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Events

 

Introduction

Except for the main class of your program (the class that contains the Main() method), every class is mostly meant to interact with other, either to request values and methods of the other classes or to provide other classes with some values or a behavior they need. When a class A requests a value or service from another class B, class A is referred to as a client of class B. This relationship is important not simply because it establishes a relationship between both classes but also because class B should be ready to provide the value or behavior that a client needs at a certain time.

While a class B is asked to provide some values or methods to another class A, many things would happen. In fact, there is an order that things should follow. For example, during the lifetime of a program, that is, while a program is running, a class may be holding a value it can provide to its client but at another time, that value may not be available anymore, for any reason; nothing strange, this is just the ways it happens. Because different things can happen to a class B while a program is running, and because only class B would be aware of these, it must be able to signal to the other classes when there is a change. This is the basis of events: An event is an action that occurs on an object and affects it in a way that its clients must be made aware of. Events are mostly familiar to those who do graphical (GUI) programming as they are able to "visually" work on Windows controls and as they are able to access the objects on which actions are happening and the objects that must know when these actions occur. Still, because events are dealt with in C#, you should be aware of their functionality.

Although events are mostly used in Windows controls programming, they can also be implemented in console applications.

Event Creation

An event is declared like a pseudo-variable but based on a delegate. Therefore, to declare an event, you must have a delegate that would implement it. Here is an example:

using System;

delegate void dlgSimple();

class Exercise
{
    public static void Welcome()
    {
	Console.WriteLine("Welcome to the Wonderful World of C# Programming!");
    }
}

To actually declare an event, you use the event keyword with the following formula:

[attributes] [modifiers] event type declarator;
[attributes] [modifiers] event type member-name {accessor-declarations};

The attributes factor can be a normal C# attribute.

The modifier can be one or a combination of the following keywords: public, private, protected, internal, abstract, new, override, static, virtual, or extern.

The event keyword is required. It is followed by the name of the delegate that specifies its behavior. If the event is declared in the main class, it should be made static. Like everything in a program, an event must have a name. This would allow the clients to know what (particular) event occurred. Here is an example:

using System;

delegate void dlgSimple();

class Exercise
{
    public static event dlgSimple Simply;

    public static void Welcome()
    {
	Console.WriteLine("Welcome to the Wonderful World of C# Programming!");
    }
}

After declaring the event, you must define a method that calls the event. Here is an example:

using System;

delegate void dlgSimple();

class Exercise
{
    public static event dlgSimple Simply;

    public static void Welcome()
    {
	Console.WriteLine("Welcome to the Wonderful World of C# Programming!");
    }

    public static void SayHello()
    {
	Simply();
    }
}

When the event occurs, its delegate would be invoked. This specification is also referred to as hooking up an event. As the event occurs (or fires), the method that implements the delegate runs. This provides complete functionality for the event and makes the event ready to be used. Before using an event, you must combine it to the method that implements it. This can be done by passing the name of the method to the appropriate delegate, as we learned when studying delegates. You can then assign this variable to the event's name using the += operator. Once this is done, you can call the event. Here is an example:

using System;

delegate void dlgSimple();

class Exercise
{
	public static event dlgSimple Simply;

	public static void Welcome()
	{
	Console.WriteLine("Welcome to the Wonderful World of C# Programming!");
	}

	public static void SayHello()
	{
		Simply();
	}

	static int Main()
	{
		Simply += new dlgSimple(Welcome);

		SayHello();

		return 0;
	}
}

Instead of the += operator used when initializing the event, you can implement add and remove of the event class. Here is an example:

using System;

delegate void dlgSimple();

class Exercise
{
	public event dlgSimple Simply
	{
		add
		{
			Simply += new dlgSimple(Welcome);
		}
		remove
		{
			Simply -= new dlgSimple(Welcome);
		}
	}

	public void Welcome()
	{
	Console.WriteLine("Welcome to the Wonderful World of C# Programming!");
	}
}
 

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