HOWTO: Override Implicit Declaration with Declared Variable

ID: Q147665

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 16-bit and 32-bit, for Windows, version 4.0

SUMMARY

Visual Basic's implicit declaration of a form variable with the same name as each form class allows form manipulation code to be written without explicitly creating an instance of each form class. This is convenient, but can also lead to confusion, especially when code is creating and manipulating multiple instances of the same form class. By overriding Visual Basic's implicit declaration with an explicit declaration, this confusion can be avoided.

MORE INFORMATION

Rather than thinking of each form listed in the Project window as an entity, one may think of a form has a template, or form class, that may be used to create concrete instances. With the ability to declare public variables, functions, and procedures on forms in Visual Basic, this analogy becomes even more solid. Public variables are just properties of the form, while public functions and procedures become the form's methods.

With this idea in mind, one can create and display two instances of an existing form class with code like this:


   Dim MyFormObject1 As Form1
   Dim MyFormObject2 As Form2

   Set MyFormObject1 = New Form1
   Set MyFormObject2 = New Form2
   MyFormObject1.Show
   MyFormObject2.Show

While this style of coding has many advantages, Visual Basic also does some work behind the scenes to allow the manipulation of a form without previously declaring a variable to hold the reference to that form. Visual Basic does the equivalent of automatically declaring an object variable with the same name as each form class. When a program with a startup form begins, or the first time a property or method of a form class is called, Visual Basic creates a form object from the form class and assigns it to the implicitly declared reference. In the example below, the caption property that is set is of the automatically declared Form2 variable and automatically created object of type Form2. Because of Visual Basic's implicit declaration and creation of the form, this code works properly.


   Private Sub Form_Load()
      Form2.Caption = "Hello VB World"
   End Sub

People familiar with the use of OLE servers from Visual Basic may have heard the statement that 'the first time a property or method of a [class] is referenced, Visual Basic creates [an object] from the [class].' This sounds suspiciously like Visual Basic's behavior when an object variable as been declared 'As New'. With this kind of declaration, Visual Basic will automatically create an object when a property or method is referenced, if an object doesn't already exist (see pages 200-201 of the Programmer's Guide for more information). Visual Basic's implicit declaration of form variables can be thought of as this sort of equivalent Visual Basic statement:


   Public MyForm As New MyForm

If one chooses to follow the convention that all form objects should be created and destroyed explicitly, it is easy to accidentally refer to a property or method of the form when the form object does not exist (either because it hasn't been explicitly created or because it has been previously destroyed). By default, because of the above implicit declaration, Visual Basic will happily create a new instance of the given form class and allow the code to proceed unheeded. This can easily lead to logic errors that can be difficult to find. To avoid this problem, one can override the implicit declaration with their own explicit declaration. Doing so will cause Visual Basic to correctly raise error 91, "Object variable not set," if a property or method of a non-existing form is referenced.

A declaration should be placed in a module for each form in the project, for example:


   Public MyForm As MyForm
   Public MySecondForm as MySecondForm

After these declarations, each form must be created with a Set statement before being used, for example:


   Sub Main()
      Set MyForm = New MyForm
      MyForm.Show
   End Sub

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Platform : NT WINDOWS
Issue type : kbhowto