Extension method syntax in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

C Sharp (C#)

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

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

AToUpper

BFirstChar

CLength

DSubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that already exists in string like ToUpper.

Not using a descriptive method name.

The method name 'FirstChar' correctly names the extension method that returns the first character of the string.

2fill in blank

medium

Complete the code to declare the extension method inside a static class named StringExtensions.

C Sharp (C#)

public static class [1] {
    public static bool IsNullOrEmpty(this string str) {
        return str == null || str == "";
    }
}

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

AStringExtensions

BMyExtensions

CHelper

DExtensions

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring the class as non-static.

Using a different class name than 'StringExtensions'.

Extension methods must be inside a static class. 'StringExtensions' is the correct class name as per the instruction.

3fill in blank

hard

Fix the error in the extension method declaration by completing the method signature correctly.

C Sharp (C#)

public static int WordCount([1] string str) {
    return str.Split(' ').Length;
}

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

Apublic

Bstatic

Cthis

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' or 'public' before the parameter instead of 'this'.

Omitting the 'this' keyword.

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

4fill in blank

hard

Complete the code to create an extension method that returns true 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 like '=='.

Choosing a method name that does not describe the functionality.

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

5fill in blank

hard

Fill all three blanks to create an extension method that returns a dictionary with words as keys and their lengths as values, filtering words longer than 3 characters.

C Sharp (C#)

public static Dictionary<string, int> [1](this string sentence) {
    return sentence.Split(' ').Where([2] => [2].Length > 3).ToDictionary([2] => [2], [3] => [3].Length);
}

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

AWordLengths

Bword

Cw

Dwords

Attempts:

3 left

💡 Hint

Common Mistakes

Using different lambda parameters inconsistently.

Choosing a method name unrelated to word lengths.

The method name 'WordLengths' fits the purpose. The lambda parameter 'w' is used consistently for keys and values.

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