主题：关于自动编号的问题

我想通过选择监测类别，自动完成项目编号，项目编号的规则是类别代码+年份+顺序号，以下代码自动生成的代码不会自动加1，请老师看看是哪里有错误？

Select e.DataCol.Name
    Case "监测类别"
        If e.DataRow.IsNull("监测类别") Then
            e.DataRow("项目编号") = Nothing
        Else
            Dim lb As String
        Select e.DataRow("监测类别")
            Case "环境现状"
                lb="H"
            Case "竣工验收"
                lb="Y"
            Case "排污许可"
                lb="P"
            Case "其他委托"
                lb="W"
            Case "来样送检"
                lb="S"
            Case "验证比对"
                lb="B"
        End Select
            If e.DataRow("项目编号").StartsWith(lb) = False '如果单据编号前缀不符
                Dim max As String
                Dim idx As Integer
                Dim y As Date= Date.Today()
                Dim bh As String = lb & y.year
                max = e.DataTable.Compute("Max(项目编号)","监测类别 = '" & bh & "' And [_Identify] <> " & e.DataRow("_Identify")) '取得该类别的最大编号
                If max > "" Then '如果存在最大编号
                    idx = CInt(max.Substring(5,3)) + 1 '获得最大编号的后三位顺序号,并加1
                Else
                    idx = 1 '否则顺序号等于1
                End If
                e.DataRow("项目编号") = bh & Format(idx,"000")
            End If
        End If
End Select