i want to work by using postgreeSQL as DB with C#.net as frontend .I want to know how we will decalre a connection string of postgreeSQL in c#.net .and how we able to amke aconnection with dot net and PostgreSQL

I have never done it myself, but have always been interested in a pg/.net combo...

With a little research I see that there is an ODBC driver availible: http://gborg.postgresql.org/project/....php?downloads

Install that and you can connect through .Net's ODBC object, shouldn't be much different than a MySQL/.net combo.