These user-defined data types are called classes. Each class contains data as well as a set of methods which manipulate the data. The data components of a class are called instance variables and one instance of a class is an object. For example, in a library system, a class could be a member, and John and Sharon could be two instances two objects of the library class.
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Classes are created according to hierarchies, and inheritance allows the structure and methods in one class to be passed down the hierarchy. That means less programming is required when adding functions to complex systems. If a step is added at the bottom of a hierarchy, then only the processing and data associated with that unique step needs to be added. Everything else about that step is inherited. Object-oriented programming allows procedures about objects to be created whose exact type is not known until runtime.
For example, a screen cursor may change its shape from an arrow to a line depending on the program mode. It also allows new shapes to be easily integrated. Visual Basic allows users to write programs that break down into modules. These modules represent the real-world objects and are knows as classes or types. An object can be created out of a class and it is known as an instance of the class. Inc lude the Dispose method but not the Finalize method.
Point out that, in a real situation, the Dispose method would be used for saving data and closing a database connection. Enter a positive integer value for the Id any number will work , and then click the Existing button. Point out that this time the constructor takes the intEmpId as a parameter, so it can load the data from a database immediately. Step through the code until the object has been instantiated, and then press F5 to allow the remaining code to run.
Click the Improved New button on the form, and then step through the code when execution halts at the preset breakpoint. Point out that the constructor takes strFirstName and strLastName as parameters so that it can create a new Employee immediately. Step through the initialization code, and then press F5 to display the form again. Click the Close button to close the form and stop the application.
Explain that this will cause all remaining objects to be destroyed, and that the Finalize methods will execute. Remind students that the Finalize method should only be used when resources need to be manually reclaimed such as database connections because it creates more work for the garbage collector. In this case the Finalize method calls the Dispose method again to ensure that the resources have been reclaimed in case the class user has forgotten to call the Dispose method explicitly. The Finalize method is not necessary if the class user has called the Dispose method.
A call to GC. SuppressFinalize within the Dispose method would have stopped the Finalize method from executing and would therefore have improved performance. Open the Inheritance. View the code for the Person class, and point out the private variables, the properties, and the methods. View the code for the Employee class, and point out that it is a simplified version of the class used in the previous demonstration in that it has only one constructor. Show that it inherits from the Person class and only stores the EmployeeId value. Also, point out that the FirstName and LastName properties are not defined in this class.
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Type values in the First Name and the Last Name boxes. Click the New Person button on the form. The code will enter break mode at the first preset breakpoint. Step through the code, explaining each line of code as you go. This will include the Dispose method and the Finalize method when the GC. Collect method is executed.
Point out that the Finalize method also calls the Dispose method of the Person by means of the MyClass object. Enter a positive integer value for the Id any number will work , and click Existing Emp. Point out that this time many of the inherited properties of the Person class are called rather than those of Employee.
Step through the code until the form appears again. Close the form and quit the Visual Studio.
Open the Polymorphism. View the code for the IPerson interface. Point out the property and method definitions and that there is no implementation code. View the code for the Employee class, and point out that it now implements Delivery Tip the IPerson interface and stores the intEmployeeId, strFName, and Explain that students can use either public or private strLName values.
Also point out the Public EmployeeId property, the methods within the class Private FirstName and Private LastName properties, and the new syntax when implementing the for the Implements keyword in each method signature. Explain that interface. Also explain that marking the properties as private is optional. They could have been marked as public, making them visible from both the IPerson interface and the Employee class. The Visual Basic. NET process will halt at the first preset breakpoint.
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Step through the code, explaining each line as you go. Point out that both the Employee class and the IPerson interface are used to access the various members.
The perPerson. Save method is called first to show what happens if you use the IPerson interface. The empEmployee. SaveEmployee method shows that you can use any name that you choose for the implemented method. Click the New Student button on the form. Point out the similarity in the calling code for the IPerson methods of both the Student and Employee objects, and explain how this aids code reuse. Open the Events. View the code for the Employee class, and point out the DataChanged event in the declarations section and its purpose.
In the FirstName property, point out the raising of the event and the purpose of the code that checks the blnCancelled value.
View the code for the frmEvents form, and point out the module-level variable that uses WithEvents. Click the Class Name combo box, and then click empEmployee. In the Method Name combo box, click DataChanged.
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Examine the default event handler code, and point out the Handles keyword in the function declaration. Click the WithEvents button on the form. This will include the RaiseEvent code in the Employee class. Click the AddHandler button on the form.
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Explain the AddHandler statement, and examine the EmployeeDataChange method at the end of the code for the form. Continue debugging as the events are raised to the EmployeeDataChange method. NET ix. Module Strategy Use the following strategy to present this module: n Defining Classes This lesson describes how to create classes in Visual Basic. Students will learn how to declare methods and properties, and how to overload class members. When you introduce students to class constructors and destructors, including multiple constructors, point out that the Finalize method should only be used when resources need to be manually reclaimed such as database connections because this method adds overhead to the disposing of objects.
Some of this lesson contains simple tasks such as how to create classes and methods. Cover these areas quickly so that more time can be spent on new features, such as the syntax for defining properties and constructors. Contrast this approach to the approach used in previous versions of Visual Basic to show the usefulness of constructors. Introduce garbage collection as an important change in the way objects are destroyed. Ensure that students understand this process, because many developers will be unaware of the potential dangers. Present the Dispose method as a suggested way to handle issues raised by garbage collection.
Point out that the notes present two common techniques for creating the Dispose method, and that the IDisposable interface provides a more consistent approach. NET documentation. Use the instructor-led demonstration to demonstrate how to create classes that contain multiple constructors. In this demonstration, you will also show how to instantiate and use classes from calling code.