主题：实现“入库单在保存时自动生成会计凭证”

Select e.DataCol.Name
    Case "制单日期","工程代码"
        If e.DataRow.IsNull("制单日期") OrElse e.DataRow.IsNull("工程代码") Then
            e.DataRow("单据编号") = Nothing
        Else
            Dim d As Date = e.DataRow("制单日期")
            Dim y As Integer = d.Year
            Dim m As Integer = d.Month
            Dim Days As Integer = Date.DaysInMonth(y,m)
            Dim fd As Date = New Date(y,m,1) '获得该月的第一天
            Dim ld As Date = New Date(y,m,Days) '获得该月的最后一天
            Dim bh As String = e.DataRow("工程代码") & "-" & Format(d,"yyyyMM") & "-" '生成编号的前缀
            If e.DataRow("单据编号").StartsWith(bh) = False '如果单据编号前缀不符
                Dim max As String
                Dim idx As Integer
                Dim flt As String
                flt = "工程代码 = '"& e.DataRow("工程代码") & "' And 制单日期 >= #" & fd & "# And 制单日期 <= #" & ld & "# And [_Identify] <> " & e.DataRow("_Identify")
                max = e.DataTable.Compute("Max(单据编号)",flt) '取得该月的相同工程代码的最大单据编号
                If max > "" Then '如果存在最大单据编号
                    idx = CInt(max.Substring(12,4)) + 1 '获得最大单据编号的后四位顺序号,并加1
                Else
                    idx = 1 '否则顺序号等于1
                End If
                e.DataRow("单据编号") = bh & Format(idx,"0000")
            End If
        End If
End Select

现在我想要实现不论什么工程代码和制单日期 单据编号自动生成，后四位永不重复，代码要怎样必写？谢谢老师指导