主题：这一例中，要加上什么代码才能不出错

If e.DataCol.Name = "客户" Then
    If e.NewValue = True Then
        Dim a As new SQLCommand
        Dim b As Row = Tables("出库").Current
        Dim c As Double
        a.C
        a.CommandText = " select sum(合计金额) from {出库} where 客户 = '"& b("客户") &"' And  结款 = '未结' "
        c = a.ExecuteScalar
        b("欠款金额") = c
    End If
End If