Robot r = new Robosnake(someConcreteMotor); // Type IPropulsion in Visual C#

Draw Code 39 Full ASCII in Visual C# Robot r = new Robosnake(someConcreteMotor); // Type IPropulsion

or by assigning it via a setter property:

Another approach to dependency injection is to use a factory method (which I discuss earlier in this chapter, in the section As a method return type, and illustrate in the section Hiding Behind an Interface ):

Using delegates to solve the callback problem Using delegates to customize a method Implementing delegates by using anonymous methods Using C# events to notify the world when interesting events happen

his chapter looks into a corner of C# that has been around since the birth of the language, but one that I ve avoided because it s challenging stuff However, if you can bear with me and I try to go as easy on you as possible the payoff is well worth it

If you ve seen the Steven Spielberg movie ET, the Extraterrestrial (1982), you watched the cute but ugly little alien stranded on Earth try to build an apparatus from old toy parts with which he could phone home He needed his ship to pick him up It s a big jump from ET to C#, but code sometimes needs to phone home, too For example, you may have wondered how the Windows progress bar works It s the horizontal bar that gradually fills up with coloring to show progress during a lengthy operation, such as copying files (On my machine, of course, good old Murphy s law Whatever can go wrong will go wrong seems to fill it up well before the task is finished Figure 9-1 shows a green progress bar (though it isn t easy being green in a black-and-white book) The progress bar is based on a lengthy operation s periodic pause to phone home In programmerese, it s a callback Usually, the lengthy operation estimates how long its task should take and then checks frequently to see how far it has progressed Periodically, the progress bar sends a signal by calling a callback method back on the mother ship the class that kicked off the long operation The mother ship can then update its progress bar The trick is that you have to supply this callback method for the long operation to use

That callback method may be on the same class as the lengthy operation such as phoning your sister on the other side of the house Or, more often, it s on another class that knows about the progress bar such as phoning Aunt Maxie in Minnesota Somehow, at its start, the lengthy operation has been handed a mechanism for phoning home sort of like giving your kid a cellphone so that she can call you at 10 pm This chapter talks about how your code can set up this callback mechanism and then invoke it to phone home when needed Callbacks are used a lot in Windows programming, typically for a piece of code, down in your program s guts, to notify a higher-level module that the task has finished, to ask for needed data, or to let that module take a useful action, such as write a log entry or update a progress bar

C# provides delegates for making callbacks and a number of other tasks Delegates are the C# way (the NET way, really, because any NET language can use them) for you to pass around methods as though they were data You re saying, Here, execute this method when you need it (and then handing over the method to execute) This chapter helps you get a handle on that concept, see its usefulness, and start using it yourself You may be an experienced coder who will recognize immediately that delegates are similar to C/C++ function pointers only much, much better But I m assuming in this section that you aren t and you don t Think of a delegate as a vehicle for passing a callback method to a workhorse method that needs to call you back or needs help with that action, as in doing the same action to each element of a collection Because the