Exercise 105 in VB.NET

Make QR Code ISO/IEC18004 in VB.NET Exercise 105
Exercise 105
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Go back to look at some of the programs you have created as exercises (look at both console and WinForms programs): a) b) c) Can you identify the rst line of code to be executed in each case Is it always this line that is executed rst How do user-interactions affect the order that things are done in
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10 n Object Modelling
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1032 System and Product Classes
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Simply put, the business objects (those that collectively make up the business tier) in an application are product classes These classes and the objects that they produce will make up the logical structure of the application; the idealized assembly of objects that will come out of analysing the use-cases de ned at the requirements speci cation stage We can call them Product classes because they are the ultimate product of the application In a sales-ordering system, they will be classes that represent customer, orders, products, order-lines, and invoices In a CAD system, they will represent drawable objects, and in an email system, they will represent emails and entries in the address book System classes are all of the other classes that are not speci cally product classes These will be form designs, controls on forms, collection classes, data access tools and any class that does not t nicely into the logical model of the application Typically, these classes will form the top and bottom tiers of a three tier application, although some system classes will occupy the middle, business tier as well No matter whether a class works as a product or a system service, it is necessary to consider its lifetime within the application When will it be created and when will
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Lifetimes of product and system objects
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104 Managing Scarce Resources
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we be able to dispense with it Typically, product classes are created on demand (eg by the user clicking on a menu item or other control) and are destroyed when the user no longer needs them or when the program ends System classes will normally either be created when an application starts and destroyed when it ends (eg a collection, a database connection or the main user-interface) or will be created to perform a speci c task and discarded when the task is completed Figure 107 shows this diagrammatically
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Objects take up memory, memory is a limited resource, and the NET framework is designed to make sure that once we are done with an object, it will be cleanly removed from memory and the space it occupied restored to the heap of available memory in the system For most applications, we can simply create objects as and when they are needed and leave it up to NET to deal with clearing up after them Microsoft s guidelines on this are pretty speci c To quote the help page: For the majority of the objects that your application creates, you can rely on the NET Framework s garbage collector to implicitly perform all the necessary memory management tasks However, some resources that we can make use of in programs cannot be automatically cleaned up by the NET runtime system These are called unmanaged resources Say, for example, you have a class in which each object creates and opens a disk le as it is constructed, and writes data to this le throughout its lifetime Pretty obviously, when you are done with this object the disk le should be closed; it is an unmanaged resource because the NET framework will not automatically close the le as the object that uses it is destroyed In a C++ program that contained an object like this, you would create a class destructor, which as its name suggests is a routine that undoes what the constructor did In the destructor, you would place code to close the le and therefore release the scarce resource that would otherwise be a drain on the operating system In Visual Basic NET, the nearest we have to a destructor is the Finalize() method If a class is given a Finalize() method, it will be called automatically when the object is destroyed This would be the place to put the code to close the le the object was holding open However, in a C++ program, the programmer must explicitly call on a class s destructor by deleting the object (actually, this is only necessary for non-local objects declared outside any function, but we don t need to bother about these details here), whereas in Visual Basic NET, we are warned (by Microsoft) to ignore the Finalize() method and leave it up to the garbage collector to call it for us That s right; the part of the system that deals with discarded objects is called the garbage collector, and it acts like its namesake in that it has the job of collecting together all of the discarded objects and reclaiming their memory: it is more like a recycler really While it is doing its job, if it clears up any objects that have a Finalize() method de ned, it executes this method We are therefore guaranteed that the le held open by our object will eventually be closed
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