LINQ with custom objects in C Sharp (C#)

Introduction

LINQ helps you easily find, filter, or sort data inside lists of your own objects, like people or products.

You have a list of students and want to find those with grades above 90.

You want to sort a list of books by their publication year.

You need to filter employees who work in a specific department.

You want to select only the names from a list of customer objects.

Syntax

C Sharp (C#)

var result = from item in collection
             where condition
             select item;

collection is your list of custom objects.

condition is how you filter the objects.

Examples

Selects all people who are 18 or older.

C Sharp (C#)

var adults = from person in people
             where person.Age >= 18
             select person;

Selects only the names from the list of people.

C Sharp (C#)

var names = from person in people
            select person.Name;

Sorts books by their publication year.

C Sharp (C#)

var sortedBooks = from book in books
                  orderby book.Year
                  select book;

Sample Program

This program creates a list of people, then uses LINQ to find all who are 18 or older. It prints their names.

C Sharp (C#)

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

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

class Program
{
    static void Main()
    {
        List<Person> people = new List<Person>
        {
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 17 },
            new Person { Name = "Charlie", Age = 25 }
        };

        var adults = from person in people
                     where person.Age >= 18
                     select person.Name;

        foreach (var name in adults)
        {
            Console.WriteLine(name);
        }
    }
}

Important Notes

You can use LINQ with any list of your own objects.

LINQ queries are easy to read and write, like asking questions about your data.

Remember to include using System.Linq; to use LINQ features.

Summary

LINQ lets you filter, sort, and select data from lists of custom objects easily.

You write simple queries that look like questions about your data.

It works well with your own classes and properties.

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