dual interface
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Hi George, Read this link about implementing Dual interfaces. I think things shoudl be clear after this. Dual Interface[^]
George_George wrote:
My confusion is dual means (IDispatch/IUnknown) or (IDispatch/customized interface)?
I think dual interface does not mean two interfaces its about the ways you can access methods. I hope it helps..
Regards, Sandip.
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Dual interface refers to supporting both IDispatch and a VTBL, which means you can call it through COM using QueryInterface or from C++ directly. Been so long since I've done it, I'm a bit fuzzy on the details.
- S 50 cups of coffee and you know it's on! A post a day, keeps the white coats away!
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I agree, but my implementation above is wrong and not dual interface, CPallini? regards, George
;) ;) Actually, "dual interface" refers to a COM class's ability to have its methods bound at compile time OR at run-time. Generally, all COM classes implement "custom" interfaces - after all, they do things that are specific to defined set of requirements, and are thus "custom". The COM class's methods are bound at compile time into a virtual function table, or VTABLE. For a program (client) to invoke the COM class's methods, it must have "knowledge" of the methods exported by the COM component at the time the client program itself is compiled into executable form. For applications where the COM object (server) and the client program are designed and built together, the client can easily have such "knowledge". I often develop COM servers and clients simultaneously, and my client programs have "intimate" knowledge of the names of the methods exported by the COM server. But what about client programs that want to use a COM server's methods at RUN TIME, but do not necessarily know the names and other properties of the methods exported by the COM server? This situation arises very often for scripting languages where the executable code is built "on the fly". The process whereby a client program "discovers" and uses the methods exported by a COM server is called "Run-time" binding, also known as "late" binding. This process allows scripting languages to identify what interfaces (methods) a COM class supports at run time, long AFTER the COM class has been compiled into executable code. This is done through QueryInterface and the IDispatch method. Thus, a COM class must support the IDispatch interface if it wants to allow client programs to bind to its methods at run-time. A COM class that supports IDispatch is thus said to be "dual interface" - a client program with "intimate" knowledge of its method's names and parameters can bind to it a compile time, OR the client can bind to its methods at run time via QueryInterface and IDispatch. Incidentally, "IDispatch" is aptly named because it is a method that "dispatches" a function call to the proper method within the COM server.
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Well, it depends on how do you implement the
IDispatchinterface (for instance, if your customized interface inherits both fromIUnknownandIDispatchthe you've a undesirable diamond inehritance path). :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Thanks CPallini, I agree the solution you mentioned -- making my component implement a customized interface, and making the IDispatch interface should be the optimum solution. :-) regards, George
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Yes, it means (1) get the
IUnknownpointer (2) get theICustomizedpointer viaIUnknown->QueryInterface(3) callICustomized->WhateverMethod()(eventually perform cleanup...) on the other hand, access viaIDispatchis quite different. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Thanks CPallini, Good answered. regards, George
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Yes, it means (1) get the
IUnknownpointer (2) get theICustomizedpointer viaIUnknown->QueryInterface(3) callICustomized->WhateverMethod()(eventually perform cleanup...) on the other hand, access viaIDispatchis quite different. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Hi CPallini, Just through of another question, to implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy, every COM component should implement it and be a dual interface. Why implementing dual interface is not mandatory -- i.e. for some other reasons, developer will not implement dual interface? regards, George
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;) ;) Actually, "dual interface" refers to a COM class's ability to have its methods bound at compile time OR at run-time. Generally, all COM classes implement "custom" interfaces - after all, they do things that are specific to defined set of requirements, and are thus "custom". The COM class's methods are bound at compile time into a virtual function table, or VTABLE. For a program (client) to invoke the COM class's methods, it must have "knowledge" of the methods exported by the COM component at the time the client program itself is compiled into executable form. For applications where the COM object (server) and the client program are designed and built together, the client can easily have such "knowledge". I often develop COM servers and clients simultaneously, and my client programs have "intimate" knowledge of the names of the methods exported by the COM server. But what about client programs that want to use a COM server's methods at RUN TIME, but do not necessarily know the names and other properties of the methods exported by the COM server? This situation arises very often for scripting languages where the executable code is built "on the fly". The process whereby a client program "discovers" and uses the methods exported by a COM server is called "Run-time" binding, also known as "late" binding. This process allows scripting languages to identify what interfaces (methods) a COM class supports at run time, long AFTER the COM class has been compiled into executable code. This is done through QueryInterface and the IDispatch method. Thus, a COM class must support the IDispatch interface if it wants to allow client programs to bind to its methods at run-time. A COM class that supports IDispatch is thus said to be "dual interface" - a client program with "intimate" knowledge of its method's names and parameters can bind to it a compile time, OR the client can bind to its methods at run time via QueryInterface and IDispatch. Incidentally, "IDispatch" is aptly named because it is a method that "dispatches" a function call to the proper method within the COM server.
Thanks Scott, I like your long and comprehensive post. Two more comments, 1. To implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy, every COM component should implement it and be a dual interface. Why implementing dual interface is not mandatory -- i.e. for some other reasons, developer will not implement dual interface? 2. For the compile time binding as you mentioned -- just clarify one point, I think COM consumer should not create and component and call its implementation method directly (call the component other than call the interface) -- but should use QueryInterface to get the interface which the COM component implements, then call the methods (using vtable) through the interface. Correct? regards, George
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Steve Echols wrote:
Been so long since I've done it, I'm a bit fuzzy on the details.
Yep, that's it, and that pretty much is the detail. ;)
led mike
Thanks led mike, As you are here. Let me just rate your reply and ask you a question. :-) To implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy, every COM component should implement it and be a dual interface. Why implementing dual interface is not mandatory -- i.e. for some other reasons, developer will not implement dual interface? regards, George
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Hi CPallini, Just through of another question, to implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy, every COM component should implement it and be a dual interface. Why implementing dual interface is not mandatory -- i.e. for some other reasons, developer will not implement dual interface? regards, George
George_George wrote:
Just through of another question, to implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy
Most of COM components implement dual interface.
George_George wrote:
So, I think since it is easy, every COM component should implement it and be a dual interface.
(1) Is not that easy if you're doing it hand-crafting (without the help of a wizard or a framework such
MFCorATL) (2) KeepingCOMrequirements minimal is (IMHO) a good design approach.George_George wrote:
i.e. for some other reasons, developer will not implement dual interface?
Because, for instance, all the clients (of the
COMserver they're building) are written inVTABLEbinding language (likeC++orVB6clients). AS you knowVTABLEbinding is more efficient thanIDispatchmechanism. Final note: if you don't need a feature, why doing efforts to implement it? :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
Hi CPallini, Just through of another question, to implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy, every COM component should implement it and be a dual interface. Why implementing dual interface is not mandatory -- i.e. for some other reasons, developer will not implement dual interface? regards, George
George_George wrote:
Why implementing dual interface is not mandatory
Keeping
COMrequirements minimal is a good design approach IMHO (and building a dual interface is not that easy, without wizards or framework, likeMFCorATL, support).George_George wrote:
i.e. for some other reasons, developer will not implement dual interface?
Because none of the intended clients need it (for instance
C++orVB6clients don't need IDispatch). As you knowIDispatchmechanism is less efficient thanVTABLEbinding. Anyway most ofCOMservers actually implement dual interface (by free choiche). :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
Thanks Scott, I like your long and comprehensive post. Two more comments, 1. To implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy, every COM component should implement it and be a dual interface. Why implementing dual interface is not mandatory -- i.e. for some other reasons, developer will not implement dual interface? 2. For the compile time binding as you mentioned -- just clarify one point, I think COM consumer should not create and component and call its implementation method directly (call the component other than call the interface) -- but should use QueryInterface to get the interface which the COM component implements, then call the methods (using vtable) through the interface. Correct? regards, George
On point 1: I rarely implement dual interfaces because I develop COM servers and COM clients as part of a package application, and my client components always have "up front" knowledge of what the server can do. So, I typically do not implement the IDispatch interface. This results in a little less complexity in my application and improved performance. My belief is that you should only implement IDispatch if a) you are developing a COM component that you are going to make publicly available to other developers so that they might use it in their applications, or b) your COM component needs to be used by a scripting language, for example VBScript within a web page, in which case you must implement IDispatch so that the VBScript can bind to your COM component at run time (late binding). The application that I support is all written in C++ (no scripting languages) and contains no components that are intended to be available for use by other developers outside my company. On point 2: You are correct, the client should always call "QueryInterface" to get a pointer to the desired interface exported by a server, then call the methods of that interface via the pointer. Since the methods of the interface are all defined as virtual functions in C++, you are implicitly invoking the methods through the virtual function table. (Of course, virtual functions are intrinsic to the C++ language and existed long before COM was created.) :)
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George_George wrote:
Just through of another question, to implement dual interface is easy, i.e. making the component implement a customized interface, and making the customized interface inherits IDispatch. So, I think since it is easy
Most of COM components implement dual interface.
George_George wrote:
So, I think since it is easy, every COM component should implement it and be a dual interface.
(1) Is not that easy if you're doing it hand-crafting (without the help of a wizard or a framework such
MFCorATL) (2) KeepingCOMrequirements minimal is (IMHO) a good design approach.George_George wrote:
i.e. for some other reasons, developer will not implement dual interface?
Because, for instance, all the clients (of the
COMserver they're building) are written inVTABLEbinding language (likeC++orVB6clients). AS you knowVTABLEbinding is more efficient thanIDispatchmechanism. Final note: if you don't need a feature, why doing efforts to implement it? :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Cool, CPallini! have a good weekend, George
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George_George wrote:
Why implementing dual interface is not mandatory
Keeping
COMrequirements minimal is a good design approach IMHO (and building a dual interface is not that easy, without wizards or framework, likeMFCorATL, support).George_George wrote:
i.e. for some other reasons, developer will not implement dual interface?
Because none of the intended clients need it (for instance
C++orVB6clients don't need IDispatch). As you knowIDispatchmechanism is less efficient thanVTABLEbinding. Anyway most ofCOMservers actually implement dual interface (by free choiche). :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Hi CPallini, I further question. If we want to support IDispatch, all types in methods' parameter/return value must be auomtation compatible? If yes, why? :-) regards, George
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On point 1: I rarely implement dual interfaces because I develop COM servers and COM clients as part of a package application, and my client components always have "up front" knowledge of what the server can do. So, I typically do not implement the IDispatch interface. This results in a little less complexity in my application and improved performance. My belief is that you should only implement IDispatch if a) you are developing a COM component that you are going to make publicly available to other developers so that they might use it in their applications, or b) your COM component needs to be used by a scripting language, for example VBScript within a web page, in which case you must implement IDispatch so that the VBScript can bind to your COM component at run time (late binding). The application that I support is all written in C++ (no scripting languages) and contains no components that are intended to be available for use by other developers outside my company. On point 2: You are correct, the client should always call "QueryInterface" to get a pointer to the desired interface exported by a server, then call the methods of that interface via the pointer. Since the methods of the interface are all defined as virtual functions in C++, you are implicitly invoking the methods through the virtual function table. (Of course, virtual functions are intrinsic to the C++ language and existed long before COM was created.) :)
Thanks Scott, 1. Why "you are developing a COM component that you are going to make publicly available to other developers" as you mentioned, I must expose IDisaptch? I could expose vtable only and let them use C++. :-) Any comments or any important points which I missed in my above statements? 2. I further question. If we want to support IDispatch, all types in methods' parameter/return value must be auomtation compatible? If yes, why? :-) regards, George
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Hi CPallini, I further question. If we want to support IDispatch, all types in methods' parameter/return value must be auomtation compatible? If yes, why? :-) regards, George
Because automation clients have no clue of other types. :)
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
Because automation clients have no clue of other types. :)
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Thanks CPallini, But can't we include the complex type definition in IDL and build the IDL into typelib and let client use the typelib? regards, George
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Thanks CPallini, But can't we include the complex type definition in IDL and build the IDL into typelib and let client use the typelib? regards, George
AFAIK automation clients don't use
typelibs. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
AFAIK automation clients don't use
typelibs. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Sorry my bad, CPallini. I always write C++ native client actually. :-) If the automation client (I think you mean client like VB or JScript) does not use typelib, what will they use to invoke? Just read some document about interface/coclass/methods signature information and use GUID or ProgID, then call IDispatch.invoke? regards, George
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Sorry my bad, CPallini. I always write C++ native client actually. :-) If the automation client (I think you mean client like VB or JScript) does not use typelib, what will they use to invoke? Just read some document about interface/coclass/methods signature information and use GUID or ProgID, then call IDispatch.invoke? regards, George
Yes,
VBScriptclients, for instance, do something like this. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
Yes,
VBScriptclients, for instance, do something like this. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Thanks CPallini, Sorry we are talking about two things. :-) I mean VB -- Visual Basic. Any way, here are my questions, let us talk VB and script language (VB Script or JScript) separately. 1. Could VB client use TLB file? 2. Could script client use TLB file? 3. If I define customized type (e.g. a structure) in IDL file, use such customized type as input parameter or return type for some methods, and generate related TLB. Could VB client use such methods? 4. If I define customized type (e.g. a structure) in IDL file, use such customized type as input parameter or return type for some methods, and generate related TLB. Could script client use such methods? regards, George
