how to write connection string in web.config file and read from it

how to write connection string in web.config file and read from it

i'm trying to write Connectionstring to Web.config like this:
<connectionStrings>
<add name="Dbconnection" connectionString="Server=localhost;
Database=OnlineShopping ; Integrated Security=True"/>
</connectionStrings >
and read from it like this:
string strcon =
ConfigurationManager.ConnectionStrings["Dbconnection"].ConnectionString;
SqlConnection DbConnection = new SqlConnection(strcon);
when run the program I get an error because of the null reference. but
when I use this code:
SqlConnection DbConnection = new SqlConnection();
DbConnection.ConnectionString = "Server=localhost; Database=OnlineShopping
; Integrated Security=True";
I don't get any error and the program works correctly! What is the problem?