use ConnectionStrings

using System.Data.SqlClient;

//WinForm
using System.Configuration;
string winFormConnectString = ConfigurationManager.ConnectionStrings["Your DB Name"].ConnectionString;

//WebForm
System.Web.Configuration;
string webFormConnectString = WebConfigurationManager.ConnectionStrings["Your DB Name"].ConnectionString;

SqlConnection connection = new SqlConnection(winFormConnectString);
//或是 SqlConnection connection = new SqlConnection(webFormConnectString);