单例模式

    class DBHelper
    {
        private static readonly string strConn = "Data Source=.;Integrated Security=true;Initial Catalog=myDB";
        private static SqlConnection sqlConn = null;
        private static DBHelper dbHelper = null;

        /// <summary>
        /// 数据库连接对象实例
        /// </summary>
        private DBHelper()
        {
            if (sqlConn == null)
            {
                sqlConn = new SqlConnection(strConn);
            }
        }

        /// <summary>
        /// 取得/创建对象实例
        /// </summary>
        /// <returns></returns>
        internal static DBHelper GetIntance()
        {
            if (dbHelper == null)
            {
                dbHelper = new DBHelper();
            }
            return dbHelper;
        }

        /// <summary>
        /// 打开数据库连接
        /// </summary>
        private void Open()
        {
            if (sqlConn.State == ConnectionState.Closed)
            {
                sqlConn.Open();
            }
        }

        /// <summary>
        /// 关闭数据库连接
        /// </summary>
        private void Close()
        {
            if (sqlConn.State == ConnectionState.Open)
            {
                sqlConn.Close();
            }
        }

        public int ExecuteSql(string strSql, SqlParameter[] para)
        {
            SqlCommand sqlCmd = new SqlCommand(strSql, sqlConn);
            if (para != null)
            {
                sqlCmd.Parameters.AddRange(para);
            }
            Open();
            int objResult = sqlCmd.ExecuteNonQuery();
            Close();
            return objResult;
        }

        public DataSet GetSqlDataSet(string strSql, SqlParameter[] para)
        {
            SqlCommand sqlCmd = new SqlCommand(strSql, sqlConn);
            if (para != null)
            {
                sqlCmd.Parameters.AddRange(para);
            }
            SqlDataAdapter sqlAdp = new SqlDataAdapter(sqlCmd);
            DataSet ds = new DataSet();
            sqlAdp.Fill(ds);
            return ds;
        }
    }

        //调用实例
        private static void GetDBList()
        {
            DBHelper dbHelper = DBHelper.GetIntance();
            DataSet ds = dbHelper.GetSqlDataSet("SELECT TOP 20 [StuId],[StuName] FROM [Student]", null);
            for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                Console.WriteLine(ds.Tables[0].Rows[i][0].ToString() + ds.Tables[0].Rows[i][1].ToString());
            }
        }