ArcGIS Engine Developer Guide

THE MICROSOFT COMPONENT OBJECT MODEL
Aggregation is important in this case since there are some hidden interfaces defined
on a feature that cannot be contained.
Visual Basic 6 does not support aggregation, so it can’t be used to create custom
features.
Although an understanding of apartments and
threading is not essential in the use of
ArcObjects, basic knowledge will help you
understand some of the implications with certain
development environments highlighted later in
this chapter.
Apartments
process space
Single threaded apartment
(main apartment)
Single threaded apartment
Single threaded apartment
Multithreaded apartment
Thread
neutral
apartment
Think of the SCM (pronounced scum) as the
COM runtime environment. The SCM interacts
with objects, servers, and the operating system
and provides the transparency between clients
and the objects with which they work.
THREADS, APARTMENTS, AND MARSHALLING
A thread is a process flow through an application. There are potentially many
threads within Windows applications. An apartment is a group of threads that
works with contexts within a process. With COM+, a context belongs to one
apartment. There are potentially many types of contexts; security is an example
of a type of context. Before successfully communicating with each other, objects
must have compatible contexts.
COM supports two types of apartments: single-threaded apartments and
multithreaded apartments (MTA). COM+ supports the additional thread-neutral
apartment (TNA). A process can have any number of STAs; each process creates
one STA called the main apartment. Threads that are created as apartments are
placed in an STA. All user interface code is placed in an STA to prevent deadlock
situations. A process can only have one MTA. A thread that is started as
multithreaded is placed in the MTA. The TNA has no threads permanently associated
with it; rather, threads enter and leave the apartment when appropriate.
In-process objects have an entry in the registry, the ThreadingModel, that informs
the COM service control manager (SCM) into which apartment to place the
object. If the object’s requested apartment is compatible with the creator’s apartment,
the object is placed in that apartment; otherwise, the SCM will find or
create the appropriate apartment. If no threading model is defined, the object
will be placed in the main apartment of the process. The ThreadingModel registry
entry can have the following values:
• Apartment. Object must be executed within the STA. Normally used by UI
objects.
• Free. Object must be executed within the MTA. Objects creating threads are
normally placed in the MTA.
• Both. Object is compatible with all apartment types. The object will be created
in the same apartment as the creator.
• Neutral. Objects must execute in the TNA. Used by objects to ensure there is
no thread switch when called from other apartments. This is only available
under COM+.
Marshalling enables a client to make interface function calls to objects in other
apartments transparently. Marshalling can occur between COM apartments on
different machines, between COM apartments in different process spaces, and
between COM apartments in the same process space (STA to MTA, for example).
COM provides a standard marshaller that handles function calls that use automation-compliant
data types (see table below). Nonautomation data types can be
handled by the standard marshaller as long as proxy stub code is generated; otherwise,
custom marshalling code is required.
Chapter 4 • Developer environments • 67