>I've got the follow UDF to get the next invoice number in a VFP table called DocsNumbers.
>

>LOCAL lnNext
>
>SET MULTILOCKS ON
>SET REPROCESS TO 0 Automatic
>
>SELECT DocsNumbers
>WAIT WINDOW "Gettign document number..." NOWAIT
>RLOCK()
>lnNext = DocsNumbers.invoice +1
>REPLACE DocsNumbers.invoice WITH lnNext
>TABLEUPDATE(.T.)
>UNLOCK IN DocsNumbers
>WAIT CLEAR
>
>RETURN lnNext
>

>
>How can I do this in T-SQL using a SQL server database ?
>
>T.I.A.

SQL Server performs table, page, and row-level locking automatically:

declare @Next int

begin transaction
    select @Next = invoice + 1 from DocsNumbers
    update DocsNumbers
        set invoice = @Next
end transaction

For more info about SQL locking, see "SET TRANSACTION ISOLATION LEVEL" in the BOL.