In .NET applications, Connection Strings are easily stored in and retrieved from a web.config file or an app.config file. Add a section to the config file, directly within the section. Within the section, place an tag for each connection string you wish to store and retrieve. This tag contains two important attributes: name and connectionString. The name attribute will be used to look up an entry, so each tag should have a unique name. The connectionString attribute contains the connection string you will use to connect to a database. If your application needs to connect to a number of different data types, it is worth setting the Provider property as well. This is the name of a database provider installed on the current machine, that will allow you to connect to a database.
An example is shown below:

  
    "MyApp_Dev" 
        connectionString="Data Source=Server01;Initial Catalog=AwesomeDB_Dev;Integrated Security=True"/>
    "MyApp_QA" 
        connectionString="Data Source=Server01;Initial Catalog=AwesomeDB_QA;Integrated Security=True"/>
    "MyApp_Prod" 
        connectionString="Data Source=Server01;Initial Catalog=AwesomeDB;Integrated Security=True"/>
  

To retrieve a connection string by its associated name attribute, use the utilities found in the System.Configuration namespace of the System.Configuration assembly. First, set a reference to System.Assembly by selecting the menu option Project | Add Reference, selecting the System.Assembly in the Component Name column of the .NET tab of the Add Reference dialog; and, clicking the OK button. Then, add the following using statement to the top of your class file.

This namespace contains the static ConfigurationManager class, which exposes a number of methods for retrieving configuration information. ConfigurationManager.ConnectionStrings returns a collection containing all connection strings settings in the config file of the current assembly. Once we have this collection, we can retrieve a single item by specifying its name. Doing so returns a ConnectionStringSettings object that contains properties retrieved from a single element of the config file. The important property is ConnectionString, which can be used with ADO.NET to connect to a database. Knowing the Provider property may also be useful if your application connects to different types of data.
Sample code is below.

ConnectionStringSettingsCollection cssCollection =
    ConfigurationManager.ConnectionStrings;
ConnectionStringSettings devCsSettings = cssCollection["MyApp_Dev"];
string devCs = devCsSettings.ConnectionString;