Extension methods for built-in types in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an extension method for the string type 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

BFirst

CGetFirst

DCharAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not clearly indicate it returns the first character.

Forgetting to make the method static or the class static.

The method name 'First' is a common and clear name for returning the first character of a string.

2fill in blank

medium

Complete the code to call the extension method 'IsEven' on an integer variable.

C Sharp (C#)

int number = 4;
bool result = number.[1]();

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

AIsEven

BCheckEven

CEvenCheck

DIsOdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong method name that does not exist.

Calling a method that checks for odd instead of even.

The extension method is named 'IsEven', so calling number.IsEven() is correct.

3fill in blank

hard

Fix the error in the extension method declaration by completing the code.

C Sharp (C#)

public static class IntExtensions {
    public static bool [1](this int value) {
        return value % 2 == 0;
    }
}

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

ACheckEven

BIsOdd

CIsEven

DEven

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the method 'IsOdd' which contradicts the logic.

Using a vague or unclear method name.

The method name 'IsEven' matches the logic that returns true if the number is even.

4fill in blank

hard

Fill both blanks to create an extension method that returns the last character of a string.

C Sharp (C#)

public static class StringHelpers {
    public static char [1](this string str) {
        return str[2];
    }
}

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

ALastChar

BLength - 1

C[str.Length - 1]

DLast

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect indexing syntax.

Choosing a method name that does not match the functionality.

The method name 'LastChar' clearly describes the purpose, and accessing str[str.Length - 1] returns the last character.

5fill in blank

hard

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

C Sharp (C#)

public static class StringExtensions {
    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

Dtimes

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names.

Using wrong method names or missing the Enumerable.Repeat call.

The method name 'Repeat' describes the action, 'count' is a common parameter name for number of repetitions, and Enumerable.Repeat is the correct method to repeat a string.

Practice

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

easy

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

B. To create a new class that inherits from a built-in type

C. To override existing methods in built-in types

D. To replace built-in types with custom implementations

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 options

   Replacing built-in types with custom implementations, creating a new class that inherits from a built-in type, or overriding existing methods are different concepts from extension methods.

3. Final Answer:

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

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:

• Confusing extension methods with inheritance
• Thinking extension methods override existing methods
• Believing extension methods replace built-in types

2. Which of the following is the correct syntax to declare an extension method for the built-in string type?

easy

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

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

C. static void MyMethod(string str) { }

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

Solution

1. Step 1: Recall extension method syntax

   Extension methods must be static methods inside static classes, with this before the first parameter type.

2. Step 2: Check options

   Only public static void MyMethod(this string str) { } uses this string str correctly. The other options miss the this keyword or static modifier.

3. Final Answer:

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

4. Quick Check:

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

Hint: Look for 'this' keyword before first parameter in static method [OK]

Common Mistakes:

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

3. Given the following code, what will be the output?

public static class StringExtensions {
    public static bool IsCapitalized(this string s) {
        return !string.IsNullOrEmpty(s) && char.IsUpper(s[0]);
    }
}

class Program {
    static void Main() {
        string word = "Hello";
        Console.WriteLine(word.IsCapitalized());
    }
}

medium

A. True

B. False

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand the extension method logic

   The method checks if the string is not null or empty and if the first character is uppercase.

2. Step 2: Analyze the input string

   The string "Hello" starts with 'H', which is uppercase, so the method returns true.

3. Final Answer:

   True -> Option A

4. Quick Check:

   First letter uppercase in "Hello" means IsCapitalized() returns True [OK]

Hint: Check if first character is uppercase and string is not empty [OK]

Common Mistakes:

• Assuming method returns false for capitalized words
• Confusing null or empty check logic
• Expecting compilation error due to extension method

4. Identify the error in this extension method code:

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

medium

A. The method must be inside a non-static class

B. The method cannot return bool

C. The method must be static and have 'this' before the first parameter

D. The method name cannot be 'IsEven'

Solution

1. Step 1: Check extension method requirements

   Extension methods must be static and the first parameter must have the 'this' keyword to specify the type being extended.

2. Step 2: Identify missing 'this' keyword

   The method parameter 'int number' lacks 'this', so it is not a valid extension method.

3. Final Answer:

   The method must be static and have 'this' before the first parameter -> Option C

4. Quick Check:

   Extension methods require 'this' keyword on first parameter [OK]

Hint: Extension method first parameter must have 'this' keyword [OK]

Common Mistakes:

• Forgetting 'this' before the first parameter
• Placing method in non-static class
• Using instance methods instead of static

5. You want to create an extension method for List<int> that returns the sum of all even numbers in the list. Which of the following implementations correctly achieves this?

hard

A. public static int SumEven(this List<int> list) { return list.Sum(n => n % 2); }

B. public int SumEven(List<int> list) { return list.Where(n => n % 2 == 0).Sum(); }

C. public static int SumEven(List<int> list) { return list.Sum(n => n % 2 == 0); }

D. public static int SumEven(this List<int> list) { int sum = 0; foreach(var n in list) { if(n % 2 == 0) sum += n; } return sum; }

Solution

1. Step 1: Verify extension method signature

   public static int SumEven(this List<int> list) { int sum = 0; foreach(var n in list) { if(n % 2 == 0) sum += n; } return sum; } and the version using list.Sum(n => n % 2) have 'this List<int> list' parameter, making them extension methods. The others lack 'this'.

2. Step 2: Check logic correctness

   public static int SumEven(this List<int> list) { int sum = 0; foreach(var n in list) { if(n % 2 == 0) sum += n; } return sum; } sums even numbers correctly using a loop and condition. public static int SumEven(this List<int> list) { return list.Sum(n => n % 2); } sums n % 2 which returns 0 or 1, not the number itself. The others are not extension methods.

3. Final Answer:

   public static int SumEven(this List<int> list) { int sum = 0; foreach(var n in list) { if(n % 2 == 0) sum += n; } return sum; } -> Option D

4. Quick Check:

   Extension method with correct sum logic is public static int SumEven(this List<int> list) { int sum = 0; foreach(var n in list) { if(n % 2 == 0) sum += n; } return sum; } [OK]

Hint: Use 'this' and sum only even numbers correctly [OK]

Common Mistakes:

• Missing 'this' keyword for extension method
• Incorrect lambda in Sum causing wrong results
• Not filtering even numbers before summing