Custom LINQ extension methods in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a custom LINQ extension method

What is the output of this C# code using a custom LINQ extension method?

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

public static class Extensions
{
    public static IEnumerable<int> DoubleEvenNumbers(this IEnumerable<int> source)
    {
        foreach (var num in source)
        {
            if (num % 2 == 0)
                yield return num * 2;
        }
    }
}

class Program
{
    static void Main()
    {
        var numbers = new List<int> {1, 2, 3, 4, 5};
        var result = numbers.DoubleEvenNumbers();
        foreach (var n in result)
            Console.Write(n + " ");
    }
}

A 4 8

B 1 3 5

C 2 4

D 1 2 3 4 5

💡 Hint

Look at which numbers are even and how they are transformed.

The method DoubleEvenNumbers yields only even numbers doubled. 2 becomes 4, 4 becomes 8.

🧠 Conceptual

intermediate

Purpose of 'this' keyword in extension methods

In C#, what does the this keyword before the first parameter in an extension method signify?

A It marks the method as static.

B It indicates the method is an extension method for the type of the parameter.

C It makes the method private.

D It means the method can only be called inside the class.

💡 Hint

Think about how extension methods add functionality to existing types.

The this keyword before the first parameter tells the compiler this method extends that type.

🔧 Debug

advanced

Identify the error in this custom LINQ extension method

What error will this code produce when compiled?

C Sharp (C#)

public static class Extensions
{
    public static IEnumerable<int> FilterOdds(this IEnumerable<int> source)
    {
        return source.Where(x => x % 2 == 1);
    }

    public static IEnumerable<int> FilterOdds(this List<int> source)
    {
        return source.Where(x => x % 2 == 1);
    }
}

A Compile-time error: Duplicate method name with same signature

B Compile-time error: Missing return statement

C Runtime error: StackOverflowException

D No error, compiles and runs fine

💡 Hint

Check method signatures and how overload resolution works for extension methods.

No error. The methods have different 'this' parameter types (IEnumerable<int> and List<int>), making them valid overloads. The compiler selects the most specific match at call sites.

📝 Syntax

advanced

Correct syntax for a custom LINQ extension method with multiple parameters

Which option shows the correct syntax for an extension method that filters strings by minimum length?

A public static IEnumerable<string> FilterByLength(IEnumerable<string> source, int minLength) { return source.Where(s => s.Length >= minLength); }

B public IEnumerable<string> FilterByLength(IEnumerable<string> source, int minLength) { return source.Where(s => s.Length >= minLength); }

C public static IEnumerable<string> FilterByLength(this IEnumerable<string> source, int minLength) { return source.Where(s => s.Length >= minLength); }

D public static IEnumerable<string> FilterByLength(this IEnumerable<string> source) { return source.Where(s => s.Length >= minLength); }

💡 Hint

Remember extension methods must be static and the first parameter uses this.

Option C correctly declares a static method with this on the first parameter and includes the extra parameter.

🚀 Application

expert

Result count after chaining custom LINQ extension methods

Given these two extension methods, what is the count of items in result?

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

public static class Extensions
{
    public static IEnumerable<int> MultiplyBy(this IEnumerable<int> source, int factor)
    {
        foreach (var n in source)
            yield return n * factor;
    }

    public static IEnumerable<int> FilterGreaterThan(this IEnumerable<int> source, int threshold)
    {
        foreach (var n in source)
            if (n > threshold)
                yield return n;
    }
}

class Program
{
    static void Main()
    {
        var numbers = new List<int> {1, 2, 3, 4, 5};
        var result = numbers.MultiplyBy(3).FilterGreaterThan(10);
        Console.WriteLine(result.Count());
    }
}

A 2

B 0

C 3

D 1

💡 Hint

Multiply each number by 3, then count how many are greater than 10.

Numbers multiplied by 3: 3,6,9,12,15. Only 12 and 15 are > 10, so count is 2.