Visual Basic projects have a special object that can be automatically loaded when
the program is run. This object is referred to as the Startup object. Figure 4.20
shows an example of a Startup object. The Startup object can now vary depending
on the type of project you are creating. A form can be selected, the Sub Main
procedure, or— new in VB5 and VB6—nothing. You would specify Nothing
when the program does not require an interface. An example of this type of project
might be an ActiveX component or an ActiveX control.
When a form is specified as the Startup object,
that form automatically loads into memory when
the application starts. No other form loads
unless it is referenced in program code or is
explicitly loaded into memory.
Use the Load statement to load a form into
memory without making it visible yet. The Load
statement will take only one argument: the name
of the object to be loaded. Take a look at the
following code:
Load Form1
Load frmTest
FIGURE 4.20 Selecting the project Startup object.
The Load statement in both cases accepts a
valid object name. This causes the object to
load into memory. Although the object loads
into memory, this does not mean that the object
will be visible to the user. This enables the
programmer to load multiple forms that may be
required and to prepare them with information
before display.
Once loaded into memory, the form's controls
and any program code can be used. When working
with forms in VB, it is important to note that
any reference to an object will cause that object
to load. The Load statement does not have to
be explicitly used before an object can be used.
An example of this would be if a form's
Caption property were set, as follows:
Form1.Caption = "My Notepad"
Notice that this is the only line of code
that you need to be concerned with. There is
no Load statement before the Caption property
is set. This code directly sets the form's
property. This single line of code automatically
causes the Form1 object to be loaded into memory.
This is often referred to as implicit loading.
Because the object must be loaded to set the
property, VB does exactly that.
Implied loading can often cause problems when
working on a multiform project. The programmer
does not notice that one form calls or sets
information on another form. The form then automatically
loads. Later when you attempt to unload by name
all forms that you remember using, your project
continues to run.
The End statement can be used to force the
application to terminate, regardless of which
forms were explicitly or implicitly loaded.
This is a fail-safe in case the program missed
a reference or miscounted how many forms were
loaded.
However, the End statement will have an undesirable
effect because it will end the application so
abruptly that the QueryUnload and Unload events
of forms will not have a chance to run.
For a more acceptable method to unload all forms, see the section in this
chapter "Using the Forms Collection to Unload All Forms."
When an individual form is no longer required,
you can unload it from memory. This will release
the graphic components from memory.
The following code unloads two forms:
Unload Form1
Unload frmTest
The Unload statement accepts a valid object
name. This causes the design time graphic components
of a form to be released. Although the form
has been unloaded from memory, it is very important
to note that any Public procedures or variables
belonging to the form are still in memory. The
graphical controls are no longer available,
but any Public members of the form can still
be called, using the syntax for calling any
object variable's members, formname.procname.
Remember, a form's Public procedures
and variables are considered to be members (i.e.,
methods and properties) of the instance of the
form object. Of course what's just been
said for Public variables also goes for a form's
Custom Properties implemented with Property
Get and Property Let/Set procedures.
Load and Unload are used to control the memory
status of a form. These two statements always
appear before the name of the object to be affected.
They are often confused with the Show and Hide
methods that take place after the object name.
Show and Hide are used to control whether a
form is visible to the user.
After you unload a form, the form's
Public procedures and its Public variables will
still be resident as mentioned above. To completely
reset the contents of these elements, call the
line
Set FormName = Nothing
in your code just after calling the Unload statement.
Creating Data Input Forms and Dialog Boxes topics
