求解释一下这个DBhelper

Question

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();
}
}

Answer 1

 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();
            }
        }

Answer 2

 /// <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?