using System.Data.SqlClient;

            string str = ConfigurationManager.AppSettings["con_MES"];

            try

            {

                using (SqlConnection conn = new SqlConnection(str))

                {

                    conn.Open();

                    if (conn.State == ConnectionState.Open)

                    {

                        MessageBox.Show("连接成功");

                    }

                }

            }

            catch (Exception ex)

            {

                MessageBox.Show(ex.Message);

            }

<connectionStrings>

  <add name="con_xx" connectionString="server=.\ds;uid=sa;pwd=sa;database=xxx"/>

</connectionStrings>

ConfigurationManager.ConnectionStrings["con_xxx"].ConnectionString;