Hi,
Not sure what you are trying to achieve here but:
(a) the usage will be confusing since cust[0] will refer to an Invoice
(b) you can't add to a list via an indexer so 'cust[0] = InvA;' will fail on an empty list.

Here's a simple example that shows one way an indexer can be helpful:

public class InvoiceList
    {
        List<Invoice> Invoices = new List<Invoice>();

        //Add a Couple of dummy Invoices:
        public InvoiceList()
        {
            Invoices.Add(new Invoice(){Id="A123"});
            Invoices.Add(new Invoice(){Id="B345"});
        }

        public Invoice this[string s]
        {
            get
            {
                Invoice i = Invoices.Find(x => x.Id == s);
                return i;
            }
        }
    }

    public class Invoice
    {
        public string Id { get; set; }
    }

Given this you can access an Invoice using the Id as an indexer. e.g.:

InvoiceList l = new InvoiceList();
Invoice v = l["A123"];

>I have this customer class:
>
>

>public class Customer
>{
>    private List<Invoice> _Invoices = new List<Invoice>();
>    public Invoice this[int item]
>    {
>        get
>        {
>            return _Invoices[item];
>        }
>        set
>        {
>            _Invoices[item] = value;
>        }
>    }
>}
>

>
>I then do this:
>
>

>static void Main(string[] args)
>{
>    Invoice InvA = new Invoice();
>    InvA.InvoiceId = "A0001";
>
>    Invoice InvB = new Invoice();
>    InvB.InvoiceId = "B0002";
>
>    Customer cust = new Customer();
>    cust[0] = InvA;
>    cust[1] = InvB;
>}
>

>
>I'm not really seeing a usage for indexers. It seems to me that an Invoices collection would be better. What's the point of indexers?