LINQ

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⚡ LINQ Interview Questions (Real-World Scenarios)

Here are some important LINQ-based coding problems, structured cleanly for learning, interviews, and quick revision.

🔢 Find the Second Most Frequent Number

📌 Problem

Given a list of integers:
[4, 5, 6, 5, 4, 6, 6, 4, 4]

Find the second most frequent number.

✅ Solution

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

class Program
{
    static void Main(string[] args)
    {
        var list1 = new List<int> { 4, 5, 6, 5, 4, 6, 6, 4, 4 };

        var frequentNumbers = list1
            .GroupBy(x => x)
            .Select(s => new { Element = s.Key, Occurence = s.Count() })
            .OrderByDescending(x => x.Occurence);

        var secondMostFrequent = frequentNumbers.Skip(1).Take(1).ToList();

        Console.WriteLine(string.Join(" ", secondMostFrequent));
        Console.ReadLine();
    }
}

👨‍💻 Filter Employees Based on Multiple Conditions

📌 Problem

Find all employees who:

  • Work in IT department

  • Have salary > 50,000

  • Are under 40 years old

✅ Solution

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

public class Employee
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Department { get; set; }
    public double Salary { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var employees = new List<Employee>
        {
            new Employee { Name = "Alice", Age = 30, Department = "IT", Salary = 56000},
            new Employee { Name = "Bob", Age = 45, Department = "HR", Salary = 50000},
            new Employee { Name = "Charlie", Age = 35, Department = "IT", Salary = 35000 },
            new Employee { Name = "David", Age = 28, Department = "Finance", Salary = 100000 }
        };

        var worksInIT = employees.Where(x => x.Department == "IT").Select(x => x.Name);
        var salaryGreaterThan50000 = employees.Where(x => x.Salary > 50000).Select(x => x.Name);
        var ageUnder40 = employees.Where(x => x.Age < 40).Select(x => x.Name);

        Console.WriteLine("Works in IT: ");
        Console.WriteLine(string.Join(", ", worksInIT));

        Console.WriteLine("Salary Greater Than 50000: ");
        Console.WriteLine(string.Join(", ", salaryGreaterThan50000));

        Console.WriteLine("Age Under 40: ");
        Console.WriteLine(string.Join(", ", ageUnder40));

        Console.ReadLine();
    }
}

🛒 Find Top N Records (Top 3 Expensive Products)

📌 Problem

Get the top 3 most expensive products.

✅ Solution

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

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

class Program
{
    static void Main(string[] args)
    {
        List<Product> products = new List<Product>
        {
            new Product { Id = 1, Name = "Laptop", Price = 1500m },
            new Product { Id = 2, Name = "Smartphone", Price = 800m },
            new Product { Id = 3, Name = "Tablet", Price = 600m },
            new Product { Id = 4, Name = "Smartwatch", Price = 200m },
            new Product { Id = 5, Name = "Headphones", Price = 300m },
            new Product { Id = 6, Name = "Monitor", Price = 400m },
        };

        var mostExpensiveProducts = products
            .OrderByDescending(x => x.Price)
            .Select(x => x.Name)
            .Take(3)
            .ToList();

        Console.WriteLine(string.Join(", ", mostExpensiveProducts));
        Console.ReadLine();
    }
}

🏆 Highest Salary in Each Department

employees
    .GroupBy(x => x.Department)
    .Select(s => s.ToList()
    .OrderByDescending(x => x.Salary)
    .First());

💰 Total Sales for Each Product

transactions
    .GroupBy(x => x.ProductName)
    .Select(x => new 
    { 
        ProductName = x.Key,
        TotalSales = x.Sum(g => g.Amount) 
    })
    .ToList();

🥈 Second Highest Salary

var result = employees
    .OrderByDescending(e => e.Salary)
    .Skip(1)
    .Take(1);

📈 Employees Earning More Than Average Salary

var averageSalary = employees.Average(x => x.Salary);

var result = employees
    .Where(x => x.Salary > averageSalary);

⏳ Employees with More Than 5 Years Experience

employees
    .Where(x => x.HireDate < DateTime.Now.AddYears(-5))
    .ToList();

🏅 Department with Highest Average Salary

employees
    .GroupBy(x => x.Department)
    .Select(s => new 
    { 
        Average = s.Average(g => g.Salary) 
    })
    .OrderByDescending(o => o.Average)
    .FirstOrDefault();

🧠 Key Takeaways

  • GroupBy() is essential for aggregation problems.

  • OrderByDescending() + Skip() + Take() helps solve ranking problems.

  • Where() is used for filtering based on conditions.

  • Select() helps shape the output.

  • LINQ provides a clean and readable way to handle complex data operations.

These LINQ problems are very common in .NET interviews and real-world applications, especially when working with collections and in-memory data processing.

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