C# Online Compiler | .NET Fiddle

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

public class Program
{
    public static void Main()
    {
        List<Sale> sales = new List<Sale>
        {
            new Sale { SaleID = 1, CustomerID = 1, ProductID = "ProductA", Date = new DateTime(2023, 1, 1), Amount = 100m },
            new Sale { SaleID = 2, CustomerID = 2, ProductID = "ProductB", Date = new DateTime(2023, 1, 2), Amount = 150m },
            new Sale { SaleID = 3, CustomerID = 1, ProductID = "ProductA", Date = new DateTime(2023, 1, 3), Amount = 200m },
            new Sale { SaleID = 4, CustomerID = 3, ProductID = "ProductC", Date = new DateTime(2023, 1, 4), Amount = 300m },
            new Sale { SaleID = 5, CustomerID = 2, ProductID = "ProductA", Date = new DateTime(2023, 1, 5), Amount = 250m }
        };

        List<Customer> customers = new List<Customer>
        {
            new Customer { CustomerID = 1, Age = 25, Gender = "Male", Location = "City A" },
            new Customer { CustomerID = 2, Age = 30, Gender = "Female", Location = "City B" },
            new Customer { CustomerID = 3, Age = 22, Gender = "Male", Location = "City A" }
        };

        // Step 1: Calculate the total sales amount and the number of sales for each product
        var salesByProduct = sales
            .GroupBy(s => s.ProductID)
            .Select(g => new { ProductID = g.Key, TotalSalesAmount = g.Sum(s => s.Amount), NumberOfSales = g.Count() })
            .ToList();

        // Step 2: Identify the top-selling product based on the total sales amount
        var topSellingProduct = salesByProduct
            .OrderByDescending(s => s.TotalSalesAmount)
            .First();

        Console.WriteLine($"Top-Selling Product: {topSellingProduct.ProductID}, Total Sales Amount: {topSellingProduct.TotalSalesAmount}");

        // Step 3: Analyze the demographic profile of customers who purchased the top-selling product
        var customerDemographics = sales
            .Where(s => s.ProductID == topSellingProduct.ProductID)
            .Join(customers, s => s.CustomerID, c => c.CustomerID, (s, c) => c)
            .GroupBy(c => c.Gender)
            .Select(g => new { Gender = g.Key, AverageAge = g.Average(c => c.Age), Count = g.Count() })
            .ToList();

        foreach (var demographic in customerDemographics)
        {
            Console.WriteLine($"Gender: {demographic.Gender}, Average Age: {demographic.AverageAge}, Count: {demographic.Count}");
        }
    }
}

public class Sale
{
    public int SaleID { get; set; }
    public int CustomerID { get; set; }
    public string ProductID { get; set; }
    public DateTime Date { get; set; }
    public decimal Amount { get; set; }
}

public class Customer
{
    public int CustomerID { get; set; }
    public int Age { get; set; }
    public string Gender { get; set; }
    public string Location { get; set; }
}

 
using System;
using System.Collections.Generic;
using System.Linq;
​
public class Program
{
    public static void Main()
    {
        List<Sale> sales = new List<Sale>
        {
            new Sale { SaleID = 1, CustomerID = 1, ProductID = "ProductA", Date = new DateTime(2023, 1, 1), Amount = 100m },
            new Sale { SaleID = 2, CustomerID = 2, ProductID = "ProductB", Date = new DateTime(2023, 1, 2), Amount = 150m },
            new Sale { SaleID = 3, CustomerID = 1, ProductID = "ProductA", Date = new DateTime(2023, 1, 3), Amount = 200m },
            new Sale { SaleID = 4, CustomerID = 3, ProductID = "ProductC", Date = new DateTime(2023, 1, 4), Amount = 300m },
            new Sale { SaleID = 5, CustomerID = 2, ProductID = "ProductA", Date = new DateTime(2023, 1, 5), Amount = 250m }
        };
​
        List<Customer> customers = new List<Customer>
        {
            new Customer { CustomerID = 1, Age = 25, Gender = "Male", Location = "City A" },
            new Customer { CustomerID = 2, Age = 30, Gender = "Female", Location = "City B" },
            new Customer { CustomerID = 3, Age = 22, Gender = "Male", Location = "City A" }
        };
​

Cached Result

r3.Rank = 2
0 1 2 3 4 7