Explanation of the States in Which Forms Can Exist (138068)
The information in this article applies to:
- Microsoft Visual Basic Standard Edition, 32-bit, for Windows 4.0
- Microsoft Visual Basic Professional Edition, 16-bit, for Windows 4.0
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows 4.0
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows 4.0
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows 4.0
This article was previously published under Q138068 SUMMARY
A Visual Basic 4.0 form object can exist in five possible states: - Unloaded
- Created but not loaded
- Invisible but loaded (hidden)
- Visible
- Unloaded but control is loaded
Four of these states (1, 3, 4, and 5) existed in Visual Basic version 3.0
also. The new state in Visual Basic version 4.0 is the "Created but not
loaded" state.
MORE INFORMATIONUnloaded
In this state the form hasn't been loaded yet. It does not exist.
Created But Not Loaded
This is new in Visual Basic version 4.0. To explain this state, it is
beneficial to think of the form as having two parts: a code part and a
visual part. Think of the code part as including all the methods and
properties that may be defined in the form module (this part is new in
Visual Basic version 4.0). Think of the visual part as all the rest such as
the form properties in the Property window (this part is same for Visual
Basic version 3.0 and Visual Basic version 4.0).
In Visual Basic version 4.0, forms are objects. When you use an auto-
instantiating variable to refer to a user-defined property or method (that
is, any member in the code part), the Form object is created but not loaded
(that is, only the code part of it is loaded). This means that the
Initialize and Terminate Events for the form are invoked.
Say there is a Sub procedure called MyMethod defined in the form module.
Then the following code illustrates the semi-loaded state of a form:
Dim x As New Form1
x.MyMethod
Invisible But Loaded (Hidden)
In this state, the form is loaded and then its Visible property is set to
False. The line of demarcation for this state is the execution of the
Form_Load() event.
Visible
The normal state of the form being loaded and then shown.
Unloaded But Control is Loaded
You get into this state by loading the form, setting a reference to a
control on the form, and then unloading the form. The reference to the
control still stays alive.
This can be a very dangerous state to leave your form in because on
unloading the form, the code in the Form_Unload() event is executed, yet
the form and its controls are not destroyed. The Form object and all the
objects it contains still exist. What's more, they will be loaded back into
memory, with all their current property values and states, the moment your
code uses that control reference, even if its just to query a property of
the "loaded" control. The dangerous part is that the Form_Load() event will
not be executed. It is as if you just made the form visible again.
| Modification Type: | Major | Last Reviewed: | 12/9/2003 |
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| Keywords: | KB138068 |
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