C#.net中封装类是怎么个意思啊 不懂 谁能和我解释下啊

usingSystem;usingSystem.Data;usingSystem.Configuration;usingSystem.Web;usingSystem.Web.Security;usingSystem.Web.UI;usingSystem.Web.UI.WebControls;usingSystem.Web.UI.WebControls.WebParts;usingSystem.Web.UI.HtmlControls;usingSystem.Data.SqlClient;usingSystem.Text;namespaceChinaSite.classes{publicclassDbAccess{SqlConnectionconn=null;SqlCommandcmd=null;publicDbAccess(){////TODO:在此处添加构造函数逻辑//conn=newSqlConnection();//conn.ConnectionString="initialcatalog=pubs;datasource=.;userid=sa;password=&q ...