Advantages of using delegates?

Question

I\'m looking to implement the Observer pattern in VB.NET or C# or some other first-class .NET language. I\'ve heard that delegates can be used for this, but can\'t figure ou

Answer 1

You're an O/S, and I'm an application. I want to tell you to call one of my methods when you detect something happening. To do that, I pass you a delegate to the method of mine which I want you to call. I don't call that method of mine myself, because I want you to call it when you detect the something. You don't call my method directly because you don't know (at your compile-time) that the method exists (I wasn't even written when you were built); instead, you call whichever method is specified by the delegate which you receive at run-time.

Answer 2

You're not thinking like a programmer.

The question is, Why would you call a function directly when you could call a delegate?

A famous aphorism of David Wheeler goes: All problems in computer science can be solved by another level of indirection.

I'm being a bit tongue-in-cheek. Obviously, you will call functions directly most of the time, especially within a module. But delegates are useful when a function needs to be invoked in a context where the containing object is not available (or relevant), such as event callbacks.

Answer 3

I'm repeating an answer I gave to this question.

I've always like the Radio Station metaphor.

When a radio station wants to broadcast something, it just sends it out. It doesn't need to know if there is actually anybody out there listening. Your radio is able to register itself with the radio station (by tuning in with the dial), and all radio station broadcasts (events in our little metaphor) are received by the radio who translates them into sound.

Without this registration (or event) mechanism. The radio station would have to contact each and every radio in turn and ask if it wanted the broadcast, if your radio said yes, then send the signal to it directly.

Your code may follow a very similar paradigm, where one class performs an action, but that class may not know, or may not want to know who will care about, or act on that action taking place. So it provides a way for any object to register or unregister itself for notification that the action has taken place.

Answer 4

Here is an example that might help.

There is an application that uses a large set of data. A feature is needed that allows the data to be filtered. 6 different filters can be specified.

The immediate thought is to create 6 different methods that each return the data filtered. For example

public Data FilterByAge(int age)

public Data FilterBySize(int size)

.... and so on.

This is fine but is a very limited and produces rubbish code because it's closed for expansion.

A better way is to have a single Filter method and to pass information on how the data should be filtered. This is where a delegate can be used. The delegate is a function that can be applied to the data in order to filter it.

public Data Filter(Action filter)

then the code to use this becomes

Filter(data => data.age > 30);

Filter(data => data.size = 19);

The code data => blah blah becomes a delegate. The code becomes much more flexible and remains open.

Answer 5

A delegate is, in effect, passing around a reference to a method, not an object... An Interface is a reference to a subset of the methods implemented by an object...

If, in some component of your application, you need access to more than one method of an object, then define an interface representing that subset of the objects' methods, and assign and implement that interface on all classes you might need to pass to this component... Then pass the instances of these classes by that interface instead of by their concrete class..

If, otoh, in some method, or component, all you need is one of several methods, which can be in any number of different classes, but all have the same signature, then you need to use a delegate.

Answer 6

Actually there was an interesting back-and-forth between Sun and Microsoft about delegates. While Sun made a fairly strong stance against delegates, I feel that Microsoft made an even stronger point for using delegates. Here are the posts:

http://java.sun.com/docs/white/delegates.html

http://msdn.microsoft.com/en-us/vjsharp/bb188664.aspx

I think you'll find these interesting reading...