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如何判断当FILTER定义的SQL语句找不到符合条件的记录时用MESSAGEBOX提示一遍？

If startdate.Value=Nothing OrElse enddate.value=Nothing Then
    Messagebox.Show("请选则起始日期或终止日期")
    Return
Else
    For Each dr As DataRow In dt2.SQLSelect("commitTime >='" & startdate.value & "' and PromiseTimeString <='" & EndDate.value & "'","","PNumber")
        Dim filter As String = "PreID like '%" & dr("PNumber") & "'"
        Dim fdr As DataRow = dt1.SQLFind(filter)
        If fdr Is Nothing Then
            Dim ndr As DataRow = dt3.AddNew
            ndr("项目编号") = dr("pNumber")
            ndr("客户名称") = dr("companyName")
        End If
    Next

End If