Configuration File(Web.Config)

You can provide a configuration file for your application. A configuration file allows you to configure application properties after the application has been deployed without recompiling your code. The configuration file is an XML file that contains information about how your application should be configured. You can also use configuration files to configure other aspects of your program.

Creating the Configuration File

A configuration file is simply an XML file with the appropriate tags and an appropriate name. A configuration file for an application will have the name <name>.<extension>.config, where <name> is the name of the application, and <extension> is the extension of the application (such as .exe). Thus, the .config file for an application named myApplication.exe would be myApplication.exe.config. A configuration file must be located in the same folder as the application assembly that it configures.
The content of a configuration file must be configured in the .config file schema. The basic structure of a configuration file is as follows:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
   <appSettings>

<add key="DBConnectionString" value="Data Source=GTC;Initial Catalog=IntangibleDemo;User Id=sa;Password=Tech8092"></add>

 </appSettings>

</configuration>

Aside from the first element, which specifies the XML version and the encoding, and the top-level <configuration> element, there is no required content for a configuration file. All other elements are optional and can be added or dispensed with as needed.
In C# Program
using System.Configuration;

string SQL_CON = System.Configuration.ConfigurationManager.            AppSettings.Get("DBConnectionString");

string SQL_CON = ConfigurationSettings.AppSettings["DBConnectionString"].ToString();

Process Class

//To open the Calculator

System.Diagnostics.Process.Start("calc");

//To open any file

System.Diagnostics.Process.Start("PathOfFileWithExtension");