SqlConnectionStringBuilder SQLConnStringBuilder = new SqlConnectionStringBuilder();
System.Security.Principal.WindowsIdentity CurrentUser = System.Security.Principal.WindowsIdentity.GetCurrent();
System.Data.Sql.SqlDataSourceEnumerator instance = System.Data.Sql.SqlDataSourceEnumerator.Instance;
System.Data.DataTable table = instance.GetDataSources();//таблица всех экземпляров локальных БД
//к локальной БД подключаюсь так:
try
   {
     SQLConnStringBuilder["Data Source"] = strDataSourceLocal; //тут типа "localhost\\SQLServerInst1"
     SQLConnStringBuilder["integrated Security"] = true;
     SQLConnStringBuilder["Initial Catalog"] = textBoxDataBase.Text; //тут имя базы (оно стандартно и только одно)
     SqlConnection myConnection = new SqlConnection(SQLConnStringBuilder.ConnectionString);
     myConnection.Open();
   }
catch (Exception)
   {
      //...
   }
...
//к сетевой БД или в пределах домена подключаюсь так:
SQLConnStringBuilder["Data Source"] = textBoxServerIP.Text;
SQLConnStringBuilder["User"] = textBoxLogin.Text;
SQLConnStringBuilder["Password"] = textBoxPassword.Text;
//SQLConnStringBuilder["integrated Security"] = true;
SQLConnStringBuilder["Initial Catalog"] = textBoxDataBase.Text; // константа
SqlConnection checkConnection = new SqlConnection(SQLConnStringBuilder.ConnectionString);
   try
      {
         checkConnection.Open();
      }
    catch (Exception ex)
      {
         MessageBox.Show("" + ex.Message, "Ошибка подключения", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
      }

public partial class MainForm : Form
    {
      BindingSource bs = new BindingSource();
      static SqlConnectionStringBuilder SQLConnStringBuilder = new SqlConnectionStringBuilder();
      DataTable tab = new DataTable();
        public MainForm()
        {
            InitializeComponent();
        }
        private void MainForm_Load(object sender, EventArgs e)
        {
            SQLConnStringBuilder["Data Source"] = Properties.Settings.Default.SettConnServerIP; ; // Сервер
            //SQLConnStringBuilder["integrated Security"] = true;
            SQLConnStringBuilder["User"] = Properties.Settings.Default.SettConnUser;
            SQLConnStringBuilder["Password"] = Properties.Settings.Default.SettConnPass;
            SQLConnStringBuilder["Initial Catalog"] = Properties.Settings.Default.SettConnDataBase;
            SqlConnection ConnectionStringDB = new SqlConnection(SQLConnStringBuilder.ConnectionString);
            try
            {
                ConnectionStringDB.Open();
                SqlCommand CommandDB = new SqlCommand("Command String", ConnectionStringDB);
                CommandDB.CommandText = "SELECT * FROM myTable"; 
                CommandDB.ExecuteNonQuery();
                dGV_Eq.DataSource = bs;
                tab.Load(CommandDB.ExecuteReader());
                bs.DataSource = tab.DefaultView;
            }
            catch (Exception ex)
            {
                MessageBox.Show("" + ex.Message);
            }
        }
}