.NET  C#

Events

To understand events, consider the following program:

using System;
namespace Sample
{
    public class mouseeventargs
    {
        public int x, y;
        public mouseeventargs(int x1, int y1)
        {
            x = x1;
            y = y1;
        }
    }

    public delegate void click(object m, mouseeventargs e);

    public class MyForm
    {
        public event click c1;
        public void mouseclick()
        {
            mouseeventargs m = new mouseeventargs(10, 20);
            c1(this, m);
        }
    }
    class MyForm1 : MyForm
    {
        public MyForm1()
        {
            c1 += new click(button_click);
            mouseclick();
        }

        static void Main(string[] args)
        {
            MyForm1 f = new MyForm1();
        }

        public void button_click(object m, mouseeventargs e)
        {
            Console.WriteLine("Mouse Coordinates: " + e.x + " " + e.y);
            Console.WriteLine("Type is: " + m.GetType().ToString());
        }
    }
}

The output of the program would be

Mouse Coordinates: 10 20
Type is: Sample.MyForm1

In this program we have declared 3 classes-mouseeventargs, MyForm and MyForm1 derived from MyForm. The mouseevntargs class corresponds to the EventArgs class. This class contains two int variables x and y and a constructor to initialize them. We have declared a multicast delegate click whose signature is same as the event handler button_click( ). The MyForm class here is an event raiser class and hence contains an event as a data member called c1 of the type click. It also contains a method called mouseclick( ). In this method we have created an object of the mouseeventargs class. In the statement,

c1(this, m);

c1 represents (is a delegate for) button_click( ). this contains the reference of MyForm object (event raiser) and m is a reference of the mouseeventargs object. In the MyForm1 class, we have defined the method button_click( ) (i.e. event handler). When the compiler encounters the statement

c1 += new click(button_click);

it creates an event object (delegate object) that wraps up button_click( ) method. So whenever the c1 event is raised (or called, as in this program), the method button _click( ) will get executed.