The difference between set and setref is subtle and annoying. It provides the basic ability to cast a instance of a com object to any of its supported interfaces. When you implement one or more userdefined interfaces in a class, visual basic provides an implementation of queryinterface that allows clients to move among interface references. Here is the idispatch interface defined in interface definition language idl notation. A lot of you probably heard the name idispatch before. Notice that in the idl fragment, the syntax seems more standard. Which brings us to the last, and most interesting, part of this exercise. Idispatch itself inherits from iunknown and that is the root interface of com. But it does extend the reach to other platforms compared to activex components.
I suspect i am missing something, but i dont know from where to pull the idispatch definition in. Iunknown definition at archived 20711 at the wayback machine. This type of implementation creates what is known as a singleton object since there. In computer programming, the iunknown interface is the fundamental interface in the. In delphi code, iunknown is simply another name for iinterface. Idispatch this interface provides namebased dynamic method dispatch for ole automation com objects. A unique number, called a dispatch identifier dispid, identifies each method. If the renewal price changes, we will notify you in advance so you always know whats going on. Descending from iunknown instead of iinterface informs the delphi compiler that the interface must be compatible with com objects.
Set x y, and set is used for regular value assignment, e. Explain some of commonly used methods provided by iunknown. Object can be exposed to unmanaged code as one of the following types. And you may even know that its required by comobjects used in autoit with the objcreate function. The separation between declaration and implementation is discussed. But if you want to do that you can use queryinterface to get idispatch pointer but that. Gettypeinfocount can be 1 or 0 and represents the number of type information interfaces that the object provides gettypeinfo returns the type information for the object. Net object type can be obtained from progid or clsid. The iunknown interface lets clients get pointers to other interfaces on a given object through the queryinterface method, and manage the existence of the object through the iunknownaddref and iunknownrelease methods. Createcomobject returns an object of the iunknown type, whereas createoleobject returns an object of the idispatch type.
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. Working with activex, com, and unmanaged code sciencedirect. A smart pointer is an object that behaves like a pointer. Vba can define a parameter as being of type iunknown by using a reference to the external library stdole2. Idispatch doesnt offer any functionality of its own. This is the biggest difference between the three translations we derived for queryinterface what type of value they returned. The only essential difference between figure 12b and c is the length of the connecting fiber. There are two com interfaces above the autocad object model that are essential to its operation. Unless otherwise stated, if a savings amount is shown, it describes the difference between the first term and renewal term subscription prices e. Best collection of com, dcom, activex tutorials, topics. What is the difference between idispatch and iunkown in com. Com calbacks idl idispatch vs dispinterface too old to reply bigfish 20050831 04. A visual basic object automatically implements queryinterface.
If you know visual basic, then the easiest way to explain it is that setref is used for vbs set keyword, e. So, today my mind has been 100 % focused on something that on the surface looks really boring. To call a method in a simple iunknown interface all you need is to know where in the vtable the function pointer is. The iunknown interface lets clients get pointers to other interfaces on a given object through the queryinterface method, and manage the existence of the object through the iunknown addref and iunknown release methods. That is, you can use an instance of a smart pointer class in many of the places you normally use a pointer. Iunknown implementations are generally the same for every com class you encounter the main difference between them is the interfaces exposed by each class. Net framework sdk and can be found under the bin subfolder of your sdk.
It is used to enable interprocess communication object creation in a large range of programming languages. Net also provides the ability to make use of late binding through the idispatch interface of a com object see listing 5. The object data types is the core connection between vba and com. The idispatch interface has methods that allow clients to ask the object what. The vtable that represents idispatch is shown in figure 37. Sep 16, 20 these include comptr a smart pointer type that represents the interface t, runtimeclass represents an instantiated class that inherits one or more interfaces, implements iunknown s methods and helps manage the overall reference count of the module and module a collection of objects that i will use in the further paragraphs for creating. 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. The idispatch interface exposes a number of methods for automation clients that require latebinding such as scripting languages.
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. Idispatchgettypeinfo, retrieves the type information for an object, which can. Therefore, it will not be very efficient on other platforms. This chapters deals with the similarities and differences between objects and interfaces.
It provides the basic ability to cast a instance of a com object to any of its supported interfaces queryinterface. However, using a smart pointer provides some advantages over using a raw pointer. Authoring and consuming classic com components with wrl. Another unit is also created in the project which contains pascal representations of the entities defined in the type library. From msdn, dispinterface is another interface inherits from idispatch. Automation relies on an interface named idispatch, which allows clients to create method bindings at runtime in a process known as late binding. Net framework data provider for ado and ole db system. Idispatch builds on iunknown adding the facility to discover at runtime the attributes and interfaces supported by the specified com object. It is usually not necessary to call iunknowns methods directly in your. Setref sets the value of a property to be a reference to the passedin value. Idispatch, the main interface in automation is described. Another unit is also created in the project which contains pascal representations of the entities defined in the type. The only difference between using iunknown and idispatch is that the type librray is required to use an iunknown derived interface. Inheritance is a very important concept in oo programming.
The typed accessor methods for the oledbdatareader are also listed. Obtain type information of idispatchbased com objects. To better understand how the idispatch interface works, it is helpful to think of it as a kind of surrogate interface. Extending iunknown, it is one of the standard interfaces that can be exposed by com objects.
In addition to the methods inherited from iunknown, server. And when we declare dispinterface someinterce, then it means someinterface. Iunknown methods can be used to switch between interfaces on an object, add references, and release objects. There is another difference between these two creation functions. Use iunknown methods to switch between interfaces on an object, add references, and release objects. 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. Base interfaces iunknown and idispatch are stripped type definitions are not imported. Its an ambassador of goodwill between the managed world and the unmanaged iunknown world. Lets start with a quick recapitulation of object oriented programming. You use different idl attributes depending on whether you are implementing a pure idispatch interface or a dual interface. Iunknown is the base interface for every com class. The central idispatch method is idispatchinvoke, which a client calls to invoke a particular method in the component. Scripting clients, such as vbscript or jscript, are unable to make use of raw interfaces and require late bound idispatch.
The vtable of a dispinterface is identical to that of idispatch itself. Programmers are not allowed to use an iunknown data type from vba this is a hidden data type. The type or namespace name idispatch could not be found are you missing a using directive or an assembly reference. I think vb 5 supports iuknown but vb 4 supports only idispatch. Idispatch interface and accessibility win32 apps microsoft docs. An iunknown or iunknown derived interface consists of a pointer to a virtual method table that contains a list of pointers to the functions that implement the functions declared in the interface, in the order that they are declared in the interface. Since the information needed for implementing idispatchinvoke is in the type library, its sensible to use the type library to implement idispatchinvoke. 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. These notes apply specifically to the case where the unknown type is iunknown. First we ask for the actual hresult value, as an int. I read some msdn document and some other forum discussions about the differences between idispatch and dispinterface, but still confused. If you are using nested classes to implement multiple interfaces, you must implement iunknown once for each interface you implement.
The interop runtime code will insert the appropriate method calls for reference counting when the managed proxy objects are created and freed, and will translate typecast, is, and as operators into calls to queryinterface. The purpose of an idispatch interface is to provide an interface for use with scripting clients. Jun 28, 2010 an idispatch interface is a special type of iunknown interface that includes methods that allow methods to be called by name at runtime. For objects that support idispatch, the type information count is always one. The two that you need to know about are idispatch interfaces and idl. An interface based solely on idispatch is called a dispinterface. Comnet object, stored in a 32bit address for the com interface. Obtain type information of idispatchbased com objects from. The central idispatch method is idispatch invoke, which a client calls to invoke a particular method in the component. The idispatch interface inherits from the iunknown interface. Idispatch gettypeinfo, retrieves the type information for an object, which can. Well the best way is to get your idispatch interface directly with its clsid. Every com class implements an interface named iunknown.
985 652 1120 1458 348 923 1568 162 1432 1416 1308 746 475 481 10 1119 937 1260 1076 1591 122 906 390 1068 774 282 573 966 878 974 411 1410 600 811 155 1103 948 840 1286 858 742 321 760 901