats the string the specified number of times by using Enumerable.Repeat and concatenating the results. So "Hi" repeated 3 times is "HiHiHi".

❓ Predict Output

intermediate

Output of extension method on int array

What is the output of this C# code that uses an extension method on an int array?

C Sharp (C#)

public static class ArrayExtensions {
    public static int SumPositive(this int[] arr) {
        int sum = 0;
        foreach (var n in arr) {
            if (n > 0) sum += n;
        }
        return sum;
    }
}

class Program {
    static void Main() {
        int[] numbers = { -1, 2, 3, -4, 5 };
        System.Console.WriteLine(numbers.SumPositive());
    }
}

A 0

B 5

C 10

D -10

💡 Hint

Add only positive numbers in the array.

The SumPositive extension method adds only positive numbers: 2 + 3 + 5 = 10.

🔧 Debug

advanced

Identify the error in this extension method

This extension method is intended to check if a string contains only digits. What error will this code cause when compiled?

C Sharp (C#)

public static class StringExtensions {
    public static bool IsDigitsOnly(string str) {
        foreach (char c in str) {
            if (!char.IsDigit(c)) return false;
        }
        return true;
    }
}

class Program {
    static void Main() {
        string s = "12345";
        System.Console.WriteLine(s.IsDigitsOnly());
    }
}

A Compile error: Extension method must have 'this' keyword on first parameter

B Runtime error: NullReferenceException

C Output: True

D Output: False

💡 Hint

Check the method signature for extension methods.

Extension methods must have the 'this' keyword before the first parameter to indicate which type they extend. This method lacks 'this' so it causes a compile error.

📝 Syntax

advanced

Which option correctly defines an extension method for List?

Which of the following options correctly defines an extension method named SumSquares that returns the sum of squares of all integers in a List?

A public static int SumSquares(this List<int> list) { int sum = 0; foreach(var x in list) sum += x * x; return sum; }

B public int SumSquares(List<int> list) { int sum = 0; foreach(var x in list) sum += x * x; return sum; }

C public static int SumSquares(List<int> list) { int sum = 0; foreach(var x in list) sum += x * x; return sum; }

D public static int SumSquares(this List list) { int sum = 0; foreach(var x in list) sum += x * x; return sum; }

💡 Hint

Extension methods must be static and have 'this' before the first parameter with the exact type.

Option A correctly uses 'static' and 'this List' as the first parameter. Option A is not static. Option A lacks 'this'. Option A uses List without generic type which is invalid here.

🚀 Application

expert

Result of chaining extension methods on string

Given these extension methods, what is the output of the program?

C Sharp (C#)

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

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

A HelloHello!!

B Hello!Hello!

C Hello!Hello

D Hello!!Hello!!

💡 Hint

Apply the methods step by step: first add exclamation, then repeat twice.

AddExclamation adds '!' to 'Hello' making 'Hello!'. RepeatTwice duplicates that string, resulting in 'Hello!Hello!'.