红嘴蓝鹊怎么养:关于下拉列表框(dropdownlist)联动的问题

第一个列表中的选值触发第二个的选项，这个能实现，但是第一个列表选项不可选，固定在第一个值（从数据库中读出）
sub page_load(sender as object,e as eventargs)
dim conn as new sqlconnection(configurationsettings.appsettings("conn"))
dim cmd as new sqlcommand("select 类型名称,类型编号 from 房间类型",conn)
conn.open()
dim dr1 as sqldatareader=cmd.executereader()
d1.datasource=dr1
d1.datatextfield="类型名称"
d1.datavaluefield="类型编号"
d1.databind()
conn.close()
end sub
sub selected(sender as object,e as eventargs)
dim conn as new sqlconnection(configurationsettings.appsettings("conn"))
dim cmd as new sqlcommand("select 房间编号 from 房间 where 类型编号='" & d1.selecteditem.value & "' and 房间状态='N'",conn)
conn.open()
dim dr2 as sqldatareader=cmd.executereader()
d2.datasource=dr2
d2.datatextfield="房间编号"
d2.databind()
conn.close()
end sub