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.".

Practice

1. What is the main purpose of an extension method in C#?

easy

A. To override existing methods in a class

B. To create new classes that inherit from existing ones

C. To add new methods to existing types without modifying their source code

D. To define constructors for existing classes

Solution

1. Step 1: Understand extension method purpose

   Extension methods allow adding new functionality to existing types without changing their original code.

2. Step 2: Compare with other options

   Options B, C, and D describe inheritance, overriding, and constructors, which are different concepts.

3. Final Answer:

   To add new methods to existing types without modifying their source code -> Option C

4. Quick Check:

   Extension methods add methods without changing original type [OK]

Hint: Extension methods add functionality without changing original code [OK]

Common Mistakes:

• Confusing extension methods with inheritance
• Thinking extension methods override existing methods
• Believing extension methods create new classes

2. Which of the following is the correct syntax to declare an extension method in C#?

easy

A. public void MyExtension(string this str) { }

B. public static void MyExtension(this string str) { }

C. static public void MyExtension(string str) { }

D. public static void MyExtension(string str) { }

Solution

1. Step 1: Check extension method declaration rules

   Extension methods must be static methods inside static classes, and the first parameter must have the 'this' keyword before the type.

2. Step 2: Analyze each option

   public static void MyExtension(this string str) { } correctly uses 'public static void' and 'this string str'. public void MyExtension(string this str) { } misplaces 'this'. static public void MyExtension(string str) { } misses 'this'. public static void MyExtension(string str) { } misses 'this' keyword.

3. Final Answer:

   public static void MyExtension(this string str) { } -> Option B

4. Quick Check:

   Extension method syntax requires 'this' before first parameter [OK]

Hint: Remember 'this' before first parameter in static method [OK]

Common Mistakes:

• Omitting 'this' keyword before the first parameter
• Placing 'this' after the parameter type
• Not making the method static

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

public static class StringExtensions {
    public static int WordCount(this string str) {
        return str.Split(' ').Length;
    }
}

string sentence = "Hello world from C#";
int count = sentence.WordCount();
Console.WriteLine(count);

medium

A. 4

B. 1

C. 0

D. Compilation error

Solution

1. Step 1: Understand the extension method behavior

   The method WordCount splits the string by spaces and returns the number of parts.

2. Step 2: Count words in the string

   The string "Hello world from C#" has 4 words separated by spaces, so the method returns 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Word count of "Hello world from C#" = 4 [OK]

Hint: Count words by splitting string on spaces [OK]

Common Mistakes:

• Forgetting to make the extension method static
• Assuming Split returns empty array for non-empty string
• Expecting compilation error due to extension method

4. Identify the error in the following extension method code:

public static class IntExtensions {
    public static bool IsEven(int number) {
        return number % 2 == 0;
    }
}

medium

A. The method is not static

B. The class is not static

C. The method should return int instead of bool

D. The first parameter is missing the 'this' keyword

Solution

1. Step 1: Check extension method parameter syntax

   Extension methods require the first parameter to have the 'this' keyword before the type to indicate the extended type.

2. Step 2: Analyze the code

   The method IsEven has 'int number' as parameter but lacks 'this' keyword, so it is not a valid extension method.

3. Final Answer:

   The first parameter is missing the 'this' keyword -> Option D

4. Quick Check:

   Extension method needs 'this' before first parameter [OK]

Hint: Extension method's first parameter must start with 'this' [OK]

Common Mistakes:

• Forgetting 'this' keyword on first parameter
• Making method non-static
• Not declaring class as static

5. You want to create an extension method IsNullOrEmpty for string that returns true if the string is null or empty. Which of the following is the correct implementation?

hard

A. public static bool IsNullOrEmpty(this string str) { return str == null || str == ""; }

B. public bool IsNullOrEmpty(string str) { return str == null || str == ""; }

C. public static bool IsNullOrEmpty(string str) { return str == null || str == ""; }

D. public static bool IsNullOrEmpty(this string str) { return str.Length == 0; }

Solution

1. Step 1: Check extension method signature

   The method must be static and have 'this' before the first parameter type to be an extension method.

2. Step 2: Verify method logic

   public static bool IsNullOrEmpty(this string str) { return str == null || str == ""; } correctly checks if the string is null or empty. public static bool IsNullOrEmpty(this string str) { return str.Length == 0; } misses null check, which causes error if str is null.

3. Final Answer:

   public static bool IsNullOrEmpty(this string str) { return str == null || str == ""; } -> Option A

4. Quick Check:

   Extension method with 'this' and null check is correct [OK]

Hint: Check null before accessing string properties in extension [OK]

Common Mistakes:

• Omitting 'this' keyword on first parameter
• Not checking for null before accessing Length
• Making method non-static