Ok, in this tutorial we are going to read appSetting and connectionStrings that are defined in web.config.
For explanation’s sake lets have a sample web.config file first

<configuration>
    <appSettings>
        <add key="Mykey" value="Some Value" />
    </appSettings>
    <connectionStrings>
        <add name="localConnStr" value="Data Source=ys2012;Initial Catalog=myDB;User ID=sa;Password=123;" />
    </connectionStrings>
</configuration>

So in the above example I have defined 1 appSetting and 1 connectionString. Now below I have used the Configuration Manager Class to read appSettings as well as connectionStrings

1. Reading AppSetting

string value = ConfigurationManager.AppSettings["Mykey"].ToString();

2. Reading ConnectionString

string value = ConfigurationManager.ConnectionStrings["localConnStr"].ToString();