ENTER DELEGATES

Delegates act like pointers that you find in other languages such as C++, but delegates go several steps further in C#. Delegates provide object-

oriented pointers to methods from other points in your project. This approach makes it easy for methods throughout your program to retrieve information from one source without having to enter that information repeatedly.

Delegates provide two key benefits. Delegates act as a central point where all pieces of your code that need objects refer to a specific method. It is quite inconvenient to have to write static methods for many different classes.

It is also inconvenient to refer to the same class for the same method, and that approach can also slow your project down when it runs. It is much more efficient to have one or a few delegates that can handle method operations for your entire project.

The second benefit of delegates is anonymity. The delegate does not care about what the method includes — whether it be static or non-static, what accessibility the method has, or any other information. The only thing the delegate cares about is if the method that it is looking for has the same signature as the delegate.

› Type a name for the file.

ˇ Click OK.

You can combine existing delegates into a multi-cast delegate.

TYPE THIS:

using System;

delegate void Del1(string x) class Class1

{

public static void On(string x)

{

Console.WriteLine("Switch goes on.", x);

}

public static void Off(string x)

{

Console.WriteLine("Switch goes off.", x);

}

public static void Main();

{

Del1 a, b, c;

a = new Del1(On); b = new Del1(Off); c = a + b;

Console.WriteLine("The two switch states:"); c;

}

}

RESULT:

The two switch states:

Switch goes on.

Switch goes off.

In object-oriented programming, an event lets clients of that class — clients can include delegates, other classes, methods, and indexers — know about something that

happens to an object in a class. That something that happens is of great interest to the clients in the class and so the event lets those clients know about it and act accordingly.

By acting accordingly, the clients give the class delegates so the delegates can retrain those objects using the modules called by those delegates. Once the appropriate module retrains the changed object to behave properly, the object goes back to the class for further processing.

When you declare an event inside of a class you must declare the delegate inside the event. The class that the event resides in is the only class that calls the event. When the class calls the event, the class checks to see if a client has hooked up a delegate to the event, and if that is true then the class processes the event.

The previous task mentioned that C# declares events using delegates. If you have come to this task wanting to learn about events but you have not learned about delegates yet, skip back four pages and read about delegates before you continue on.

■ The New Project

Á Type the code that establishes your event delegate, interface, and class.

‡ Type the code for the Output class that outputs the string when the event fires.