Notes

public class YourClassName
{
// Define a data structure to store last invoice numbers for each unique combination of company, PO number, and client contact
private Dictionary<string, int> lastInvoiceNumbers = new Dictionary<string, int>();

// Method to generate invoice number
private string GenerateInvoiceNumber(string companyId, string poNumber, string clientContact)
{
string key = $"{companyId}-{poNumber}-{clientContact}";

if (!lastInvoiceNumbers.ContainsKey(key))
{
// Start invoice number from 1000 for new combination
lastInvoiceNumbers[key] = GetNextInvoiceNumber();
}

return lastInvoiceNumbers[key].ToString();
}

// Method to get the next available invoice number
private int GetNextInvoiceNumber()
{
// Find the maximum invoice number used so far
int maxInvoiceNumber = lastInvoiceNumbers.Values.DefaultIfEmpty(999).Max();

// Increment by 1 for the next invoice number
return maxInvoiceNumber + 1;
}

// Your existing code...
// GetSalariedEmployeeListForInvoice method and other methods remain unchanged
}