Unit – 3 Objects in Visual Basic
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Unit – 3 Objects in Visual Basic An object is a combination of code and data that can be treated as a unit. An object can be a piece of an application, like a control or a form. An entire application can also be an object. Objects are things that you can program with, things that make programming easier. They contain a set of methods and properties that allow you to make the object do certain things without actually having to look at the objects code. For example, you have probably used the following statement many times: Text1.Text = " " Remember that? Well, what you are doing is setting the property Text, of the text box object Text1 to equal nothing. Objects are really easy, In fact, all the controls you see on your VB toolbox are all objects. For a better look at objects, either press F2 or click View, Object Browser. This nifty little window allows you to see all the properties and methods of the objects currently loaded. Although Visual Basic is not a completely object orientated language it does allow us to use objects in our code. You can declare an object using either the Dim, Private or Public keyword. As with other variables, the Private and Public keywords can only be used in the General Declarations procedure. You will probably know that declaring something as Public allows anyone to access the variable, and declaring it as Private only lets the current module of code access the variable. Code: Private m_strName As String Well, here is a simple object variable declaration: Code: Private m_clsClass1 As Class1 VB allows you to use the New keyword when you declare an object variable: Code: Private m_clsClass1 As New Class1 When your program starts, the object is automatically created so you can use its methods etc immediately. If your object fires up a connection a database and gets some records when you start, or if your object simply contains a large amount of code, then you may be causing the users machine to run slower then needed. If you don't make a call on the object for a few minutes, then you are wasting memory by having the object in memory. So, what can you do? Well, simply avoid using the New keyword when declaring the object. Every time you want to use it, create a new instance of it and then destroy it When dealing with objects you use the Set keyword to perform some operations. When we want to destroy an object, we set it to nothing, quite literally: Code: Set m_clsClass1 = Nothing This way, we only have it loaded in memory when we need to use it. If you have a lot of users using a database object for example, then you may want to open and close connections for each user. This is called object 'pooling' where you pool all your resources together and dish them out when requested. Microsoft makes this easier by giving you Microsoft Transaction Server. Objects let you declare variables and procedures once and then reuse them whenever needed. For example, if you want to add a spelling checker to an application you could define all the variables and support functions to provide spell-checking functionality. If you create your spelling checker as a class, you can then reuse it in other applications by adding a reference to the compiled assembly. Better yet, you may be able to save yourself some work by using a spelling checker class that someone else has already developed. Each object in Visual Basic is defined by a class. A class describes the variables, properties, procedures, and events of an object. Objects are instances of classes; you can create as many objects you need once you have defined a class. To understand the relationship between an object and its class, think of cookie cutters and cookies. The cookie cutter is the class. It defines the characteristics of each cookie, for example size and shape. The class is used to create objects. The objects are the cookies. Two examples in Visual Basic might help illustrate the relationship between classes and objects. • The controls on the Toolbox in Visual Basic represent classes. When you drag a control from the Toolbox onto a form, you are creating an object — an instance of a class. • The form you work with at design time is a class. At run time, Visual Basic creates an instance of the form's class — that is, an object. Objects newly created from a class are often identical to each other. Once they exist as individual objects, however, their variables and properties can be changed independently of the other instances. To create an object from a class 1. Determine from which class you want to create an object. 2. Write a Dim Statement to create a variable to which you can assign a class instance. The variable should be of the type of the desired class. Dim nextCustomer As customer 3. Add the New Operator keyword to initialize the variable to a new instance of the class. Dim nextCustomer As New customer 4. You can now access the members of the class through the object variable. nextCustomer.accountNumber = lastAccountNumber + 1 How to build Object models: You can create your object model from an existing database and use the model in its default state. You can also customize many aspects of the model and its behavior. If you are using Visual Studio, you can use the Object Relational Designer to create your object model. If you do not have an existing database and want to create one from an object model, you can create your object model by using your code editor and Create Database Documentation for the O/R Designer provides examples of how to generate a Visual Basic object model by using the O/R Designer. UNIT -4 Data Access Objects: Data Access Over the years, Visual Basic has become an application development tool used for developing Client/Server applications. This is an area of Visual Basic that seems to be getting better and better with every new version. For instance, Visual Basic 6.0 has introduced a new way of accessing data through ADO & OLEDB. And Microsoft says you stick to ADO because that is the future. Old methods like DAO and RDO are already obsolete. If you are wondering about all the stuff I mentioned, I would say, "do not worry". All that is part of data access and we will thoroughly discuss about DAO, RDO and ADO in the next couple of chapters. What is Data Access? Data access is a feature of Visual Basic that allows you to access and manipulate any database from Visual Basic. The database may be either MS-Access database or FoxPro database or it may also be any of the relational databases such as Oracle. You can develop applications in Visual Basic that can access data in a database. The database may be on the same machine as the application or it may be on database server that is far away from Visual Basic application. Whatever may be the case, you can access and manipulate the data using applications written in Visual Basic. The following are the various ways of access database: Data Access Objects (DAO) This is an object model that has a collection of objects using which you can access a database. This model gives complete control on the database. This model uses Jet Engine, which is the native database engine used by Visual Basic and MS-Access. This was the first model to be used in Visual Basic. Though it is possible to access any database using this, it is particularly suitable for MS-Access database and not suitable for ODBC data sources such as Oracle and MS-SQL Server. So, Microsoft later introduced RDO. Remote Data Objects (RDO) These objects are only used to access ODBC data sources such as Oracle. These objects access databases that are on remote machine (database server). This object model has less number of objects compared with DAO and more suitable for accessing remote databases. We will discuss more about RDOs in chapter 18. ActiveX Data Objects (ADO) Microsoft has introduced a new object model called ActiveX Data Objects (ADO), which is based on ActiveX technology, for the first time in Visual Basic 6.0. This object model has very few objects and it is based on OLE DB interface. OLE DB interface is a new interface (replacing ODBC and others), through which you can access data of all formats in the same manner. ADO uses OLE DB providers to access the data. That means each database is accessed through OLE DB provider. And ADO provides the programming framework to access OLE DB. ADO is also much easier to deal with. That is all about the introduction to data access methods. If you could not understand every part of that, don’t worry. As we unfold things, you will start understanding all about data access. But one thing is for sure. Visual Basic has got so many ways of accessing data (various object models and controls). It is certainly going to confuse a beginner. Also remember, that there is no single method that is efficient in all circumstances. Given the task on hand, choose the best-suited method. My personal suggestion is – use ADO as much as you can. Because that is what Microsoft heralds as the future. Accessing MS-Access database First, let us understand how to access data using a very simple and old method – using data control.