using System;

using System.Collections.Generic;

using System.Linq;

​

var invoices = new List<Invoice>

{

    new Invoice("Germany",  BillingMethod.Paper,        InvoiceCount: 4,    CustomerCount: 5,   1234),

    new Invoice("Germany",  BillingMethod.Electronic,   InvoiceCount: 52,   CustomerCount: 34,  4567),

    new Invoice("Greece",   BillingMethod.Paper,        InvoiceCount: 8,    CustomerCount: 75,  78955),

    new Invoice("Denmark",  BillingMethod.Paper,        InvoiceCount: 32,   CustomerCount: 15,  7890),

    new Invoice("Norway",   BillingMethod.Electronic,   InvoiceCount: 4,    CustomerCount: 52,  65898),

};

​

Console.WriteLine("Invoices:");

foreach (var invoice in invoices)

    Console.WriteLine(invoice);

​

var flattened = invoices.GroupBy(invoice => invoice.Country)

                        .Select(group => new CountryInvoice(group));

​

Console.WriteLine("Flattened Invoices:");

foreach (var invoice in flattened)

    Console.WriteLine(invoice);

​

​

enum BillingMethod { None, Paper, Electronic }

​

record Invoice(string Country,

               BillingMethod BillingMethod,

               int InvoiceCount,

               int CustomerCount,

               decimal Amount);

​

record CountryInvoice(string Country,

                      int PaperInvoiceCount,

                      int PaperCustomerCount,

                      decimal PaperAmount,

                      int ElectronicInvoiceCount,

                      int ElectronicCustomerCount,

                      decimal ElectronicAmount)

{

    public CountryInvoice(IGrouping<string, Invoice> group)

        : this(group.Key, 0, 0, 0, 0, 0, 0)

    {

        foreach(var invoice in group)

        {

            switch(invoice.BillingMethod)

            {

                case BillingMethod.Paper:

                    PaperInvoiceCount = invoice.InvoiceCount;

                    PaperCustomerCount = invoice.CustomerCount;

                    PaperAmount = invoice.Amount;

                    break;

​

                case BillingMethod.Electronic:

                    ElectronicInvoiceCount = invoice.InvoiceCount;

                    ElectronicCustomerCount = invoice.CustomerCount;

                    ElectronicAmount = invoice.Amount;

                    break;

            }

        }

    }

}

​

​