Excel VBA : Auto numbering - vba

Question

I'm creating a database on Excel, and encountered some problems as I tried to assign auto number to each row.
Requirements are:
generate auto number to each row(on the column A) when column B is not blank.
the number should be unique and must always be connected to the contents of the same row even when the column is sorted or when new rows are inserted, etc.
when a new row is inserted (anywhere on the same column), a new number should be assigned (the newest number should be the biggest number)
if
possible, the auto number should have a prefix, and number should be displayed in four digits (e.g. 0001, 0011)
I have tried some VBA codes I found from other people's questions (e.g. Excel VBA : Auto Generating Unique Number for each row).
So far, the code below has worked the best, but the requirement (3) and (4) couldn't be solved by that code.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim maxNumber
If Not Intersect(Target, Range("B:B")) Is Nothing Then
' don't run when more than one row is changed
If Target.Rows.Count > 1 Then Exit Sub
' if column A in the current row has a value, don't run
If Cells(Target.Row, 1) > 0 Then Exit Sub
' get the highest number in column A, then add 1 and write to the
' current row, column A
maxNumber = Application.WorksheetFunction.Max(Range("A:A"))
Target.Offset(0, -1) = maxNumber + 1
End If
End Sub
I'm short of the knowledge of VBA and I hope someone could help me this.
Many thanks.

Answer 1

Alternative via CustomDocumentProperties
Instead of using a hidden sheet as proposed by #TimWilliams, one can assign incremented values to a user defined custom document property (CDP), naming it e.g. "InvNo" holding the newest invoice number. The cdp remain stored in the saved workbook.
The function below gets the current number saved to this workbook related property and returns the next number by adding 1 to the current value. It uses a help procedure RefreshCDP to assign the new value (could be used of course independantly to reset values programmaticaly to any other value). - If the cdp name isn't passed as (optional) argument, the function assumes "InvNo" by default.
Note that code requires some error handling to check if the cdp exists.
Example call
Dim InvoiceNumber as Long
InvoiceNumber = NextNumber("InvNo") ' or simply: NextNumber
Public Function NextNumber(Optional CDPName As String = "InvNo") As Long
'a) get current cdp value
Dim curVal As Long
On Error Resume Next
curVal = ThisWorkbook.CustomDocumentProperties(CDPName)
If Err.Number <> 0 Then Err.Clear ' not yet existing, results in curVal of 0
'b) increment current cdp value by one to simulate new value
Dim newVal As Long
newVal = curVal + 1
'Debug.Print "Next " & CDPName & " will be: " & newVal
'c) assign new value to custom document property
RefreshCDP CDPName, newVal, msoPropertyTypeNumber
'Debug.Print "New " & CDPName & " now is: " & ThisWorkbook.CustomDocumentProperties(CDPName)
NextNumber = newVal
End Function
Help procedure RefreshCDP
Sub RefreshCDP(CDPName As String, _
newVal As Variant, docType As Office.MsoDocProperties)
On Error Resume Next
ThisWorkbook.CustomDocumentProperties(CDPName).Value = newVal
'If cdp doesn't exist yet, create it (plus adding the new value)
If Err.Number > 0 Then
ThisWorkbook.CustomDocumentProperties.Add _
Name:=CDPName, _
LinkToContent:=False, _
Type:=docType, _
Value:=newVal
End If
End Sub
Related links
MS help: Excel.Workbook.CustomDocumentProperties
Check if BuiltInDocumentProperty is set without error trapping
Chip Pearson: Document Properties
How to add a DocumentProperty to CustomDocumentProperties in Excel?

Answer 2

Do not use Max() to find the next number - use instead a hidden sheet or name to store the current number, and increment it each time a new Id is required.
For example:
Public Function NextNumber(SequenceName As String)
Dim n As Name, v
On Error Resume Next
Set n = ThisWorkbook.Names(SequenceName)
On Error GoTo 0
If n Is Nothing Then
'create the name if it doesn't exist
ThisWorkbook.Names.Add SequenceName, RefersTo:=2
v = 1
Else
'increment the current value
v = Replace(n.RefersTo, "=", "")
n.RefersTo = v + 1
End If
NextNumber = v
End Function
This allows you to use multiple different sequences as long as you give each one a distinct name.
Dim seq
seq = NextNumber("seqOne")
'etc