Understanding COM/ActiveX
Jeff Paulter
Staff Software Engineer
Thurs Aug 17
10:15-11:30 a.m., 1:45-3:00 p.m.
Ash (10A)
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Outline
What is ActiveX?
Why use COM/ActiveX
What is COM?
What are ActiveX/COM
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Servers
Objects
Interfaces
More
What is ActiveX all about?
Demonstrations
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Document objects
Automation
Controls
Containers
What is ActiveX?
The name for technologies built on top of
COM
Examples include:
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Document objects
ActiveX controls
Automation
OLE for process control (OPC)
Grew out of OLE
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Motivation for COM/ActiveX
Component software development
• Reuse of binary software components
• Create and use components from various
languages
• Update components without recompiling or
breaking existing clients
Object-based model
Distributed software
Location transparency
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What About DLLs?
Calling conventions can vary
DLLs are not object based (Flat API)
Versioning can be difficult
Memory management concerns
Linkage issues
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What is COM?
Component object model
Binary and network standard
Run-time system
• Calls across network
• Location transparency
• Security
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COM Servers
Can be DLLs or EXEs
Provide objects for clients
• A single server can provide implementations for
more than one object
• The set of objects that a server provides and the
relationships between them is referred to as an
object hierarchy
Are activated by the COM run-time services
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Object Hierarchy
Top Level
Objects
(Creatable)
Lower Level
Objects
(Non-Creatable)
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COM Objects
Clients view objects as black boxes
Clients communicate with objects only
through interfaces
Objects can handle more than one interface
Interface1
Interface2
Interface3
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Object
COM Interfaces
Sets of related methods
Immutable contracts
Implementations not described
• NO member data (functions only)
COM defines a standard for in-memory layout
of interfaces
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Interface Benefits
Compiler neutral
Language neutral
Object implementations can change without
affecting clients
• Clients never need to be recompiled
Versioning
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Sample Interfaces
 IAnimal
• Eat
• Sleep
• Walk
 IDog : IAnimal
• Bark
• Fetch
• ChaseCat
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 IGuard
• SoundAlarm
• AttackBadGuy
Sample Object
IAnimal
IDog
IGuard
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Guard Dog Object
IUnknown Interface
Every COM interface inherits from IUnknown
IUnknown includes functionality needed by
every object
• Provides a mechanism for clients to obtain one
interface from another interface
• Provides object lifetime manangement
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IUnknown Interface
 Mechanism to get from one interface on an object to
another interface on the object
HRESULT QueryInterface (REFIID riid, void **ppvObject);
 Mechanism to manage object lifetimes
• Clients do not delete COM objects directly
• COM objects are responsible for deleting themselves
• COM objects are reference counted
ULONG AddRef ( );
ULONG Release ( );
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Registration of Servers
Servers are registered with the system
HKEY_CLASSES_ROOT
CLSID
{E312522E-A7B7-11D1-A52E-0000F8751BA7}
Default = “Guard Dog Object”
InprocServer32 = "D:\Examples\AnimalLib\AnimalLib.dll"
AnimalLib.GuardDog
CLSID
Default = “{E312522E-A7B7-11D1-A52E0000F8751BA7}”
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Event Interfaces
Interface provides communication from the
server back to the client
Server defines an interface that it calls to
notify the client that an event occurred
Client supplies server with a pointer to the
interface
Client’s code is executed when the server
calls methods in the event interface
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Example Event Interface
IDogEvents
• AmHungry
• WantAttention
• NeedToGoOutside
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IDispatch Interface
Automation servers must handle IDispatch
ActiveX controls must handle IDispatch
IDispatch gives scripting clients access to
COM objects
• Microsoft Office
• Internet Explorer
• Old versions of Visual Basic
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IDispatch Interface
 IDispatch::Invoke provides access to an entire
interface through a single function
 Clients use the Invoke function to call all methods
• Specify method by method ID
• Pass parameters in a single large structure
 Invoke understands only Automation types
• Yes: int, double, enum, boolean, structures
• Yes: BSTR, SafeArray, Variant
• No: unions, C style arrays, C style strings
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COM/ActiveX Programming
Many environments provide some tools to
make writing COM servers and clients easier
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Wrappers for accessing COM servers
Wizards to create skeleton servers or controls
Tools to add interfaces, methods, properties
Free implementations of some standard interfaces
Measurement Studio
• Visual Basic, Visual C++, LabWindows/CVI
LabVIEW™
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ActiveX and NI Products
 ActiveX controls
• Measurement Studio
 Containers
• LabVIEW, BridgeVIEW™
 Automation servers
• LabVIEW, Measurement Studio, HiQ™, DataSocket™
 Clients
• LabVIEW, BridgeVIEW, Measurement Studio, TestStand™
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COM/ActiveX Resources
Essential COM
• Don Box (Addison-Wesley, 1998)
Inside COM
• Dale Rogerson (Microsoft Press, 1997)
Various MSDN Articles
NI Product Examples
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