DCOM Interface

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DCOM Interfaces

IUnknown

IDispatchIClassFactory


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IUnknown

Enables clients to get pointers to other interfaces

Manage the existence of the object through the

AddRefand Release methods.

All other COM interfaces are inherited, directly

or indirectly, from IUnknown.

Therefore, the three methods in IUnknown are

the first entries in the VTable for every interface.

implement IUnknown as part of every interface.
http://msdn.microsoft.com/en-us/library/windows/desktop/ms691379(v=vs.85).aspxhttp://msdn.microsoft.com/en-us/library/windows/desktop/ms682317(v=vs.85).aspxhttp://msdn.microsoft.com/en-us/library/windows/desktop/ms682317(v=vs.85).aspxhttp://msdn.microsoft.com/en-us/library/windows/desktop/ms691379(v=vs.85).aspx


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HRESULT QueryInterface(

[in] REFIID riid,

[out] void **ppvObject

);

Retrieves pointers to the supported interfaces on an object. This method calls IUnknown::AddRefon the pointer it

returns.

Return value

returns S_OK ->interface is supported

E_NOINTERFACE otherwise.

IfppvObjecti= NULL,returns E_POINTER.
http://msdn.microsoft.com/en-us/library/windows/desktop/ms691379(v=vs.85).aspxhttp://msdn.microsoft.com/en-us/library/windows/desktop/ms691379(v=vs.85).aspxhttp://msdn.microsoft.com/en-us/library/windows/desktop/ms691379(v=vs.85).aspxhttp://msdn.microsoft.com/en-us/library/windows/desktop/ms691379(v=vs.85).aspx


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Set of interfaces accessible on an object throughQueryInterface must be static, not dynamic.

if a call to QueryInterface for a pointer to a

specified interface succeeds the first time, it mustsucceed again, and if it fails the first time, it mustfail on all subsequent queries.

Reflexive

Symmetric

Transitive next


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must be reflexive if a client holds a pointer

to an interface on an object, and queries for

that interface, the call must succeed.

back


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must be symmetric if a client holding a

pointer to one interface queries successfully

for another, a query through the obtained

pointer for the first interface must succeed.

back


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must be transitive if a client holding a

pointer to one interface queries successfully

for a second, and through that pointer queries

successfully for a third interface, a query for

the first interface through the pointer for the

third interface must succeed.

back


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Binding is the term given to the process of matchingthe location of a function given a pointer to an object.

COM supports three types of binding:

Late. This is where type discovery is left until runtime.

Method calls made by the client but not implementedby the object will fail at execution time.

ID. Method IDs are stored at compile time, butexecution of the method is still performed through a

higher-level function. Custom vTable (early). Binding is performed at compile

time. The client can then make method calls directlyinto the object.


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IDispatch

The IDispatch interface supports late- and ID-binding languages. Late binding requires a call to the object to retrieve the list of method IDs;

The client must then construct the call to the Invoke method and call it.

The Invoke method must then unpack the method parameters and call thefunction.

All these steps add significant overhead to the time it takes to execute a method.


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ID binding offers a slight improvement over

late binding in that the method IDs are cached

at compile time, which means the initial call to

retrieve the IDs is not required.

However, there is still significant call overhead

because the IDispatch::Invoke method is still

called to execute the required method on theobject.


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Early binding, often referred to as custom vTablebinding, does not use the IDispatch interface.

Instead, a type library provides the required

information at compile time to allow the client toknow the layout of the server object.

At runtime, the client makes method calls directlyinto the object.

This is the fastest method of calling objectmethods and also has the benefit of compile-timetype checking.


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dual

Objects that support both IDispatch and custom vTable are referred to as dual

interface objects


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IClassFactory

When to implement

For every class that you register in the systemregistry and to which you assign a CLSID, so

objects of that class can be created. IClassFactory::CreateInstance : creates an

instance of component using CLSID

IClassFactory::LockServer : increment the refcount ,server stays in memory and doesnotshutdown before client finishes its work.


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IClassInterface


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Before a client creates an instance of a COM object,

1. It creates an instance of the object's class factory,(using CoGetClassObject()).

2. CoGetClassObject() calls the DllGetClassObject() in

the DLL.3. It creates the classFactory instance using CLSID and

IID and returns interface pointer to client

4. Using ClassFactory Instance the client then calls the

class factory's ClassFactory::CreateInstance method.5. ClassFactory::CreateInstance() creates a new instance

of the component and return the interface pointer tothe client

DCOM Interface

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