The vtable that represents idispatch is shown in figure 37. The only difference between using iunknown and idispatch is that the type librray is required to use an iunknown derived interface you cannot use iunknown for scripting languages. The two that you need to know about are idispatch interfaces and idl. This new unit needs to define a class that will act as the event sink. Extending iunknown, it is one of the standard interfaces that can be exposed by com objects. You should be able to queryinterface for iunknown and compare the pointers. From msdn, dispinterface is another interface inherits from idispatch. Which excerpt is an example of pathos from the damnation of a canyon. Idispatch interface and accessibility win32 apps microsoft docs. When the ccomptr is destructed, it will release the pointer, undoing the addref that was performed by the assignment operator. The reflexive property refers to the ability for the queryinterface call on a given interface with the interfaces id to return the same instance of the interface. The separation between declaration and implementation is discussed. The central idispatch method is idispatch invoke, which a client calls to invoke a particular method in the component.
Notice that in the idl fragment, the syntax seems more standard. The idispatch interface was initially designed to support automation. There is a requirement, however, that there is type info available for the interfaces. Idispatchimpl provides a default implementation of the idispatch interface portion of any dual. Is there an equivalent to the following vb code to get the idispatch interface from an object in vbscript. You use different idl attributes depending on whether you are implementing a pure idispatch interface or a dual interface. What is the difference between idispatch and iunkown in com. All com interfaces are inherited from iunknown interface.
Cdispatchptr is a subclass of idispatchptr, with the getsetsetrefinvoke methods added. Iunknown is the base interface for every com class. The idispatch interface has methods that allow clients to ask the object what methods it supports. If it does, however, the basic enumerating involves calling itypeinfogettypeattr to get the typeattr for the interface and looking at the number of implemented methods cfuncs and variables cvars and looping over these and calling itypeinfogetfuncdesc or itypeinfogetvardesc. I think vb 5 supports iuknown but vb 4 supports only idispatch. What type of intermolecular forces are expected between pooh3 molecules3. How do i compare 2 objects which reference com objects. It is used in multiple inheritance where the various interfaces can share one implementation of iunknown. While its a very simple interface, compared to some of the others well get to later, it does highlight a number of interesting and important behaviors that the. Dynamic creation of idispatch interfaces using simple. Com makes the differences transparent to the client.
Idispatch provides some methods that allow a scripting client to dynamically discover its methods and properties at runtime instead of compile time. Idispatch is the pascal definition of the windows dispatch interface definition. A dual interface is simply idispatch with functions pasted onto the. The only difference between spanish castilian and spanish modern is the sort. Surely if all interfaces implement iunknown then there is no difference a. To implement multiinterface dispatch on one or more pure iunknownderived interfaces, use puredispatchimpl. Only com interop supports marshaling for object types.
The type library already shows a difference, there is no dual interface implemented. But if you want to do that you can use queryinterface to get idispatch pointer but that is not converting you will be getting a new pointer. Iunknown interface provides clients like directx control of to get pointers to other interfaces. Tinterfacedobject implements the three iunknown methods already so that is a suitable class to inherit from, however we must reimplement the queryinterface method to indicate runtime support for the events interface as well. Some of the basic technics used in the com samples for assembly masm syntax are. Iunknown implementations are generally the same for every com class you encounter the main difference between them is the interfaces exposed by each class. Idispatch itself inherits from iunknown and that is the root interface of com. An interface based solely on idispatch is called a dispinterface.
The idispatch interface exposes a number of methods for automation clients that require latebinding such as scripting languages. The object data types is the core connection between vba and com. It doesnt need to be an idispatch or even iunknown object. The following table lists the different threading models you can specify. Difference between dual interface and dispatch only. Jun 28, 2010 the two that you need to know about are idispatch interfaces and idl. Though there are few such objects, we anticipated that there might be one someday. As in the case of iunknown, visual basic programmers never deal with this interface directly. The implementation adds a single idispatch implementation to the class, and uses typeinfo to dispatch against the pure interfaces. You cant change the inheritance list on a published interface anymore that you can change the list of methods or their arguments. Com calbacks idl idispatch vs dispinterface too old to reply bigfish 20050831 04. It is based directly on idispatch and will handle any incoming property query or interface call. Object can be exposed to unmanaged code as one of the following types.
The central idispatch method is idispatchinvoke, which a client calls to invoke a particular method in the component. Unfortunately, an idispatch implementation is not obligated to provide type info about the methods and properties it implements. Iobjectwithsite this com interface allows a parentchild pair of objects to connect to each other to implement a browser helper object bho iinspectable the comderived windows runtime winrt uses this iunknown derived interface as its. So what is the practical difference then between things like. Of course, you can provide access through both idispatch and through a custom interface, but that involves bookkeeping. Well the best way is to get your idispatch interface directly with its clsid. If they both use the same mechanism then it becomes impossible to build an idispatchex object which is both a collection and enumerates its properties. And when we declare dispinterface someinterce, then it means. Observe that assigning a t to a ccomptr addrefs the incoming pointer and releases the old pointer if any. Only some classes support idispatch which derives from iunknown. The decision made sense at the time, if you create multiple objects of the same type, why would you want to create copies of an already existing. Lets start with a quick recapitulation of object oriented programming.
Contribute to go olego ole development by creating an account on github. A unique number, called a dispatch identifier dispid, identifies each method. In computer programming, the iunknown interface is the fundamental interface in the. Doing so would cause the vtable slots to shift down to make space for the idispatch methods. It provides the basic ability to cast a instance of a com object to any of its supported interfaces. Did you change your main interface to now inherit from idispatch instead of inheriting from iunknown. This only makes sense if the passedin value is of type idispatch or iunknown.
If the renewal price changes, we will notify you in advance so you always know whats going on. Implement the idispatch interface in comenabled classes that have to act as automation controllers. Jan 29, 2019 while this shouldnt make any difference, it is not surprising and uncommon for servers to be buggy. To call a method in a simple iunknown interface all you need is to know where in the vtable the function pointer is.
So i think it unlikely that the iideaclientopendatabase method returns anything other than idispatch. Gettypeinfocount can be 1 or 0 and represents the number of type information interfaces that the object provides. Vba can define a parameter as being of type iunknown by using a. Idispatch doesnt offer any functionality of its own. Idispatch, the main interface in automation is described. To better understand how the idispatch interface works, it is helpful to think of it as a kind of surrogate interface. In addition to the methods inherited from iunknown, server developers.
Find answers to iunknown to idispatch from the expert community at experts exchange. Idispatch this interface provides namebased dynamic method dispatch for ole automation com objects. I need to search for a method with a specific name and, once i have the dispid, invoke the method i know the parameters the method takes. Extending iunknown, it is one of the standard interfaces that can be exposed by com. Limitedtime offer applies to the first charge of a new subscription only. The only difference between using iunknown and idispatch is that the. The vtable of a dispinterface is identical to that of idispatch itself. Now instead of deriving from iunknown, the isum interface derives from idispatch, which is itself derived from iunknown, so we still get the queryinterface, addref, and release methods added to the vtable. However, this process is less efficient than using iunknown, as the client has to make extra method calls before calling the desired com objects method. Explain some of commonly used methods provided by iunknown.
Idispatch derives from iunknown and extends its set of three methods addref, release and queryinterface with an additional four methods gettypeinfocount, gettypeinfo, getidsofnames and invoke. Vba can define a parameter as being of type iunknown by using a reference to the external library stdole2. Mar 20, 2007 the i in idispatch refers to interface. Jan 24, 2009 to implement multiinterface dispatch on one or more pure iunknown derived interfaces, use puredispatchimpl. On the client side the difference between an interface and an object is only small. Arcobjects is based on microsofts component object model com. Two wizards, in particular, ease the process of creating simple com objects. Idispatch is the interface that exposes the ole automation protocol. The iunknown interface id is defined as a guid with the value of 0000000000000000c000000000000046 a com components interfaces are required to exhibit the reflexive, symmetric, and transitive properties. Idispatchex, an extension of the idispatch interface, supports features appropriate for dynamic languages such as scripting languages. However, the efficiency difference between using a unicode string and an ansi string is very small, so dont be afraid to use ansi strings if you want to.
Idispatch builds on iunknown adding the facility to discover at runtime the attributes and interfaces supported by the specified com object. This chapters deals with the similarities and differences between objects and interfaces. Iunknown and retval rewriting today, were going to finish out the definition of iuknown that we started last time, by adding the three methods to the interface. As for going to iunknown first and then to icomdatabase, it doesnt work as shown here. In my implementation, i decided to make the cdynamicmethoddata a pointer instead of a an actual object.
Hi, whats the difference between an interface declared with. It is one of the standard interfaces that can be exposed by com objects. Oct 07, 2004 1 because theres a difference between enumerating the elements of a collection and the properties of an object. It is expected that readers are familiar with idispatch and have access to the idispatch documentation. But just as you no longer need to understand assembly language to get software working these days, pretty soon, youll no longer need to understand all the nuances of iunknown and com to. This is the only real difference between a dispinterface and a dual interface. Automation clients use the two methods getidsofnames and invoke to achieve late binding. Unless otherwise stated, if a savings amount is shown, it describes the difference between the first term and renewal term subscription prices e.
An idispatch interface is a special type of iunknown interface that includes methods that allow methods to be called by name at runtime. What is the difference between dispatch and despatch answers. The difference between assignment and attachment with atl. Ideally i would like to do this using smart com pointers ccomptr. Every object must have an implementation of iunknown to control its own lifetime. Using atl vs2008 how can i enumerate the available methods available on a given idispatch interface idispatch. Idispatchimplex template classes for multiinterface. The default behavior is to marshal objects to com variants. It provides the basic ability to cast a instance of a com object to any of its supported interfaces queryinterface. Dispinterface is an idispatch plus an agreement on what each dispid means and what parameters it should come with. Activex or com objects can implement the idispatch interface for access by. Programmers are not allowed to use an iunknown data type from vba this is a hidden data type. The documentation goes into detail here, but the gist is that object by default marshals as variant, which is almost always not what we want. This section describes the idispatchex interface itself, the differences between idispatch and idispatchex, and the rationale for the extensions.