“DD、登记时间、编号”都是订单表的列，

DataColChanged事件代码：

Select e.DataCol.Name
    Case "DD","登记时间"
        If e.DataRow.IsNull("登记时间") OrElse e.DataRow.IsNull("DD") Then
            e.DataRow("编号") = Nothing
        Else
            Dim d As Date = e.DataRow("登记时间")
            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("DD") & "-" & Format(d,"yyyyMM") & "-" \'生成编号的前缀
            If e.DataRow("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 = "DD = \'"& e.DataRow("DD") & "\' And 登记时间 >= #" & fd & "# And 登记时间 <= #" & ld & "# And [_Identify] <> " & e.DataRow("_Identify")
                max = e.DataTable.Compute("Max(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("编号") = bh & Format(idx,"0000")
            End If
        End If
End Select

按上述设置后，第一次新增行时没有问题，第二次开始出错。

提示信息：

.NET Framework 版本：2.0.50727.3053
Foxtable 版本：2014.11.11.1
错误所在事件：表，订单，DataColChanged
详细错误信息：
Exception has been thrown by the target of an invocation.
Index and length must refer to a location within the string.
Parameter name: length

根据这句代码：

Dim bh As String = e.DataRow("DD") & "-" & Format(d,"yyyyMM") & "-" \'生成编号的前缀

你变好的前缀是9位，不是12位，

所以:

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

应该改为：

idx = CInt(max.Substring(9,4)) + 1 \'获得最大单据编号的后四位顺序号,并加1