全局 SqlConnection

 class SqlHelper
    {
        public static SqlConnection conn;

        public static SqlConnection Open(string connStr)
        {
            if (!string.IsNullOrEmpty(connStr))
            {
                SqlConnection conn = new SqlConnection(connStr);
                try
                {
                    conn.Open();
                    return conn;
                }
                catch (Exception)
                {
                }
            }
            return null;
        }

        public static bool Open(SqlConnection conn)
        {
            if (conn != null && conn.State != ConnectionState.Open)
            {
                try
                {
                    conn.Open();
                    return true;
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                    return false;
                }
            }
            return false;
        }

        public static DataTable DB_Select(SqlConnection conn, string text, params SqlParameter[] parameters)
        {
            if (Open(conn))
            {
                DataTable dt = new DataTable();
                SqlDataAdapter da = new SqlDataAdapter(text, conn);
                da.Fill(dt);
                return dt;
            }
            return null;
        }

        public static void Close(SqlConnection conn)
        {
            if (conn != null && conn.State != ConnectionState.Closed)
                conn.Close();
        }
    }