Open App.config File in Project Solution Explorer write connectionString using appsettings

<?xml version=”1.0″ encoding=”utf-8″ ?>
<configuration>

<configSections>
<sectionGroup name=”applicationSettings” type=”System.Configuration.ApplicationSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089″ >
<section name=”WinFormApp.Properties.Settings” type=”System.Configuration.ClientSettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089″ requirePermission=”false” />
</sectionGroup>
</configSections>
<appSettings>
<add key=”ConnectionString” value=”Persist Security Info=False;Data Source=lokesh;Initial Catalog=winForms;User ID=sa;Password=xyz;” />
</appSettings>

<applicationSettings>
<WinFormApp.Properties.Settings>
<setting name=”WinFormApp_localhost_Service1″ serializeAs=”String”>
<value>http://localhost:2557/Service1.asmx</value>
</setting>
</WinFormApp.Properties.Settings>
</applicationSettings>
</configuration>

ConnectionString is the name of the connection used to retrieve connection into class files and form files

DataSource=lokesh: It is the Database server name if you are using localhostthen DataSource=localhost/SQLEXPRESS

Initial Catalog=winForms: Winform is Database name

Getting app.config file connection string into Class file

using System.Data.SqlClient;
using System.Configuration; //add this namespace

SqlConnection objConn = new SqlConnection(ConfigurationSettings.AppSettings["ConnectionString"]);
SqlCommand objCmd = new SqlCommand(”select *from tblCategory”, objConn);