fire user control event from aspx.cs file

Hi All,
   I have a scenario like below, please help me how do I go about.

   I have a user control with two drop down, on selection change of first dropdown second dropdown should get populated. Now when I place this user control in a aspx file, I need to access the second dropdown value on change of send dropdown .
  eg: in ascx file
           Department drop down and employee dropdown. on change of department dropdown, employee dropdown should populate. (which is working fine for me)

in aspx.cs file
    on change of employee dropdown, I need to get the employee id and so some other operation. (I am struggling here)

Is there any way where I can fire usercontrol  event from aspx.cs file?

Please help...
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