求解释一下这个DBhelper

namespace Web.DAL
{
public class DBHelper
{
private static SqlConnection _conn;
static DBHelper()
{
if (_conn == null)
{
string strConn = ConfigurationManager.ConnectionStrings["SQLConn"].ConnectionString;
_conn = new SqlConnection(strConn);
}
}
public static bool ExecuteSql(string sql)
{
try
{
if (_conn.State != ConnectionState.Open)
_conn.Open();
SqlCommand cmd = _conn.CreateCommand();
cmd.CommandText = sql;
cmd.ExecuteNonQuery();
return true;
}
catch
{
return false;
}
finally
{
_conn.Close();
}
}
public static DataTable GetTable(string sql, string tableName)
{
try
{
SqlDataAdapter adp = new SqlDataAdapter(sql, _conn);
DataSet ds = new DataSet();
adp.Fill(ds, tableName);
return ds.Tables[0];
}
catch
{
return null;
}
}
        public static DataTable GetTableByPages(string sql, int startIndex,int pagesize,string tableName)
        {
            try
            {
                SqlDataAdapter adp = new SqlDataAdapter(sql, _conn);
                DataSet ds = new DataSet();
                adp.Fill(ds, startIndex, pagesize, tableName);
                return ds.Tables[0];
            }
            catch
            {
                return null;
            }
        }
public static SqlDataReader GetReader(string sql)
{
try
{
if (_conn.State != ConnectionState.Open)
_conn.Open();
SqlCommand cmd = _conn.CreateCommand();
cmd.CommandText = sql;
return cmd.ExecuteReader(CommandBehavior.CloseConnection);
}
catch
{
return null;
}
}

public static object GetScalar(string sql)
{
try
{
if (_conn.State != ConnectionState.Open)
_conn.Open();
SqlCommand cmd = _conn.CreateCommand();
cmd.CommandText = sql;
return cmd.ExecuteScalar();
}
catch
{
return null;
}
finally
{
_conn.Close();
}
} --------------------编程问答-------------------- 自己先理解一些，不明白的拿出来问。 --------------------编程问答-------------------- 我是个菜鸟，基于答辩之用，求大神指导 --------------------编程问答--------------------



 public class DBHelper

    {

        /// <summary>

        /// 数据库连接对象

        /// </summary>

        private static SqlConnection _conn;





        //构造函数

        static DBHelper()

        {

            if (_conn == null)

            {

                //数据库连接串

                string strConn = ConfigurationManager.ConnectionStrings["SQLConn"].ConnectionString;

                _conn = new SqlConnection(strConn);

            }

        }

        public static bool ExecuteSql(string sql)

        {

            try

            {

                if (_conn.State != ConnectionState.Open)

                    //打开数据库连接

                    _conn.Open();

                //创建命令

                SqlCommand cmd = _conn.CreateCommand();

                cmd.CommandText = sql;

                //指定sql语句

                cmd.ExecuteNonQuery();

                return true;

            }

            catch

            {

                return false;

            }

            finally

            {

                _conn.Close();

            }

        }



        /// <summary>

        /// 查询数据

        /// </summary>

        /// <param name="sql"></param>

        /// <param name="tableName"></param>

        /// <returns></returns>

        public static DataTable GetTable(string sql, string tableName)

        {

            try

            {

                //数据适配器对象

                SqlDataAdapter adp = new SqlDataAdapter(sql, _conn);

                DataSet ds = new DataSet();

                //填充数据

                adp.Fill(ds, tableName);

                return ds.Tables[0];

            }

            catch

            {

                return null;

            }

        }



        /// <summary>

        /// 分页查询数据

        /// </summary>

        /// <param name="sql"></param>

        /// <param name="startIndex"></param>

        /// <param name="pagesize"></param>

        /// <param name="tableName"></param>

        /// <returns></returns>

        public static DataTable GetTableByPages(string sql, int startIndex, int pagesize, string tableName)

        {

            try

            {

                SqlDataAdapter adp = new SqlDataAdapter(sql, _conn);

                DataSet ds = new DataSet();

                adp.Fill(ds, startIndex, pagesize, tableName);

                return ds.Tables[0];

            }

            catch

            {

                return null;

            }

        }



        /// <summary>

        /// 获得DataReader对象

        /// </summary>

        /// <param name="sql"></param>

        /// <returns></returns>

        public static SqlDataReader GetReader(string sql)

        {

            try

            {

                if (_conn.State != ConnectionState.Open)

                    _conn.Open();

                SqlCommand cmd = _conn.CreateCommand();

                cmd.CommandText = sql;

                return cmd.ExecuteReader(CommandBehavior.CloseConnection);

            }

            catch

            {

                return null;

            }

        }



        /// <summary>

        /// 获得第一行第一列的值

        /// </summary>

        /// <param name="sql"></param>

        /// <returns></returns>

        public static object GetScalar(string sql)

        {

            try

            {

                if (_conn.State != ConnectionState.Open)

                    _conn.Open();

                SqlCommand cmd = _conn.CreateCommand();

                cmd.CommandText = sql;

                return cmd.ExecuteScalar();

            }

            catch

            {

                return null;

            }

            finally

            {

                _conn.Close();

            }

        }

--------------------编程问答--------------------

引用 3 楼的回复:

C# code

public class DBHelper
    {
        /// <summary>
        /// 数据库连接对象
        /// </summary>
        private static SqlConnection _conn;

        //构造函数
        static DBHelper()
        ……

这个详细 --------------------编程问答-------------------- /// <summary>
        /// 分页查询数据
        /// </summary>
        /// <param name="sql"></param>
        /// <param name="startIndex"></param>
        /// <param name="pagesize"></param>
        /// <param name="tableName"></param>
        /// <returns></returns>
        public static DataTable GetTableByPages(string sql, int startIndex, int pagesize, string tableName)
        {
            try
            {
                SqlDataAdapter adp = new SqlDataAdapter(sql, _conn);
                DataSet ds = new DataSet();
                adp.Fill(ds, startIndex, pagesize, tableName);
                return ds.Tables[0];
            }
            catch
            {
                return null;
            }
        }

这种分页效率会有问题吧?

补充：.NET技术 ,  C#