Need help, I am trying to make an array list which will add all of the invoice name in the database to a drop down list, however, getting errors

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

namespace MyClassLibrary
{
    class clsInvoiceNameCollection
    {
        //this function is used to populate the invoice class
        Int32 DisplayInvoice()
        {
            //create an instance of the invoice class
            clsInvoice Invoice = new clsInvoice();
            //var to store the invoice no primary key 
            string InvoiceNo;
            //var to store the name of the invoice 
            string InvoiceName;
            //var to store the index for the loop
            Int32 Index = 0;
            //while the index is less that the number of records to process
            while (Index < Invoices.Count)
            {
                //get the invoice number from the database
                InvoiceNo = Convert.ToString(Invoices.AllInvoices[Index].InvoiceNo);
                //get the invoice name from the database 
                Invoice = Invoice.AllInvoices[Index].Invoice;
                //set up the new row to be added to the list
                ListItem NewInvoice = new ListItem(Invoice, InvoiceNo);
                //add the new row to the list
                ddlInvoice.Items.Add(NewInvoice);
                //increment the index to the next record
                Index++;
            }
            //return the number of records found
            return Invoice.Count;
        }
    }
}

Error	1	The name 'Invoices' does not exist in the current context	
Error	2	The name 'Invoices' does not exist in the current context	
Error	3	'MyClassLibrary.clsInvoice' does not contain a definition for 'AllInvoices' and no extension method 'AllInvoices' accepting a first argument of type 'MyClassLibrary.clsInvoice' could be found (are you missing a using directive or an assembly reference?)	
Error	4	The type or namespace name 'ListItem' could not be found (are you missing a using directive or an assembly reference?)	
Error	5	The type or namespace name 'ListItem' could not be found (are you missing a using directive or an assembly reference?)
Error	6	The name 'ddlInvoice' does not exist in the current context	
Error	7	Cannot convert method group 'Count' to non-delegate type 'int'. Did you intend to invoke the method?	

- while (Index < Invoices.Count) 'Invoices' part
- InvoiceNo = Convert.ToString(Invoices.AllInvoices[Index].InvoiceNo); 'Invoices' part
- Invoice = Invoice.AllInvoices[Index].Invoice; 'AllInvoices' part
- ListItem NewInvoice = new ListItem(Invoice, InvoiceNo); Both the 'ListItem' parts
- ddlInvoice.Items.Add(NewInvoice); The 'ddlInvoice' part
- return Invoice.Count; 'Invoice.Count' part


}