Select clause projection in C Sharp (C#) - Mini Project: Build & Apply

Select Clause Projection in C#

📖 Scenario: You work in a bookstore and have a list of books with their details. You want to create a simple list showing only the book titles and their prices.

🎯 Goal: Build a C# program that uses LINQ's select clause to create a new list containing only the titles and prices of books.

📋 What You'll Learn

Create a list of books with title, author, and price

Create a variable to hold the minimum price filter

Use a LINQ query with select clause to project only title and price

Print the projected list showing title and price

💡 Why This Matters

🌍 Real World

Selecting and projecting data is common when working with collections, such as filtering products in a store or showing user details in an app.

💼 Career

Understanding LINQ and data projection is important for C# developers working on data processing, reporting, or UI display tasks.

1

Create the list of books

Create a list called books of type List<Book> with these exact entries: new Book("The Hobbit", "J.R.R. Tolkien", 15.99m), new Book("1984", "George Orwell", 12.50m), new Book("Clean Code", "Robert C. Martin", 33.99m). Also define the Book class with public properties Title, Author, and Price.

C Sharp (C#)

// Define the Book class and create the books list
// Your code here

Hint

Define a class Book with properties for title, author, and price. Then create a List<Book> with the given books.

2

Add a minimum price filter

Create a decimal variable called minPrice and set it to 13.00m.

C Sharp (C#)

using System;
using System.Collections.Generic;

public class Book
{
    public string Title { get; set; }
    public string Author { get; set; }
    public decimal Price { get; set; }

    public Book(string title, string author, decimal price)
    {
        Title = title;
        Author = author;
        Price = price;
    }
}

class Program
{
    static void Main()
    {
        List books = new List
        {
            new Book("The Hobbit", "J.R.R. Tolkien", 15.99m),
            new Book("1984", "George Orwell", 12.50m),
            new Book("Clean Code", "Robert C. Martin", 33.99m)
        };
        // Create minPrice variable
        // Your code here
    }
}

Hint

Just create a decimal variable named minPrice and assign it the value 13.00m.

3

Use LINQ select clause to project title and price

Write a LINQ query called selectedBooks that selects from books only those with Price greater than or equal to minPrice. Use the select clause to create an anonymous type with properties Title and Price.

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

public class Book
{
    public string Title { get; set; }
    public string Author { get; set; }
    public decimal Price { get; set; }

    public Book(string title, string author, decimal price)
    {
        Title = title;
        Author = author;
        Price = price;
    }
}

class Program
{
    static void Main()
    {
        List books = new List
        {
            new Book("The Hobbit", "J.R.R. Tolkien", 15.99m),
            new Book("1984", "George Orwell", 12.50m),
            new Book("Clean Code", "Robert C. Martin", 33.99m)
        };

        decimal minPrice = 13.00m;

        // Write LINQ query to select title and price
        // Your code here
    }
}

Hint

Use a LINQ query with from, where, and select clauses. The select clause should create an anonymous type with Title and Price.

4

Print the selected book titles and prices

Use a foreach loop to print each book's Title and Price from selectedBooks. Format the output as: Title: [Title], Price: [Price].

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

public class Book
{
    public string Title { get; set; }
    public string Author { get; set; }
    public decimal Price { get; set; }

    public Book(string title, string author, decimal price)
    {
        Title = title;
        Author = author;
        Price = price;
    }
}

class Program
{
    static void Main()
    {
        List books = new List
        {
            new Book("The Hobbit", "J.R.R. Tolkien", 15.99m),
            new Book("1984", "George Orwell", 12.50m),
            new Book("Clean Code", "Robert C. Martin", 33.99m)
        };

        decimal minPrice = 13.00m;

        var selectedBooks = from book in books
                            where book.Price >= minPrice
                            select new { book.Title, book.Price };

        // Print the selected books
        // Your code here
    }
}

Hint

Use a foreach loop to go through selectedBooks and print each book's title and price using Console.WriteLine with string interpolation.

Practice

1. What does the select clause do in a LINQ query in C#?

easy

A. It groups the data by a key.

B. It filters the data based on a condition.

C. It sorts the data in ascending order.

D. It chooses and shapes the data to return from the query.

Solution

1. Step 1: Understand the role of select in LINQ

   The select clause defines what data to return from the query, shaping the output.

2. Step 2: Differentiate from other clauses

   Filtering is done by where, sorting by orderby, grouping by group by. Only select shapes the output.

3. Final Answer:

   It chooses and shapes the data to return from the query. -> Option D

4. Quick Check:

   Select clause = shapes output [OK]

Hint: Select clause shapes output data, not filters or sorts [OK]

Common Mistakes:

• Confusing select with where clause
• Thinking select sorts data
• Mixing select with group by

2. Which of the following is the correct syntax for a LINQ query using select to get only the names from a list of Person objects?

easy

A. var names = from p in people select p.Name;

B. var names = from p in people where p.Name;

C. var names = from p in people orderby p.Name select p;

D. var names = from p in people group p by p.Name;

Solution

1. Step 1: Identify correct select syntax

   The correct syntax to project only the Name property is select p.Name.

2. Step 2: Check other options for errors

   var names = from p in people where p.Name; uses where incorrectly without a condition. var names = from p in people orderby p.Name select p; selects whole object, not just names. var names = from p in people group p by p.Name; groups data, not selects.

3. Final Answer:

   var names = from p in people select p.Name; -> Option A

4. Quick Check:

   Select projects properties correctly [OK]

Hint: Select clause syntax: from x in y select x.Property [OK]

Common Mistakes:

• Using where without condition
• Selecting whole object instead of property
• Confusing group by with select

3. What is the output of this code?

var numbers = new[] {1, 2, 3};
var result = from n in numbers select n * 2;
foreach(var r in result) Console.Write(r + " ");

medium

A. 1 2 3

B. 3 4 5

C. 2 4 6

D. Syntax error

Solution

1. Step 1: Understand the select projection

   The query multiplies each number by 2, so 1 becomes 2, 2 becomes 4, 3 becomes 6.

2. Step 2: Trace the output loop

   The foreach prints each doubled number followed by a space: "2 4 6 "

3. Final Answer:

   2 4 6 -> Option C

4. Quick Check:

   Select doubles numbers = 2 4 6 [OK]

Hint: Select transforms each item; multiply 1,2,3 by 2 = 2,4,6 [OK]

Common Mistakes:

• Thinking select filters instead of transforms
• Expecting original numbers
• Confusing output format

4. Identify the error in this LINQ query:

var people = new List<Person>();
var names = from p in people select p.Name.ToUpper;

medium

A. select clause must come before from clause.

B. Missing parentheses after ToUpper method call.

C. List<Person> cannot be queried with LINQ.

D. Cannot select Name property directly.

Solution

1. Step 1: Check method usage in select

   ToUpper is a method and requires parentheses: ToUpper().

2. Step 2: Validate other options

   Cannot select Name property directly. is wrong because selecting Name is valid. List<Person> cannot be queried with LINQ. is wrong because List supports LINQ. select clause must come before from clause. is wrong because from must come first.

3. Final Answer:

   Missing parentheses after ToUpper method call. -> Option B

4. Quick Check:

   Method calls need () [OK]

Hint: Method calls need parentheses, e.g., ToUpper() [OK]

Common Mistakes:

• Forgetting () after method names
• Misordering LINQ clauses
• Thinking List can't use LINQ

5. Given a list of products with properties Name and Price, how would you use a LINQ select clause to create a new list of anonymous objects containing only Name and a discounted price (10% off)?

hard

A. var discounted = from p in products select new { p.Name, DiscountedPrice = p.Price * 0.9 };

B. var discounted = from p in products select p.Name, p.Price * 0.9;

C. var discounted = from p in products where p.Price > 0.9 select p.Name;

D. var discounted = from p in products select new List { p.Name, p.Price * 0.9 };

Solution

1. Step 1: Understand anonymous object creation in select

   To create new shaped data, use new { } with property assignments inside select.

2. Step 2: Check each option's correctness

   var discounted = from p in products select new { p.Name, DiscountedPrice = p.Price * 0.9 }; correctly creates anonymous objects with Name and discounted price. var discounted = from p in products select p.Name, p.Price * 0.9; syntax is invalid (multiple values without new). var discounted = from p in products where p.Price > 0.9 select p.Name; filters but does not create discounted price. var discounted = from p in products select new List { p.Name, p.Price * 0.9 }; tries to create a List incorrectly.

3. Final Answer:

   var discounted = from p in products select new { p.Name, DiscountedPrice = p.Price * 0.9 }; -> Option A

4. Quick Check:

   Select new anonymous objects with discounted price [OK]

Hint: Use new { } in select to create shaped objects [OK]

Common Mistakes:

• Trying to select multiple values without new
• Using where instead of select for projection
• Incorrect anonymous object syntax