我在运行过程中碰到这种问题该怎么解决呢 程序是这样的 sub page_load(sender as object,e as eventargs) dim conn as new oledbconnection() dim cmd as new oledbcommand() dim objreader as oledbdatareader dim i as integer conn.connectionstring="provider=microsoft.jet.oledb.4.0;data source="&

server.mappath("guestbook.mdb") conn.open() cmd.connection=conn cmd.commandtext="select * from book" objreader=cmd.executereader() do while objreader.read() if request("artid")=objreader.item("id") then username.text=objreader.item("name") email.text=objreader.item("email") posttime.text=objreader.item("posttime") content.text=objreader.item("text") exit do end if loop objreader.close() conn.close() end sub 提示这样的错误 System.InvalidCastException: 运算符对 'Nothing' 和 类型“DBNull” 无效