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When we use the term event, we are referring to something that is an object considered important to communicate to the world For example, imagine a coffee-travel-mug object: the specific object we have is blue in color with a black lid The object has two methods, drink() and spill(); two parameters, noSpill (closes the lid) and level (indicates how much coffee there is in the cup) This can be a very common, everyday travel mug, or it can be a software object that emulates a travel mug either way we can interact with the mug object Here is our task: We want to make sure we can refill the mug object as soon as it is empty, so we need to perform a loop asking is it empty that reads the level parameter every so many seconds When the level indicates that the mug is empty, the mug should be refilled We could have produced a procedural software structure similar to this pseudo-code

do while (true) { if (muglevel <= 0) { refill(mug) } }

Which lockset function from the following listing would be described as outside knob always locked, entry by key only, inside knob always free A Entry B Storeroom C Classroom D Vestibule

.

This type of looping structure was the mainstay of the procedural world; we used to draw lovely flowcharts so we would know the flow of execution through the application This type of processing is synchronous, with everything happening in a predetermined order, and only in that order In our travel mug example, the end result of all that checking would be that coffee most likely would be removed from the daily food list it is too much work for the person checking the level in the mug The natural question is whether there is a better way, and fortunately there is The event model gives us the ability to have two or more objects communicating with each other asynchronously We will bestow our travel mug with two events For now we will just define an event as something that is important to the object, and in this case we will create the empty event and the full event The mug will cause the empty event when the mug is empty and the full event when the mug is close to overflowing To take advantage of this new better travel-mug model, we need to do one extra thing we need to tell the mug that we would like to be told when these events take place This request is a registration of interest We give the mug a sticky note that says Tell me when the empty event happens and gives an address where we can be contacted Now we can go about our day doing what we do best, and when the mug is empty we will get a message from the mug telling us that it is empty This process is performed in the NET environment every time we use an event Actually this is the processing that took place in previous versions of the Windows operating system, as well In the software world, we call the event routing a callback The receiver of the event registers an address that should be called when the event fires (happens) The code that is called when the event fires is called the event handler Now let s have a look at how we perform event handling in the NET environment: we use a delegate to connect the event with the code that will handle the event.

Previous languages in the C family use the concept of the function pointer; every function occupies a block of memory, and the starting address is the function pointer Being able to refer to a function by its memory address (pointer) is very powerful, but it is also fraught with dangers: if a pointer changes, the call to the location of the pointer will most likely run code that will crash the application, or even the operating system The design of the C# language intentionally removed the ability to use pointers and perform pointer operations that C and C++ permitted and that led to many difficult-to-locate bugs Instead, the C# designers created the equivalent of the function pointer in a type-safe OO way: they created the delegate Here s the one-sentence definition of a delegate: The delegate object encapsulates a reference to a method Let s look at how you declare and instantiate a delegate The first step is to declare the delegate, defining the return data type as well as the parameters The following code defines a couple of delegates:

Which lockset must be removed from the door to release the cylinder A Arrow Mil B Schlage A53 C Kwikset 400 D Best E None of the above