LINQ with custom objects in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

LINQ Query Output with Custom Objects

What is the output of the following C# code using LINQ on a list of custom Person objects?

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; }
}

public class Program {
    public static void Main() {
        var people = new List<Person> {
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 25 },
            new Person { Name = "Charlie", Age = 35 },
            new Person { Name = "Diana", Age = 28 }
        };

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

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

A

Alice
Diana
Charlie

B

Charlie
Alice

C

Bob
Diana

D

Alice
Charlie

💡 Hint

Look at the filter condition Age > 28 and then the ordering by Name.

The code filters people older than 28: Alice (30) and Charlie (35). Then it orders them by name alphabetically: Alice, then Charlie. So the output is those two names in that order.

🧠 Conceptual

intermediate

Understanding LINQ Select Projection

Given a list of Product objects, which LINQ query correctly projects only the product names into a list of strings?

C Sharp (C#)

public class Product {
    public int Id { get; set; }
    public string Name { get; set; }
    public double Price { get; set; }
}

var products = new List<Product> {
    new Product { Id = 1, Name = "Pen", Price = 1.5 },
    new Product { Id = 2, Name = "Notebook", Price = 3.0 },
    new Product { Id = 3, Name = "Eraser", Price = 0.5 }
};

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

B var names = products.Where(p => p.Name).ToList();

C var names = products.Select(p => p.Price).ToList();

D var names = products.OrderBy(p => p.Name).ToList();

💡 Hint

Projection means selecting a specific property from each object.

Select is used to transform each object into something else. Here, p => p.Name picks the name property. Where filters, so option A is wrong. Option A selects prices, not names. Option A orders but does not project names.

🔧 Debug

advanced

Fix the LINQ Query to Avoid Runtime Error

The following code throws a runtime exception. Which option fixes the code so it runs without error and returns the names of employees older than 40?

C Sharp (C#)

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

public class Employee {
    public string Name { get; set; }
    public int? Age { get; set; }
}

public class Program {
    public static void Main() {
        var employees = new List<Employee> {
            new Employee { Name = "John", Age = 45 },
            new Employee { Name = "Jane", Age = null },
            new Employee { Name = "Mark", Age = 50 }
        };

        var result = employees.Where(e => e.Age > 40).Select(e => e.Name);

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

A var result = employees.Where(e => e.Age != null).Select(e => e.Name);

B var result = employees.Where(e => e.Age.HasValue && e.Age > 40).Select(e => e.Name);

C var result = employees.Where(e => e.Age.Value > 40).Select(e => e.Name);

D var result = employees.Where(e => e.Age > 40).Select(e => e.Name);

💡 Hint

Nullable types need to be checked before comparing their values.

Option B checks if Age has a value before comparing, preventing a InvalidOperationException. Option B causes runtime error because it compares nullable without check. Option B throws if Age is null. Option B filters non-null but does not check age > 40.

📝 Syntax

advanced

Identify the Syntax Error in LINQ Query

Which option contains a syntax error in the LINQ query that selects all customers with balance greater than 1000?

C Sharp (C#)

public class Customer {
    public string Name { get; set; }
    public decimal Balance { get; set; }
}

var customers = new List<Customer> {
    new Customer { Name = "Anna", Balance = 1200 },
    new Customer { Name = "Ben", Balance = 800 },
    new Customer { Name = "Cara", Balance = 1500 }
};

A var richCustomers = customers.Where(c => c.Balance > 1000).Select(c.Name);

B var richCustomers = customers.Where(c => c.Balance > 1000).Select(c => c.Name);

C var richCustomers = from c in customers where c.Balance > 1000 select c.Name;

D var richCustomers = customers.Where(c => c.Balance > 1000).Select(c => c.Name).ToList();

💡 Hint

Check the syntax of the Select method usage.

Option A uses Select(c.Name) which is invalid syntax. The Select method requires a lambda expression like c => c.Name. Options B, C, and D are syntactically correct.

🚀 Application

expert

Count Unique Cities from a List of People Using LINQ

Given a list of Person objects where each has a City property, which LINQ expression correctly counts how many unique cities are in the list?

C Sharp (C#)

public class Person {
    public string Name { get; set; }
    public string City { get; set; }
}

var people = new List<Person> {
    new Person { Name = "Tom", City = "New York" },
    new Person { Name = "Sara", City = "Boston" },
    new Person { Name = "Mike", City = "New York" },
    new Person { Name = "Anna", City = "Chicago" }
};

A int count = people.GroupBy(p => p.City).Count();

B int count = people.Count(p => p.City.Distinct());

C int count = people.Select(p => p.City).Distinct().Count();

D int count = people.Select(p => p.City).Count();

💡 Hint

Think about how to get unique values and then count them.

Option C selects all cities, removes duplicates with Distinct(), then counts them. Option C is invalid because p.City.Distinct() is invalid on string like that. Option C groups by city and counts groups, which also works but is not the correct answer here because it is not the option asked for. Option C counts all cities including duplicates.