主题：[求助]编号问题

以下是我模仿写的编号代码，不知问题出在哪，两三台机子同时工作时，依然会出现编号重复！求老师指点！谢谢！

Select e.DataCol.Name
    Case "投保日期","社区代码"
        If e.DataRow.IsNull("投保日期") OrElse e.DataRow.IsNull("社区代码") Then
            e.DataRow("保单编号") = Nothing
        Else
            Dim d As Date = e.DataRow("投保日期")
            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,1,1) '获得该月的第一天
            Dim ld As Date = New Date(y,12,31) '获得该月的最后一天
            Dim bh As String = e.DataRow("社区代码") & "-" & Format(d,"yyyy") & "-" '生成编号的前缀
            If e.DataRow("保单编号").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 = "社区代码 = '"& e.DataRow("社区代码") & "' And 投保日期 >= '" & fd & "' And 投保日期 <= '" & ld & "' And [_Identify] <> " & e.DataRow("_Identify")
                max = e.DataTable.SQLCompute("Max(保单编号)",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(18,4)) + 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("保单编号") = bh & Format(idx,"0000")
            End If
        End If
End Select