ni.com What is COM?

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) ni.com

Outline What is ActiveX? Why use COM/ActiveX What is COM? What are ActiveX/COM • • • • ni.com

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: • • • •

Document objects ActiveX controls Automation OLE for process control (OPC)

Grew out of OLE ni.com

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 ni.com

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 ni.com

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 ( ); ni.com

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}” ni.com

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 ni.com

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 • • • •

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™ ni.com

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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