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Visual Basic Tutorial Part 9

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Yes

Yes

GlobalMultiUse Yes
Yes

SingleUse
Yes


GlobalSingleUse
Yes



Dependent Objects (PublicNotCreatable)
The value of the Instancing property determines the
part an object plays in your component's object model,
as discussed in "Organizing Objects: The Object
Model."
If the Instancing property of a class is
PublicNotCreatable, objects of that class are called
dependent objects. Dependent objects are typically
parts of more complex objects.
For example, you might allow a client application to
create multiple Library objects, but you might want
Book objects to exist only as parts of a Library. You
can make the Book class PublicNotCreatable, and let
the user add new books to a Library object by giving
the Library class a Books collection with an Add
method that creates new books only within the
collection.
Your component can support as many dependent objects


as necessary. You can write code in the Add method of
a collection class to limit the number of objects in
the collection, or you can allow the number to be
limited by available memory.
For More Information Dependent objects are discussed
in detail in "Dependent Objects," later in this
chapter.
Externally Creatable Objects
All values of the Instancing property besides
PublicNotCreatable and Private define externally
creatable objects -- that is, objects that clients can
create using the New operator or the CreateObject
function.
MultiUse vs. SingleUse
In ActiveX DLLs, Instancing for an externally
creatable class will most commonly be MultiUse. This
setting allows an in-process component to supply any
number of instances of the class to the client
executable, and to any other in-process component.
For ActiveX EXEs, the Instancing values SingleUse and
MultiUse define very different behaviors for a class.
MultiUse makes the most efficient use of memory,
because it allows one instance of your component to
provide multiple objects to multiple client
applications without duplication of resources or
global data.
For example, suppose the SmallMechanicals component
provides a Widget class, and the Instancing property
of the class is set to MultiUse. If one client
application creates two Widget objects, or if two
client applications each create a Widget object, all
the Widgets will be supplied from one instance of your
component.
If the Instancing property of the Widget class is set
to SingleUse, the result of both scenarios above is
that a separate copy of your component will be loaded
into memory for each Widget created. The uses and
limitations of this behavior are discussed in Chapter
8, "Building Code Components," and in Appendix A,
"ActiveX Component Standards and Guidelines."The uses
and limitations of this behavior are discussed in
"Building Code Components," and in "ActiveX Component
Standards and Guidelines."
MultiUse and Multithreading
If your component is an ActiveX EXE marked for
unattended execution (that is, it has no user
interaction whatever), and the Instancing property of
the Widget class is set to MultiUse, the result of
both scenarios above is that two Widget objects are
created in same copy of SmallMechanicals, each on its
own thread of execution.
Apartment Model threading is used, meaning that each
thread is like an apartment, and objects in different
apartments are unaware of each other's existence. This
is accomplished by giving each Widget its own copy of
the SmallMechanicals component's global data.
For More Information The use of multithreading or

SingleUse instancing to avoid blocked execution is
discussed in Chapter 8, "Building Code Components."The
use of multithreading or SingleUse instancing to avoid
blocked execution is discussed in "Building Code
Components."
Global Objects
Frequently it's useful to have utility functions that
users of your component can employ without first
creating an instance of one of your objects. In
out-of-process components, such functions are
frequently implemented as properties or methods of the
Application object.
If the Instancing property for a class is marked
GlobalMultiUse or GlobalSingleUse, then properties and
methods of the class can be invoked without explicitly
creating an instance of the object.
For example, suppose you want your SmallMechanicals
component to provide a GlobalUtility object whose
methods are general-purpose functions. You can add a
class module to the SmallMechanicals project, set the
Name property to GlobalUtility, and set the Instancing
property to GlobalMultiUse.
Now you can add properties and methods to the class
module. For example, you might implement a
ReversePolarity method and a read-only WidgetCount
property:
Public Sub ReversePolarity()
' (Code to reverse polarity on all Widgets.)
End Sub
In the client application, the ReversePolarity method
can be invoked without first creating a GlobalUtility
object:
Private Sub Command1_Click()
' No object variable is required.
ReversePolarity
End Sub
Note The properties and methods of a GlobalMultiUse
object are not part of the global name space of the
component that provides the object. For example,
within a project that contains the GlobalUtility
class, you must explicitly create an instance of
GlobalUtility in order to use the object's properties
and methods. Other limitations of global objects are
listed in "Global Objects and Code Libraries," in
Chapter 8, "Building Code Components."Other
limitations of global objects are listed in "Global
Objects and Code Libraries," in "Building Code
Components."
Be careful when choosing names for the properties and
methods of global objects. Using common or obvious
names may result in name collisions with other
components. Name conflicts must be resolved by
qualifying the property or method with the type
library name:
Private Sub Command1_Click()
SmallMechanicals.ReversePolarity
Esalen.ReversePolarity

End Sub
Important The "global" in global objects refers to
the fact that all of the object's properties and
methods are available in the global name space of your
project. It does not mean that one object is
automatically shared by all clients. Each client that
uses your component gets its own global object.
For More Information "Providing Named Constants for
Your Component," later in this chapter, discusses the
use of global objects to provide string constants and
non-integer constants. Code components are discussed
in depth in Chapter 8, "Building Code Components."Code
components are discussed in depth in "Building Code
Components."

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