Object-oriented (single-column mode)

usingSystem;usingSystem.Collections.Generic;usingSystem.Linq;usingSystem.Text;usingSystem.Threading.Tasks;namespacesingle-column mode {//single-row mode//control a class can instantiate only one object//class Test//General Class//{    //public string name; //}    //Data Access Classes    classDbda { Public stringhost;  Public stringdatabase; //a static member that is used to store the object of the class         Public StaticDBDA db =NULL; //let the class not be instantiated        PrivateDbda () {}//provides a method of making objects, static, to allow outside access to         Public StaticDbda Duixiang () {if(db = =NULL) {db=NewDbda (); }            returnDB; }    }    classProgram {Static voidMain (string[] args) {            //Test T1 = new test (); //Test t2 = new Test (); //Dbda a = new Dbda (); Error, private constructor//Dbda db = Dbda.            Duixiang (); //db.host = "localhost"; //Dbda db1 = Dbda.            Duixiang (); //Console.WriteLine (); //Console.ReadLine (); //Interview Questions//three main features of object-oriented//Design Patterns        }        Static voidTest (intAintb) {Console.WriteLine (a+b); }        //speech Function Method        Static voidSpeak (intN) {//America (); //China (); //if (n = = 0)//{            //America (); //}            //else if (n = = 1)//{            //China (); //}            //else if (n = = 2)//{            //Hanyu (); //}        }        Static voidAmerica () {Console.WriteLine ("Hello"); }        Static voidChina () {Console.WriteLine ("Hello"); }        Static voidHanyu () {Console.WriteLine ("1234"); }    }}

Object-oriented (single-column mode)