LINQ with custom objects in C Sharp (C#) - Mini Project: Build & Apply

LINQ with custom objects

📖 Scenario: You work in a bookstore and want to organize a list of books by their price. Each book has a title and a price.

🎯 Goal: You will create a list of book objects, set a price limit, use LINQ to find books cheaper than that limit, and then print their titles.

📋 What You'll Learn

Create a Book class with Title and Price properties

Create a list of Book objects with exact titles and prices

Create a priceLimit variable to filter books

Use LINQ query with where to select books cheaper than priceLimit

Print the titles of the filtered books

💡 Why This Matters

🌍 Real World

Filtering and selecting items from a list of custom objects is common in apps like online stores, libraries, or inventory systems.

💼 Career

Understanding LINQ with custom objects helps you write clean, readable code to query data collections efficiently in many C# jobs.

1

Create the Book class and list of books

Create a Book class with Title and Price properties. Then create a list called books with these exact entries: "C# Basics" priced 30, "LINQ in Action" priced 45, "ASP.NET Core" priced 50, "Entity Framework" priced 40.

C Sharp (C#)

// Define Book class and create list books with given entries
// Your code here

Hint

Define a class with two properties. Then create a list and add new Book objects with the exact titles and prices.

2

Set the price limit

Inside the Main method, create an integer variable called priceLimit and set it to 45.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Book
{
    public string Title { get; set; }
    public int Price { get; set; }
}

class Program
{
    static void Main()
    {
        List books = new List
        {
            new Book { Title = "C# Basics", Price = 30 },
            new Book { Title = "LINQ in Action", Price = 45 },
            new Book { Title = "ASP.NET Core", Price = 50 },
            new Book { Title = "Entity Framework", Price = 40 }
        };
        // Create priceLimit variable and set to 45
        // Your code here
    }
}

Hint

Just create an integer variable named priceLimit and assign it the value 45.

3

Use LINQ to find books cheaper than priceLimit

Use a LINQ query with var cheapBooks = from book in books where book.Price < priceLimit select book; inside the Main method, after defining priceLimit.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Book
{
    public string Title { get; set; }
    public int Price { get; set; }
}

class Program
{
    static void Main()
    {
        List books = new List
        {
            new Book { Title = "C# Basics", Price = 30 },
            new Book { Title = "LINQ in Action", Price = 45 },
            new Book { Title = "ASP.NET Core", Price = 50 },
            new Book { Title = "Entity Framework", Price = 40 }
        };
        int priceLimit = 45;
        // Use LINQ query to find books cheaper than priceLimit
        // Your code here
    }
}

Hint

Use a LINQ query expression with from, where, and select to filter books cheaper than priceLimit.

4

Print the titles of the filtered books

Use a foreach loop to print the Title of each book in cheapBooks.

C Sharp (C#)

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

class Book
{
    public string Title { get; set; }
    public int Price { get; set; }
}

class Program
{
    static void Main()
    {
        List books = new List
        {
            new Book { Title = "C# Basics", Price = 30 },
            new Book { Title = "LINQ in Action", Price = 45 },
            new Book { Title = "ASP.NET Core", Price = 50 },
            new Book { Title = "Entity Framework", Price = 40 }
        };
        int priceLimit = 45;
        var cheapBooks = from book in books
                         where book.Price < priceLimit
                         select book;
        // Print titles of cheapBooks
        // Your code here
    }
}

Hint

Use a foreach loop to go through cheapBooks and print each book's Title with Console.WriteLine.

Practice

1. What does LINQ primarily help you do with custom objects in C#?

easy

A. Create new classes automatically

B. Compile code faster

C. Filter, sort, and select data easily

D. Manage memory manually

Solution

1. Step 1: Understand LINQ's purpose

   LINQ is designed to query collections like lists of objects easily.

2. Step 2: Identify LINQ's main features

   It helps filter, sort, and select data without manual loops.

3. Final Answer:

   Filter, sort, and select data easily -> Option C

4. Quick Check:

   LINQ = Filter, sort, select [OK]

Hint: Remember LINQ is for querying data collections [OK]

Common Mistakes:

• Thinking LINQ creates classes
• Confusing LINQ with compilation
• Assuming LINQ manages memory

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

easy

A. var names = people.Select(p => p.Name());

B. var names = people.Select(p.Name);

C. var names = people.Select(p => p);

D. var names = people.Select(p => p.Name);

Solution

1. Step 1: Understand Select syntax

   Select expects a lambda expression to pick a property, like p => p.Name.

2. Step 2: Check each option

   var names = people.Select(p => p.Name); uses correct lambda syntax. var names = people.Select(p.Name); misses lambda. var names = people.Select(p => p); selects whole object. var names = people.Select(p => p.Name()); wrongly calls Name as method.

3. Final Answer:

   var names = people.Select(p => p.Name); -> Option D

4. Quick Check:

   Select needs lambda with property [OK]

Hint: Use lambda syntax p => p.Property for Select [OK]

Common Mistakes:

• Omitting lambda arrow =>
• Calling property as method
• Selecting whole object instead of property

3. Given the class Person { public string Name; public int Age; } and list people with three persons: Alice(30), Bob(25), and Carol(35), what does this LINQ query return?

var result = people.Where(p => p.Age > 28).Select(p => p.Name).ToList();

medium

A. ["Alice", "Carol"]

B. ["Bob"]

C. ["Alice", "Bob", "Carol"]

D. Empty list

Solution

1. Step 1: Filter people older than 28

   Alice is 30 (yes), Bob is 25 (no), Carol is 35 (yes).

2. Step 2: Select their names

   Names selected are "Alice" and "Carol".

3. Final Answer:

   ["Alice", "Carol"] -> Option A

4. Quick Check:

   Age > 28 filters Alice and Carol [OK]

Hint: Filter first, then select property [OK]

Common Mistakes:

• Including Bob who is younger
• Selecting whole object instead of names
• Confusing > with <

4. What is wrong with this LINQ query?

var adults = people.Where(p => p.Age >= 18).Select(p => p.Name);
foreach(var name in adults) Console.WriteLine(name);

medium

A. The query is correct and will print all names of adults

B. The lambda expression syntax is incorrect

C. The query is missing ToList() or ToArray(), so it won't compile

D. The Where clause should be after Select

Solution

1. Step 1: Check LINQ query syntax

   Where and Select are used correctly with proper lambda syntax.

2. Step 2: Check foreach usage

   LINQ returns IEnumerable<string>, which foreach can iterate without ToList().

3. Final Answer:

   The query is correct and will print all names of adults -> Option A

4. Quick Check:

   IEnumerable works with foreach directly [OK]

Hint: IEnumerable can be iterated without ToList() [OK]

Common Mistakes:

• Thinking ToList() is mandatory before foreach
• Misplacing Where and Select order
• Incorrect lambda syntax

5. You have a list of Product objects with properties Name (string) and Price (decimal). How do you create a dictionary with product names as keys and prices as values, but only include products costing more than 50 using LINQ?

hard

A. products.ToDictionary(p => p.Name, p => p.Price).Where(p => p.Value > 50);

B. products.Where(p => p.Price > 50).ToDictionary(p => p.Name, p => p.Price);

C. products.Select(p => new {p.Name, p.Price > 50}).ToDictionary(p => p.Name, p => p.Price);

D. products.Where(p => p.Price > 50).Select(p => p.Name).ToDictionary();

Solution

1. Step 1: Filter products with price > 50

   Use Where to keep only products costing more than 50.

2. Step 2: Convert filtered list to dictionary

   Use ToDictionary with key selector p.Name and value selector p.Price.

3. Final Answer:

   products.Where(p => p.Price > 50).ToDictionary(p => p.Name, p => p.Price); -> Option B

4. Quick Check:

   Filter then ToDictionary with key and value selectors [OK]

Hint: Filter first, then use ToDictionary with key and value [OK]

Common Mistakes:

• Trying to filter after ToDictionary (invalid)
• Selecting anonymous types instead of original objects
• Calling ToDictionary without key/value selectors