C#

tabControl.SelectedIndex = ...; // index of the tab

// or

tabControl.SelectedTab = ...; // an actual tab control instance

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Text;
 
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string test = ConfigurationManager.AppSettings["test"];
        }
    }
}

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Text;
 
namespace DataAccessLayer
{
    public class DataAccess
    {
        private string connectionString;
 
        public DataAccess()
        {
            this.connectionString = ConfigurationManager.AppSettings["connectionString"];
 
            // you could also use the following, which is the "more correct" way
            // this.connectionString = ConfigurationManager.ConnectionStrings["connectionString"];
        }
 
        public string ConnectionString
        {
            get
            {
                return this.connectionString;
            }
        }
    }
}

this.connectionString = ConfigurationManager.AppSettings["connectionString"];