Delegation

[Abalfazl]

Hello

http://www.csharp-station.com/Tutorials/Lesson14.aspx

 using System;
using System.Drawing;
using System.Windows.forms;

// custom delegate
public delegate void Startdelegate();

class Eventdemo : form
{
    // custom event
    public event Startdelegate StartEvent;

    public Eventdemo()
    {
        Button clickMe = new Button();

        clickMe.Parent = this;
        clickMe.Text = "Click Me";
        clickMe.Location = new Point(
            (ClientSize.Width - clickMe.Width) /2,
            (ClientSize.Height - clickMe.Height)/2);

        // an EventHandler delegate is assigned
        // to the button's Click event
        clickMe.Click += new EventHandler(OnClickMeClicked);

        // our custom "Startdelegate" delegate is assigned
        // to our custom "StartEvent" event.
        StartEvent += new Startdelegate(OnStartEvent);

        // fire our custom event
        StartEvent();
    }

    // this method is called when the "clickMe" button is pressed
    public void OnClickMeClicked(object sender, EventArgs ea)
    {
        MessageBox.Show("You Clicked My Button!");
    }

    // this method is called when the "StartEvent" Event is fired
    public void OnStartEvent()
    {
        MessageBox.Show("I Just Started!");
    }

    static void Main(string[] args)
    {
        Application.Run(new Eventdemo());
    }
}

And this from :http://www.codeguru.com/csharp/cshar...eventhandling/

using System;
//Step 1 Create delegate object
public delegate void MyHandler1(object sender,MyEventArgs e);
public delegate void MyHandler2(object sender,MyEventArgs e);

//Step 2 Create event handler methods
class A{
  public const string m_id="Class A";
  public void OnHandler1(object sender,MyEventArgs e){
    Console.WriteLine("I am in OnHandler1 and MyEventArgs is {0}",
                      e.m_id);
  }
  public void OnHandler2(object sender,MyEventArgs e){
    Console.WriteLine("I am in OnHandler2 and MyEventArgs is {0}",
                      e.m_id);
  }

  //Step 3 create delegates, plug in the handler and register
  //       with the object that will fire the events
  public A(B b){
    MyHandler1 d1=new MyHandler1(OnHandler1);
    MyHandler2 d2=new MyHandler2(OnHandler2);
    b.Event1 +=d1;
    b.Event2 +=d2;
  }
}

//Step 4 Calls the encapsulated methods through the
//       delegates (fires events)
class B{
  public event MyHandler1 Event1;
  public event MyHandler2 Event2;
  public void FireEvent1(MyEventArgs e){
    if(Event1 != null){
      Event1(this,e);
    }
  }
  public void FireEvent2(MyEventArgs e){
    if(Event2 != null){
      Event2(this,e);
    }
  }
}
public class MyEventArgs  EventArgs{
  public string m_id;
}
public class Driver{
  public static void Main(){
    B b= new B();
    A a= new A(b);
    MyEventArgs e1=new MyEventArgs();
    MyEventArgs e2=new MyEventArgs();
    e1.m_id ="Event args for event 1";
    e2.m_id ="Event args for event 2";
    b.FireEvent1(e1);
    b.FireEvent2(e2);
  }
}

I have 2 questions,

In first example ,It didn't used



But in second example,It is used
Why?

When do we use EventArgs?