Why extension methods are needed in C Sharp (C#) - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an extension method for string that returns the first character.

C Sharp (C#)

public static class StringExtensions {
    public static char [1](this string str) {
        return str[0];
    }
}

Drag options to blanks, or click blank then click option'

AFirstChar

BGetFirst

CStartChar

DCharAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not clearly describe its purpose.

The method name 'FirstChar' clearly describes the extension method's purpose.

2fill in blank

medium

Complete the code to call the extension method on a string variable.

C Sharp (C#)

string name = "Alice";
char first = name.[1]();

Drag options to blanks, or click blank then click option'

AGetFirst

BCharAt

CStartChar

DFirstChar

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call the method as a static method instead of an instance method.

The extension method 'FirstChar' is called like a normal method on the string instance.

3fill in blank

hard

Fix the error in the extension method definition by completing the missing keyword.

C Sharp (C#)

public static class StringExtensions {
    public static char FirstChar([1] string str) {
        return str[0];
    }
}

Drag options to blanks, or click blank then click option'

Apublic

Bstatic

Cthis

Dreadonly

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the 'this' keyword, which causes the method not to be an extension method.

The 'this' keyword before the first parameter marks it as an extension method.

4fill in blank

hard

Complete the code to create an extension method that checks if a string contains only digits.

C Sharp (C#)

public static bool [1](this string str) {
    return str.All(c => char.IsDigit(c));
}

Drag options to blanks, or click blank then click option'

AIsDigitsOnly

B=>

C=>=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect lambda syntax or method name.

The method name 'IsDigitsOnly' describes the check, and '=>' is the lambda arrow used in the All method.

5fill in blank

hard

Fill all three blanks to create an extension method that repeats a string n times.

C Sharp (C#)

public static string [1](this string str, int [2]) {
    return string.Concat(Enumerable.[3](str, [2]));
}

Drag options to blanks, or click blank then click option'

ARepeat

Bcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names or method names.

Not using Enumerable.Repeat correctly.

The method name is 'Repeat', the parameter is 'count', and the Enumerable method is 'Repeat' to repeat the string.

Practice

1. Why do we need extension methods in C#?

easy

A. To replace inheritance completely

B. To add new functionality to existing types without modifying their source code

C. To create new classes from scratch

D. To make variables mutable

Solution

1. Step 1: Understand the purpose of extension methods

   Extension methods allow adding new methods to existing types without changing their original code or creating a new derived class.

2. Step 2: Compare with other options

   Options B, C, and D do not describe the purpose of extension methods correctly. They do not replace inheritance or create new classes, nor do they affect variable mutability.

3. Final Answer:

   To add new functionality to existing types without modifying their source code -> Option B

4. Quick Check:

   Extension methods add new actions without changing original types [OK]

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

Common Mistakes:

• Thinking extension methods replace inheritance
• Confusing extension methods with new class creation
• Believing extension methods change variable mutability

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

easy

A. public static void MyMethod(this int number) { }

B. public void MyMethod(int this number) { }

C. static public void MyMethod(int number) { }

D. public void this MyMethod(int number) { }

Solution

1. Step 1: Identify extension method syntax

   An extension method must be static, inside a static class, and the first parameter must have the this keyword before the type it extends.

2. Step 2: Check each option

   public static void MyMethod(this int number) { } correctly uses public static void MyMethod(this int number). Options A, B, and C have incorrect placement or missing this keyword.

3. Final Answer:

   public static void MyMethod(this int number) { } -> Option A

4. Quick Check:

   Extension method syntax requires static + this on first parameter [OK]

Hint: Look for 'static' and 'this' on first parameter [OK]

Common Mistakes:

• Omitting 'static' keyword
• Placing 'this' incorrectly
• Declaring method as instance method

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

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

int number = 4;
Console.WriteLine(number.Square());

medium

A. Compilation error

B. 4

C. Runtime error

D. 16

Solution

1. Step 1: Understand the extension method behavior

   The method Square extends int and returns the number multiplied by itself.

2. Step 2: Calculate the output for number = 4

   Calling 4.Square() returns 4 * 4 = 16, which is printed.

3. Final Answer:

   16 -> Option D

4. Quick Check:

   4 squared = 16 [OK]

Hint: Extension method acts like instance method on type [OK]

Common Mistakes:

• Expecting original number instead of squared
• Confusing extension method call syntax
• Thinking code causes errors

4. Identify the error in this extension method declaration:

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

medium

A. Extension method cannot extend int type

B. Extension method must be inside a non-static class

C. Extension method must be static

D. Method cannot have parameters

Solution

1. Step 1: Check method declaration rules

   Extension methods must be declared as static methods inside static classes.

2. Step 2: Identify the error in code

   The method Double is not static, which violates extension method rules.

3. Final Answer:

   Extension method must be static -> Option C

4. Quick Check:

   Extension methods require static keyword on method [OK]

Hint: Extension methods must be static methods [OK]

Common Mistakes:

• Forgetting to mark method as static
• Thinking class must be non-static
• Believing extension methods can't extend primitive types

5. You want to add a method IsPalindrome() to the string type without modifying the original string class. Which approach best uses extension methods to achieve this?

hard

A. Create a static class with a static method IsPalindrome(this string s) that checks palindrome logic

B. Inherit from string and add IsPalindrome() method

C. Modify the string class source code to add IsPalindrome()

D. Create a new class with a method IsPalindrome(string s) and call it manually

Solution

1. Step 1: Understand extension method usage

   Extension methods allow adding new methods to existing types without modifying or inheriting from them.

2. Step 2: Evaluate options

   Create a static class with a static method IsPalindrome(this string s) that checks palindrome logic correctly uses a static class and static method with this string s parameter to extend string. Options A and C are not possible because string is sealed and source code cannot be changed. Create a new class with a method IsPalindrome(string s) and call it manually requires manual calls and is not an extension method.

3. Final Answer:

   Create a static class with a static method IsPalindrome(this string s) that checks palindrome logic -> Option A

4. Quick Check:

   Extension methods add methods without inheritance or source changes [OK]

Hint: Use static class and static method with 'this' parameter [OK]

Common Mistakes:

• Trying to inherit from sealed string class
• Attempting to modify string source code
• Not using 'this' keyword in method parameter