End Get Set(ByVal Value As Integer) End Set
Note that because VB .NET supports function overloading (discussed later in this chapter), we can define multiple constructors in a single class, provided each constructor has a unique argument signature. We can then invoke any of the custom constructors simply by supplying the correct number and type of arguments for that constructor. Note also that once we define one or more custom constructors, we can no longer invoke the default (that is, parameterless) constructor with a statement such as: Dim APerson As New CPerson( ) Instead, to call a parameterless constructor, we must specifically add the constructor to the class module: ‘ Default constructor Sub New( ) … End Sub 3.3.8 Finalize, Dispose, and Garbage Collection In VB 6, a programmer can implement the Class_Terminate event to perform any clean up procedures before an object is destroyed. For instance, if an object held a reference to an open file, it might be important to close the file before destroying the object itself. In VB .NET, the Terminate event no longer exists, and things are handled quite differently. To understand the issues involved, we must first discuss garbage collection. When the garbage collector determines that an object is no longer needed (which it does, for instance, when the running program no longer holds a reference to the object), it automatically runs a special destructor method called Finalize. However, it is important to understand that, unlike with the Class_Terminate event, we have no way to determine exactly when the garbage collector will call the Finalize method. We can only be sure that it will be called at some time after the last reference to the object is released. Any delay is due to the fact that the .NET Framework uses a system called reference-tracing garbage collection, which periodically releases unused resources. Finalize is a Protected method. That is, it can be called from a class and its derived classes, but it is not callable from outside the class, including by clients of the class. (In fact, since the Finalize destructor is automatically called by the garbage collector, a class should never call its own Finalize method directly.) If a class’ Finalize method is present, then it should explicitly call its base class’ Finalize method as well. Hence, the general syntax and format of the Finalize method is: Overrides Protected Sub Finalize( ) ‘ Cleanup code goes here MyBase.Finalize End Sub The benefits of garbage collection are that it is automatic and it ensures that unused resources are always released without any specific interaction on the part of the programmer. However, it has the disadvantages that garbage collection cannot be initiated directly by application code and some resources may remain in use longer than necessary. Thus, in simple terms, we cannot destroy objects on cue. We should note that not all resources are managed by the Common Language Runtime. These resources, such as Windows handles and database connections, are thus not subject to garbage collection without specifically including code to release the resources within the Finalize method. But, as we have seen, this approach does not allow us or clients of our class to release resources on
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