LINQ method syntax in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this LINQ method syntax code?

Consider the following C# code using LINQ method syntax. What will be printed to the console?

C Sharp (C#)

var numbers = new List<int> { 1, 2, 3, 4, 5 };
var result = numbers.Where(n => n % 2 == 0).Select(n => n * n);
foreach(var num in result) {
    Console.Write(num + " ");
}

A 2 4

B 1 9 25

C 4 16

D 1 2 3 4 5

💡 Hint

Think about which numbers are even and what happens to them in the Select step.

The code filters the list to only even numbers (2 and 4), then squares each. So output is 4 and 16.

🧠 Conceptual

intermediate

Which LINQ method syntax expression counts elements greater than 10?

You have a list of integers. Which of the following LINQ method syntax expressions correctly counts how many elements are greater than 10?

A numbers.Select(n => n > 10).Count()

B numbers.Count(n => n > 10)

C numbers.Where(n => n > 10).Count()

D numbers.Count().Where(n => n > 10)

💡 Hint

Count can take a condition directly or you can filter first then count.

Option B uses Count with a predicate, which counts elements matching the condition directly. Option B also works but is less efficient. Option B counts all elements because Select returns booleans. Option B is invalid syntax.

🔧 Debug

advanced

What error does this LINQ method syntax code produce?

Examine the code below. What error will it cause when compiled or run?

C Sharp (C#)

var words = new List<string> { "apple", "banana", "cherry" };
var result = words.Select(word => word.Length > 5 ? word : );
foreach(var w in result) {
    Console.WriteLine(w);
}

A InvalidOperationException: Sequence contains no elements

B NullReferenceException at runtime

C No error, prints all words

D SyntaxError: Missing expression after ':' in ternary operator

💡 Hint

Look carefully at the ternary operator syntax inside Select.

The ternary operator requires both 'true' and 'false' expressions. Here, the false part is missing, causing a syntax error.

❓ Predict Output

advanced

What is the output of this LINQ method syntax with grouping?

Given the code below, what will be printed to the console?

C Sharp (C#)

var fruits = new List<string> { "apple", "apricot", "banana", "blueberry", "cherry" };
var groups = fruits.GroupBy(f => f[0]);
foreach(var group in groups) {
    Console.Write(group.Key + ":");
    Console.WriteLine(string.Join(",", group));
}

A

a:apple,apricot
b:banana,blueberry
c:cherry

B

apple:apricot
banana:blueberry
cherry:

C

a:apple
apricot
b:banana
blueberry
c:cherry

D

a:apple,banana
b:apricot,blueberry
c:cherry

💡 Hint

GroupBy groups by the first letter of each fruit.

The fruits are grouped by their first letter: 'a' group has apple and apricot, 'b' group has banana and blueberry, 'c' group has cherry. The output prints each group key and its items separated by commas.

❓ Predict Output

expert

What is the value of 'result' after this LINQ method syntax chain?

Analyze the code below. What is the value of the variable 'result' after execution?

C Sharp (C#)

var data = new List<int> { 3, 6, 9, 12, 15 };
var result = data.Aggregate(1, (acc, val) => val % 3 == 0 ? acc * val : acc);

A 29160

B 0

C 1

D 45

💡 Hint

Aggregate starts with 1 and multiplies values divisible by 3.

All numbers in data are divisible by 3, so the accumulator multiplies all: 1*3=3, 3*6=18, 18*9=162, 162*12=1944, 1944*15=29160.

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