Role level connection

Hi,
I have asp.net application (.NET 3.5) where i am connecting with sql server using windows authentication. After connection I am getting user role from user table. Based on the role i would like to create a Role level connection with Sql Server so user can do the operation on db objects which they have permission.
How can we make connectionstring in asp.net with Role level details?
 
Thnx,
Viki
 
 
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