insert and update in vb.net with access index out of range - vb.net

Question

i have application i want it to do
1- insert the first statement .
2-get the (maxid) of the invoice
3-insert the detail of the invoice with the Id .
it give me this error
index out of range .
Private Sub insert()
Dim invoiceday As Date = Today
Dim userid As Integer
Dim clientid As Integer
Dim note As String
If clients.Visible = True Then
userid = 1
clientid = 2
note = "cash"
Else
userid = 2
clientid = 6
note = "credit"
End If
Dim query1 As String = " insert into invoices([purchasedate],clientid,[Note],userID,total,disq) Values ( '" & CDate(invoiceday) & "','" & clientid & "','" & note & "','" & userid & "','" & totalprice.Text & "','" & txtdis.Text & "')"
samselect2(query1)
Dim maxinvoice As Integer
maxinvoice = invoiceid()
For Each row As DataGridViewRow In dvsale.Rows
Dim query As String = " insert into invoicede(invoiceid,barcode,doaname,Qty,Price,qtyprice) Values ('" & maxinvoice + 1 & "','" & row.Cells("barcode").Value & "','" & row.Cells("doaname").Value & "','" & row.Cells("qty").Value & "','" & row.Cells("price").Value & "','" & row.Cells("tqty").Value & "')"
samselect2(query)
Next
End Sub
this the class of the samselect2
Public Sub samselect2(ByVal sql As String)
Try
con.Open()
With cmd
.Connection = con
.CommandText = sql
End With
Catch ex As Exception
MsgBox(ex.Message, MsgBoxStyle.Exclamation)
End Try
con.Close()
da.Dispose()
End Sub
and the invoiceid max number
Private Function invoiceid()
checkConnection()
Dim strQ As String = "SELECT max(invoiceID)as MaxIDbatch from invoices "
Dim cmdQ As OleDbCommand = New OleDbCommand(strQ, con)
Dim result = cmdQ.ExecuteScalar()
If result IsNot Nothing Then
Dim x As Integer = 0
Return x
Else
Return result
End If
End Function

Answer 1

Although I don't fully understand your question, I notice a couple things: 1) It appears that InvoiceID is not an auto-incrementing identity field, and it probably should be. (Check out "Is Identity".) If it were, you could insert and specify all fields except that one, and the number would increment automatically. 2) In your for...each, you are not increasing maxinvoice at all with each iteration through the loop. Although you're using maxinvoice+1, it will still be the same number each time.
Still, all of that should be unnecessary if you make InvoiceID an identity field.