我的数据是外部access,按页加载的 。同一页时不会报错，同一工地在不同页时会报错，请帮忙改一下代码

Select e.DataCol.Name
    Case "工地"
        If e.DataRow.IsNull("工地") Then
            e.DataRow("工地批次") = Nothing
        Else
            Dim lb As String = e.DataRow("工地")
            If e.DataRow("工地批次").StartsWith(lb) = False \'如果单据编号前缀不符
                Dim max As String
                Dim idx As Integer
                max = e.DataTable.Compute("Max(工地批次)","工地 = \'" & lb & "\' 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(max.length-3)) + 1

                Else
                    idx = 1 \'否则顺序号等于1
                End If
                e.DataRow("工地批次") = lb & Format(idx,"000")
            End If
        End If
End Select