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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all even numbers from the list using LINQ method syntax.

C Sharp (C#)

var evens = numbers.Where(n => n [1] 2 == 0).ToList();

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

A%

B/

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using division operator '/' instead of modulo '%'.

Using multiplication '*' or addition '+' operators incorrectly.

The modulo operator % returns the remainder. To find even numbers, we check if n % 2 == 0.

2fill in blank

medium

Complete the code to order the list of names alphabetically using LINQ method syntax.

C Sharp (C#)

var sortedNames = names.[1](name => name);

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

AGroupBy

BWhere

CSelect

DOrderBy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Where which filters instead of sorting.

Using Select which projects elements.

Using GroupBy which groups elements.

The OrderBy method sorts elements in ascending order based on a key.

3fill in blank

hard

Fix the error in the code to find the first number greater than 10 using LINQ method syntax.

C Sharp (C#)

var first = numbers.[1](n => n > 10);

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

AFirstOrDefault

BOrderBy

CSelect

DWhere

Attempts:

3 left

💡 Hint

Common Mistakes

Using Where which returns a collection, not a single element.

Using Select which projects elements.

Using OrderBy which sorts elements.

FirstOrDefault returns the first element matching the condition or default if none found. Where returns a collection, not a single element.

4fill in blank

hard

Fill both blanks to create a dictionary from a list of words where the key is the word and the value is its length.

C Sharp (C#)

var wordLengths = words.ToDictionary([1] => [1], [2] => [2].Length);

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

Aword

Bw

Citem

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using variable names not matching in both selectors.

We use the same variable name word for both key and value selector functions.

5fill in blank

hard

Fill all three blanks to filter numbers greater than 5, order them descending by their squares.

C Sharp (C#)

var result = numbers.Where([1] => [1] [2] 5).OrderByDescending([3] => [3] * [3]).ToList();

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

An

B>

Cx

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for filtering.

Mixing variable names between lambdas.

Not squaring the number in the ordering.

We filter numbers greater than 5 using n > 5, then order descending by their squares using x => x * x.

Practice

1. What does the LINQ method Where do in method syntax?

easy

A. Transforms each item in a collection

B. Counts the number of items in a collection

C. Sorts the collection in ascending order

D. Filters a collection based on a condition

Solution

1. Step 1: Understand the purpose of Where

   The Where method is used to select only those items from a collection that satisfy a given condition.

2. Step 2: Differentiate from other LINQ methods

   Unlike Select which transforms items, or OrderBy which sorts, Where filters items based on a predicate.

3. Final Answer:

   Filters a collection based on a condition -> Option D

4. Quick Check:

   Where = Filter [OK]

Hint: Remember: Where filters items by condition [OK]

Common Mistakes:

• Confusing Where with Select (transformation)
• Thinking Where sorts the collection
• Assuming Where counts items

2. Which of the following is the correct syntax to select all even numbers from a list numbers using LINQ method syntax?

easy

A. numbers.Select(n => n % 2 == 0)

B. numbers.Where(n => n % 2 == 0)

C. numbers.OrderBy(n => n % 2 == 0)

D. numbers.Count(n => n % 2 == 0)

Solution

1. Step 1: Identify method for filtering

   To get only even numbers, we need to filter the list, which is done by Where.

2. Step 2: Check lambda expression correctness

   The lambda n => n % 2 == 0 correctly tests if a number is even.

3. Final Answer:

   numbers.Where(n => n % 2 == 0) -> Option B

4. Quick Check:

   Filter even numbers = Where with condition [OK]

Hint: Use Where for filtering with a condition [OK]

Common Mistakes:

• Using Select instead of Where for filtering
• Using OrderBy which sorts, not filters
• Using Count which returns a number, not a collection

3. What is the output of the following code?

var numbers = new List<int> {1, 2, 3, 4, 5};
var result = numbers.Where(n => n > 3).Select(n => n * 2).ToList();
Console.WriteLine(string.Join(", ", result));

medium

A. 1, 2, 3, 4, 5

B. 4, 6, 8, 10

C. 8, 10

D. 2, 4, 6, 8, 10

Solution

1. Step 1: Filter numbers greater than 3

   The Where method selects numbers 4 and 5 from the list.

2. Step 2: Multiply filtered numbers by 2

   The Select method transforms 4 to 8 and 5 to 10.

3. Final Answer:

   8, 10 -> Option C

4. Quick Check:

   Filter >3 then double = 8, 10 [OK]

Hint: Filter first, then transform with Select [OK]

Common Mistakes:

• Applying Select before Where (wrong order)
• Including numbers not greater than 3
• Printing original list instead of result

4. Identify the error in this LINQ method syntax code:

var words = new List<string> {"apple", "banana", "cherry"};
var result = words.Where(w => w.Length > 5).Select(w => w.ToUpper);
Console.WriteLine(string.Join(", ", result));

medium

A. Missing parentheses after ToUpper method

B. Where method should be Select

C. Cannot use string.Join with LINQ

D. Lambda expression syntax is incorrect

Solution

1. Step 1: Check usage of ToUpper method

   The code uses w.ToUpper without parentheses, but ToUpper is a method and needs ().

2. Step 2: Confirm other parts are correct

   The Where and Select methods and lambda syntax are correct; string.Join works with IEnumerable.

3. Final Answer:

   Missing parentheses after ToUpper method -> Option A

4. Quick Check:

   Method calls need () [OK]

Hint: Remember to add () after method names like ToUpper [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Confusing Where and Select roles
• Thinking string.Join can't handle LINQ results

5. Given a list of students with their names and scores:

var students = new List<(string Name, int Score)>
{
    ("Alice", 85), ("Bob", 92), ("Charlie", 78), ("Diana", 92)
};

Which LINQ method syntax query returns a dictionary with scores as keys and list of student names who have that score as values?

hard

A. students.GroupBy(s => s.Score).ToDictionary(g => g.Key, g => g.Select(s => s.Name).ToList())

B. students.ToDictionary(s => s.Score, s => s.Name)

C. students.Select(s => new { s.Score, s.Name }).ToDictionary()

D. students.Where(s => s.Score > 80).ToDictionary(s => s.Name, s => s.Score)

Solution

1. Step 1: Group students by their score

   The GroupBy method groups students sharing the same score.

2. Step 2: Convert groups to dictionary with score keys and list of names

   ToDictionary uses group key as dictionary key and selects student names as list for values.

3. Final Answer:

   students.GroupBy(s => s.Score).ToDictionary(g => g.Key, g => g.Select(s => s.Name).ToList()) -> Option A

4. Quick Check:

   GroupBy + ToDictionary for grouped lists [OK]

Hint: Use GroupBy then ToDictionary for grouping collections [OK]

Common Mistakes:

• Using ToDictionary directly without grouping (loses duplicates)
• Swapping keys and values in dictionary
• Using Select without grouping for this task