Vb.net - call child method from parent class (vb.net Polymorphism) -

i'm newbie vb.net , try perform polymorphism.

it's interface "imyconnection"

imports system.data.common  interface imyconnection     sub openconnection()     sub closeconnection()     function getconnection() dbconnection     function executequery(byval queryselect string) dbcommand     function executeupdate(byval queryupdate string) boolean end interface 

in main class, create new instance this

dim connection imyconnection connection = new accessconnection() 

but when call function never declare in imyconnection

connection.setdatasource("d:\access db\mdb\akademik.mdb") 

i error message

'setdatasource' not member of 'windowsapplication1.imyconnection' 

so handle error, create new object this

dim accessconnection accessconnection = connection accessconnection.setdatasource("d:\access db\mdb\akademik.mdb") 

please me solve problem, can't find keyword find better solutions thanks