The power of delegate

Last time I have an opportunity to build a small example which shown me the big power of delegate.This example has focused on getting data from database. I wanted to build a method which would show me all data from any table. The first thing which I did, was
building a class which mapes the database table into class for example:

8 public class Person

9 {

10 int id;

11 string name;

12 string surname;

13 List<Address> addresses;

14

15 public List<Address> Addresses

16 {

17 get { return new AddressDAO().GetAddressesByPersonId(this.id); }

18 set { addresses = value; }

19 }

20

21 public int Id

22 {

23 get { return id; }

24 set { id = value; }

25 }

26

27 public string Name

28 {

29 get { return name; }

30 set { name = value; }

31 }

32

33 public string Surname

34 {

35 get { return surname; }

36 set { surname = value; }

37 }

38

39 public Person(int id, string name, string surname)

40 {

41 this.id = id;

42 this.name = name;

43 this.surname = surname;

44 }

45 }

When I have it done the next thing was building a DAOTemplate class which has couple methods like: query, execute. In fact I will just focused on query method. I want to use this method to get a data from any database table. This method takes two parameters string query where we put our query and delegate which will create a generic list with our data.

The query method looks like:

20 public void Query(string query, RowCallBackHandler del)

21 {

22 this.con.Open();

23 OdbcCommand cmd = this.con.CreateCommand();

24 cmd.CommandText = query;

25

26 OdbcDataReader rdr = cmd.ExecuteReader();

27 while (rdr.Read())

28 {

29 del(rdr);

30 }

31 this.con.Close();

32 }

As you can see, I open connection, next create command and execute it. Next I launch a while loop to get all results from datareader and pass them to delegate. It is pretty easy.

The last step is building ADO layer for Person table:

13 public List<Person> GetPeople()

14 {

15 List<Person> people = new List<Person>();

16 DAOTemplate template = DAOManager.GetDaoTemplate();

17

18 template.Query("select * from Person", delegate( OdbcDataReader rdr)

19 {

20 Person person = new Person (rdr.GetInt32(0), rdr.GetString(1), rdr.GetString(2));

21 people.Add(person);

22 });

23

24 return people;

25 }

Now when I invoke the method GetPeople it means I will invoke the template.Query method inside, with two parameters, first - the query which gives me all person and the second parameter - anonymous callback method. This second parameter it responsibles for adding a new person to my list of people.

The best way to see the true power of callbacks is execute this example yourself.