主题：这是怎么回事?

Select Case e.DataCol.Name
    Case "入库数量","出库数量"
        Dim mr As DataRow = e.DataRow
        Dim dr As DataRow
        Dim drs As List(of DataRow)
        dr = e.DataTable.Find("[_SortKey] < " & mr("_SortKey") & "And[类别] = '" & e.DataRow("类别") & "' And [品名] = '" & e.DataRow("品名")& "'And [规格与色号] = '" & e.DataRow("规格与色号") & "'" , "[_SortKey] Desc")
        If dr Is Nothing Then '如果没有上一行,说明本行就是同产品的第一行"[IO] = '" & e.DataRow("IO") & "' And [口岸] = '" & e.DataRow("口岸")
            mr("结存数量") = mr("入库数量") - mr("出库数量")
            dr = mr
        End If
        drs = e.DataTable.Select("[_SortKey] >= " & dr("_SortKey") & "And[类别] = '" & e.DataRow("类别") & "' And [品名] = '" & e.DataRow("品名")& "'And [规格与色号] = '" & e.DataRow("规格与色号") & "'")
        For i As Integer = 1 To drs.Count  - 1 '重算余下行的余额
            drs(i)("结存数量") =  drs(i-1)("结存数量")  + drs(i)("入库数量") - drs(i)("出库数量")
        Next
End Select

  我按照例子的高效率流水账做的东西,为什么提示说没有"_sortkey"这一列