VB.NET Language in a Nutshell, Second Edition

Object Variables and Their Binding

In VB.NET, classes and their objects are everywhere. Of course, there are the classes and objects that we create in our own applications. There are also the classes in the .NET Framework Class Library. In addition, many applications take advantage of the objects that are exposed by other applications, such as ActiveX Data Objects (ADO), Microsoft Word, Excel, Access, various scripting applications, and more. The point is that for each object we want to manipulate, we will need to declare a variable of that class type. For instance, if we create a class named CPerson, then in order to instantiate a CPerson object, we must declare a variable:

Dim APerson As CPerson

Similarly, if we decide to use the ADO Recordset object, we will need to declare a variable of type ADO.Recordset:

Dim rs As ADO.Recordset

Even though object variables are declared in the same manner as nonobject variables, there are some significant differences. In particular, the declaration:

Dim obj As MyClass

does not create an object variable — it only binds a variable name with a class name. To actually construct an object and set the variable to refer to that object, we need to call the constructor of the class. This function, discussed in detail in Chapter 4, is responsible for creating objects of the class.

Constructors are called using the New keyword, as in:

Dim obj As MyClass = New MyClass(  )

or:

Dim obj As MyClass
obj = New MyClass(  )

VB.NET also provides a shortcut that does not ...