1. Add a reference

using System.Configuration;

2. Code

string strConnectionString=ConfigurationManager.AppSettings["LocalConnectionString"].ToString()

3. web. config

<configuration>
 <appSettings>
 <add key="LocalConnectionString" value="User id=sa;Password=123456;Database=Users;Server=192.168.1.1;Connect Timeout=50;Max Pool size=200;Min pool Size=5"/>
 </appSettings>
</configuration>