Problem in using Dropdownlist

hi all,

   I am developing a web page. I have used one dropdownlist.I  have used that dropdownlist box in both .aspx & .aspx.cs  file. But in .aspx  when i tried to access the item from the dropdownlist, the dropdownlist is giving error. i.e Null reference exception was unhandled by user code.How can i solve this?Can anybody help me regarding this.

thanks
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