LINQ method syntax in C Sharp (C#) - Mini Project: Build & Apply

Filter and Select with LINQ Method Syntax

📖 Scenario: You work in a bookstore and have a list of books with their prices. You want to find all books that cost less than $20 and get their titles.

🎯 Goal: Build a C# program that uses LINQ method syntax to filter books priced below $20 and select their titles.

📋 What You'll Learn

Create a list of books with exact titles and prices

Create a price limit variable

Use LINQ method syntax with Where and Select to filter and select book titles

Print the filtered book titles

💡 Why This Matters

🌍 Real World

Filtering and selecting data from collections is common in apps like online stores, libraries, and inventory systems.

💼 Career

Knowing LINQ method syntax helps you write clear and efficient queries in C# for data processing tasks.

1

Create the list of books

Create a list of tuples called books with these exact entries: ("C# Basics", 15.99), ("LINQ in Action", 25.50), ("ASP.NET Core", 18.75), ("Entity Framework", 22.30), ("Clean Code", 19.99).

C Sharp (C#)

// Create the list of books with titles and prices
// Your code here

Hint

Use new List<(string Title, double Price)> { ... } to create the list.

2

Set the price limit

Create a double variable called priceLimit and set it to 20.0.

C Sharp (C#)

var books = new List
{
    ("C# Basics", 15.99),
    ("LINQ in Action", 25.50),
    ("ASP.NET Core", 18.75),
    ("Entity Framework", 22.30),
    ("Clean Code", 19.99)
};

// Create the price limit variable
// Your code here

Hint

Use double priceLimit = 20.0; to set the limit.

3

Filter and select book titles using LINQ method syntax

Create a variable called affordableBooks that uses LINQ method syntax on books to filter books where Price < priceLimit using Where, then select only the Title using Select.

C Sharp (C#)

var books = new List
{
    ("C# Basics", 15.99),
    ("LINQ in Action", 25.50),
    ("ASP.NET Core", 18.75),
    ("Entity Framework", 22.30),
    ("Clean Code", 19.99)
};

double priceLimit = 20.0;

// Use LINQ method syntax to filter and select titles
// Your code here

Hint

Chain Where and Select methods with lambda expressions.

4

Print the filtered book titles

Use a foreach loop to print each title in affordableBooks on its own line.

C Sharp (C#)

var books = new List
{
    ("C# Basics", 15.99),
    ("LINQ in Action", 25.50),
    ("ASP.NET Core", 18.75),
    ("Entity Framework", 22.30),
    ("Clean Code", 19.99)
};

double priceLimit = 20.0;

var affordableBooks = books.Where(book => book.Price  book.Title);

// Print each title in affordableBooks
// Your code here

Hint

Use foreach (var title in affordableBooks) { Console.WriteLine(title); } to print.

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