Notes

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Threading.Tasks;

public class YourClassName
{
private readonly YourDatabaseContext dbContext; // Assuming you have a database context class

public YourClassName(YourDatabaseContext dbContext)
{
this.dbContext = dbContext;
}

public async Task<InvoiceMasterModelForSalariedEmployee> GetSalariedEmployeeListForInvoice(string startDate, string endDate, string companyId)
{
int lastUsedInvoiceNumber = RetrieveLastUsedInvoiceNumberFromDatabase();

List<int> generatedInvoiceNumbers = new List<int>();

// Your existing code here

foreach (var item in getExternalConsultingSalaryList.DistinctBy(x => new { x.CandidateId, x.BillingStartId, x.BillingRecId }))
{
// Your existing code here

if (/* Conditions */)
{
// Your existing code here

// Increment the last used invoice number
lastUsedInvoiceNumber++;

// Add the generated invoice number to the list
generatedInvoiceNumbers.Add(lastUsedInvoiceNumber);

// Set the invoice number for the current invoice
invoiceModelForSalariedEmployee.InvoiceNumber = lastUsedInvoiceNumber.ToString();

// Your existing code here
}
}

// Save the generated invoice numbers to the database
SaveGeneratedInvoiceNumbersToDatabase(generatedInvoiceNumbers);

// Save the last used invoice number to the database for next month
SaveLastUsedInvoiceNumberToDatabase(lastUsedInvoiceNumber);

// Your existing code here
}

private int RetrieveLastUsedInvoiceNumberFromDatabase()
{
// Retrieve the last used invoice number from the database
// For example:
var lastInvoiceNumberRecord = dbContext.InvoiceNumberRecords.FirstOrDefault();
if (lastInvoiceNumberRecord != null)
{
return lastInvoiceNumberRecord.LastUsedInvoiceNumber;
}
else
{
// If no record exists, return the starting invoice number
return 1000;
}
}

private void SaveGeneratedInvoiceNumbersToDatabase(List<int> generatedInvoiceNumbers)
{
// Save the generated invoice numbers to the database
// For example:
foreach (int invoiceNumber in generatedInvoiceNumbers)
{
dbContext.GeneratedInvoiceNumbers.Add(new GeneratedInvoiceNumber { InvoiceNumber = invoiceNumber });
}

dbContext.SaveChanges();
}

private void SaveLastUsedInvoiceNumberToDatabase(int invoiceNumber)
{
// Update or insert the last used invoice number record in the database
// For example:
var lastInvoiceNumberRecord = dbContext.InvoiceNumberRecords.FirstOrDefault();
if (lastInvoiceNumberRecord != null)
{
lastInvoiceNumberRecord.LastUsedInvoiceNumber = invoiceNumber;
}
else
{
dbContext.InvoiceNumberRecords.Add(new InvoiceNumberRecord { LastUsedInvoiceNumber = invoiceNumber });
}

dbContext.SaveChanges();
}
}