To specify connection string in Web.config

Connection string may change from time to time , specially when u r coding and after u deployed the application. It is generally specified in web.config.

In configuration section, in appsetting tag, add a key value pair specifying the name and actual connection string value. Like this
<configuration>

<appSettings>

<add key="SqlConnection" value="Data Source =NSLDB;DataBase=Company;Uid=sa;Pwd=;" />

</appSettings>
</configuration>
 
To use it in your code :-
string source = ConfigurationManager.AppSettings["SqlConnection"].ToString();