how to use a com object

obtain object's interface

Call QueryInterface to Request a New Interface:

Interface Definition Language (IDL)

Interface definition language (IDL) files are a structured way to define interfaces and objects.

ex:

[

object,

uuid(),

helpstring("IWDFObject Interface"),

local,

restricted,

pointer_default(unique)

]

interface IWDFObject : IUnknown

{

HRESULT

DeleteWdfObject(

void

);

HRESULT

AssignContext(

[in, unique, annotation("__in_opt")] IObjectCleanup * pCleanupCallback,

[in, unique, annotation("__in_opt")] void * pContext

);

HRESULT

RetrieveContext(

[out, annotation("__out")] void ** ppvContext

);

void

AcquireLock(

void

);

void

ReleaseLock(

void

);

};

Active Template Library (ATL)

a set of template-based C++ classes that are often used to simplify the creation of COM objects

HRESULT & com

a 32-bit type

three field:

(1) severity bit:

indicate success or error

(2) facility

(3) return code

success codes are assigned names that begin with S, such as S_OK

failure codes are assigned names that begin with E

use FAILED(hr) or SUCCEEDED(hr) to check success or failure

GUID & com

Globally unique identifiers (GUIDs)

1.Interface ID (IID)

a GUID that uniquely identifies a particular COM interface

2.Class ID (CLSID)

a GUID that uniquely identifies a particular COM object

com & interface

An interface is the COM mechanism for exposing public methods on an object

If an object exposes a standard interface, the object must implement every method in that interface.

A typical COM object exposes at least two and sometimes many interfaces.

IUnknown

IUnknown is the key COM interface. Every COM object must expose this interface