Select e.DataCol.Name
    Case "类别"
        If e.DataRow.IsNull("类别") Then
            e.DataRow("编号") = Nothing
        Else
            Dim lb As String = e.DataRow("类别")
            If e.DataRow("编号").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(编号)","类别 = \'" & 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(2,3)) + 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("编号") = lb & Format(idx,"000")
            End If
        End If
End Select

DataColChanged按类别生成编码  怎么转换成sql语句写法呢？

Dim lb As String = "DY"
Dim max As String
Dim idx As Integer
Dim cmd As New SQ LCommand
cmd.Conn ectio nNa me = "主数据源"
cmd.Comm andText = "Se lect * From 台账"
Dim dt As DataTable = cmd.ExecuteReader()
Dim dr As DataRow = dt.DataRows(dt.DataRows.Count - 1) \'获得最后一行
output.show(dr("_Identify"))
max = dt.Compute("Max(编号)","分类代码 = \'" & lb & "\'And [_Identify] <> " & dr("_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(2,3)) + 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
output.show(lb & Format(idx,"000"))

---------------------------
版本:2022.1.30.2
---------------------------
代码执行出错，错误信息：

System.InvalidCastException: 从字符串“ZBD”到类型“Integer”的转换无效。 ---> System.FormatException: 输入字符串的格式不正确。

   在 Microsoft.VisualBasic.CompilerServices.Conversions.ParseDouble(String Value, NumberFormatInfo NumberFormat)

   在 Microsoft.VisualBasic.CompilerServices.Conversions.ToInteger(String Value)

   --- 内部异常堆栈跟踪的结尾 ---

   在 Microsoft.VisualBasic.CompilerServices.Conversions.ToInteger(String Value)

   在 UserCode.Test()
---------------------------
确定  
---------------------------

Dim lb As String = "DY"
Dim idx As Integer
Dim cmd As New  SQLCommand
cmd.Con ne ctio n Name = "主数据源"
cmd.CommandText = "Se lect * From 办案装备_管理台账"
Dim dt As DataTable = cmd.ExecuteReader()
Dim dr As DataRow = dt.DataRows(dt.DataRows.Count - 1) \'获得最后一行
output.show(dr("_Identify"))
Dim max As String = "sele ct Max(编号) from 办案装备_管理台账 where 分类代码 = \'" & lb & "\' And [_Identify] <> " & dr("_Identify")
output.show(max)
If max > "" Then \'如果存在最大编号
    idx = CInt(max.Substring(2,3)) + 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
output.show(lb & Format(idx,"000"))

输出得到：

8
sel ect Max(编号) from 办案装备_管理台账 where 分类代码 = \'DY\' And [_Identify] <> 8

谢谢老师  按照老师指导 修正如下  问题得以解决！

Dim lb As String = "DY"
Dim idx As Integer
Dim cmd As New SQLCommand
cmd.Con nec tio nNa me = "主数据源"
cmd.CommandText = "Sel ect * From 办案装备_管理台账"
Dim dt As DataTable = cmd.ExecuteReader()
Dim dr As DataRow = dt.DataRows(dt.DataRows.Count - 1) \'获得最后一行
output.show(dr("_Identify"))
cmd.CommandText = "sel ect Max(编号) from 办案装备_管理台账 where 分类代码 = \'" & lb & "\' And [_Identify] <> " & dr("_Identify")
Dim max As String
max = cmd.ExecuteScalar()
output.show(max)
If max > "" Then \'如果存在最大编号
    idx = CInt(max.Substring(6,3)) + 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
output.show("BAZB" & lb & Format(idx,"000"))