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Hello,
I am new to COM/ATL programming and have a question.
If I want to inherit one interface from another I have previously written (e.g. IB : IA ) do I need to implement all of IA's methods in IB? if so is there any way I can use some technique that allows you to just directly call the IA method instead of having to re-write (as in C++).
Thanks.
Paul.
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I have had a look at aggregation - could I override methods from my base interface using this?
e.g. where IB : IA
IA* pIA = null
pIB->QueryInterface(IID_IA, &pIA)
1) pIA->methodA()
2) pIB->methodA()
would 1) call the same thing as 2) -- I think it would if these were C++ classes (using cast instead of QI).
help!
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Sure.
1) and 2) will call same function implementation.
I don't quite catch your intention.
This is just simple class or struct inhertance.
This is nothing to do with COM as I understand.
What do you want to do?
Regards,
Ryan
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Hi,
Can anybody tell me how to find out if a COM DLL has implemented a specific interface. All I know is the name of the DLL and the CLSID of the interface. I gues Microsfot is doing something similar when they implements add-ins to both Developer Studio and office?
Martin Eskildsen
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Ok - I also thought about QueryInterface in the first place, both I can't figure out how to use it on a spefic DLL (only given the filename of a COM DLL to query the specific DLL wheter it supports the requested interface since).
A short example might make my question more clear:
I have an executable (MyProg.exe) and two COM DLL's (MyDllA.dll and MyDllB.dll). Both Dll's is allowed to support the requested interface (like add-ins in Developer Studio). In this sample both DLL's supports the interface called IMyInterface, with CLSID CLSID_MyInterface.
How can I use QueryInterface in combination with the information above to verify that both DLL's support the interface IMyInterface?
Martin Eskildsen
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You're confusing DLLs vs. COM servers, and CLSIDs vs. IIDs.
First, when creating a COM object, you reference the server by its CLSID, not its filename. CoCreateInstance() takes the CLSID.
You also can request an interface in CoCreateInstance(). So if you have the CLSID of one of the servers, and the IID they support (say IID_IMyInterface) you pass both of those to CoCreateInstance() and check the return value to see if it succeeded.
--Mike--
http://home.inreach.com/mdunn/
The Signature, back by popular demand: Buffy. Pajamas.
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In the event that the CLSID of the CoClass object is not known, you can:
- use the COM category manager (ICatInformation) and enumerate the objects' CLSIDs at runtime
- do category management under your own registry key(s), basically you store the list of COM object CLSIDs and you call CoCreateInstance() on those CLSID and get your interface using QueryInterface() after that
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Here we have a Custom OLEDB provider. We have problems
dealing with the null values. The OLEDB provider is in
VC. Here we get the records as an variant array and
then moving it to a stream object and then assigning
it to a BSTR pointer. We are reading that as an ADO
recordset from VB. All the values are ok in VB. But
when we come across null values, we are not able to
process it properly. When we write NULL to the stream
object (which is bytes), we are reading it as 0 in the
VB recordset.(since in vc NULL has a value 0). So i need to read that value as null in
the VB.
In VC i tried writing VT_NULL and VT_EMPTY. But i
could not read that as null in VB.
if i use the isnull(fieldname) , i need to get the
result as true if the value is null. I dont know how
the null value is treated internally.
We use this OLEDB provider not only from VB, we use
even from Crystal reports for reports.
So can anybody please let me know how do it or let me
know any source where i can get the help from.
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Hi,
I have Q concerning ADO/OLEDB.
I am writing an application which has to access MS-Access databases selected
at runtime by the user thru a file open dialog, the format of the data bases
is same but the databases may be located in different places.
1. Can I use ADO/OLEDB to open these DB's when a path is specified
( This capability like that of DAO, the path can be spec and
opened )
2. Or do i need the DSN and not specify the actual location
so that the DSN will ask the user to locate the DB.
(Actually here the application will use ODBC again when i want to
use ADO)
Since you are already working in OLEDB I hope you can help me in
this context.
Any suggestions or awnsers will be helpful.
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Do you set the DBSTATUS_S_ISNULL in the status field for the accessor in your custom OLEDB provider??? I guess that is why VB does not cosider it as NULL properly.
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Hi,
I'm new to COM and ATL and was wondering why do I need another set of windowing classes for COM development since i can easily embed MFC code (tried only for dialogs so far though, maybe i cannot do everything?)
Thanks
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Hello everyone.
This is my first ATL/COM project, and I think the progress is "behaving" so far (happy to be one week behind the deadline). However, there is one dead-end issue that I cannot resolve:
Within my ATL component, I have to use a couple of in-proc Visual Basic - based COM .dll's. I use #import "XXXX.dll" in my source code, but compiler spits out the error "fatal error C1084: Cannot read type library file: 'XXXX.dll': Element not found."
Note that regsvr32 has successfully registered that .dll and .dll's path is correct.
Another spooky observation: OLE View fails to open .dll's IDL file giving me "Could not decompile selected item. Element not found" error message.
If anyone can help, please save me from this miseary.
BIG thanks in advance.
Kirill Sherman
shermank@glstrat.com
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it sounds like you have not compiled your typelibrary into your application. What you need to do is in your .rc file create a custom resource called "TYPELIB" and point it to your .tlb file. This will inclued the tlb in your final compiled DLL so that it can be imported. Your other alternative is to import the actual tlb file instead of the DLL. Hope this helps.
Joseph Dempsey
jdempsey@cox.rr.com
Joseph.Dempsey@thermobio.com
"Software Engineering is a race between the programmers, trying to make bigger and better fool-proof software, and the universe trying to make bigger fools. So far the Universe in winning."
--anonymous
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What about your IDL definition?
If you don't mind, show your idl definition.
MIDL compiler don't generate type library without proper keywords.
Regards,
Ryan
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Thanks for your concern, but I asked the author of those VB components to recompile his works.
Don't understand why, but after recompilation, I could view all the IDL's with OLE Viewer and #import them without any problems.
As long as it works - I am happy.
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I have a DLL component with some User Interface sitting on my server machine. My test program is on the client machine. I want my test program to activate the DLL component on the server machine. But I want the UI to be visible on the client machine.
Anyone knows how to do it?
Thanks a lot!
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Hi,
I use a COM object(lot of classes and functions) that does not support the multi thread call.
The program i do is multithread and then i have problem with the COM object.
i would like to serialize all the calls i have to do to this object in a single thread. From the creation of the object to its release.
Can someone help me please?
thanks!
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If you are using ATL, try to use CComCriticalSection object. Create the data member of that type in your class and initialize it in your object's constructor. Use CComCriticalSection::Lock() and Use CComCriticalSection::Unlock() methods in code blocks that require only-one-at-a-time execution (a.k.a. serialized access or thread-safe code). This object is well documented in MSDN library. The alternative to it is Win32 API:
CRITICAL_SECTION hMyCS;
InitializeCriticalSection(&hMyCS);
***
***
EnterCriticalSection(&hMyCS);
// your thread-safe code begings
***
***
***
// your thread-safe code ends
LeaveCriticalSection(&hMyCS);
As opposed to events, semaphores, and mutexes, critcal sections are the easiest and fastest synchronization objects.
Second, events, semaphores and mutexes are mostly used for inter-process syncrhonization and communication, but you still can use them at intra-process level.
When Critical Sections are used, all client threads that access your component will be automatically queued in First-Come-First-Served fashion. Windows and COM runtime will take care of that for you.
Hope you find it helpful.
Kirill Sherman
shermank@glstrat.com
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All right for that,
but even if the threads are queued in critical section, the critical sections are accessed by several threads, not at the same time sure!
I have a class:
Class CObj: public COleDispatchDriver{
public:
LPDISPATCH fct1(COleVariant vrIndex);
short fct2();
};
what i want is that a thread controls the access to fct1 and fct2 as a consequence, only this thread will use these functions!
even if other threads call them, i want to execute these functions always in the same thread!
In fact, it's a problem that i have with an ActiveX i use, its threading model is appartment, but sometime, when i access classes of the activeX with different threads, i have an error (the error is done by the activeX i use, not a window exception). Then, i thought that the best way was to create a wrapper class to the ActiveX, and this wrapper class will always work in one thread. So, all calls to the activeX are serialized.
Thanks!
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That's where COM apartment may help. Your ActiveX control is apartment threaded, which means that all calls to that object always must be executed on the same thread. Looks like your client also apartment threaded, but probably when you use your ActiveX object on other thread you didn't marhsall you interface pointer (CoMarshalInterface or CoMarshalInterThread..).
One possible fix, that won't require much changes, is to put all client threads in free threaded apartment (each thread should calls CoInitialiseEx(NULL, COINIT_MULTITHREADED)). There will be slight perfomance hit, since all the calls to your object will be marshalled. If it's not acceptable ( or the client must be apartment threaded), than you'll need to use CoMarshal../Unmarshall or Global interface table (see IGlobalInterfaceTable, CLSID_StdGlobalInterfaceTable) to marshal interface pointer between apartments. In this case the calls from apartment that created object won't be marshalled.
Hope this helps.
Edward.
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How do you position a chart that was add from excel to the second page. I used the AddOleObject method and i did not specify an anchor. Does anyone know how to set up an anchor?? Any help will be appreciated.
DrunkerII
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I define this struct in my IDL:
typedef struct PATIENT_INFO
{
[string] wchar_t wszLastName[ 50 ];
[string] wchar_t wszPatientID[ 30 ];
} PATIENT_INFO;
When I compile I get this error:
midl\oleaut32.dll : error MIDL2020 : error generating type library : LayOut failed on struct : __MIDL___MIDL_itf_PatientTestSvr_0000_0001
When I take out the [string] attribute it compiles fine.
It is my understanding that the [string] attribute means only the NULL terminated string itself will get marshalled and not the whole buffer (which is the behavior I want).
Is my understanding correct and how can I get this to compile?
Glenn Wardius
GE Medical Systems
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first rid yourself of the [50]'s cause you don't need them. the string attribute tells MIDL to look for the null terminator and it does size calculations on its own. Next get rid of the typedef. MIDL doesn't need it. just use
<br />
struct PATIENT_INFO<br />
{<br />
[string] wchar_t wszPatientID;<br />
[string] wchar_t wszLastName;<br />
};<br />
Hope this helps ya out.
Joseph Dempsey
jdempsey@cox.rr.com
Joseph.Dempsey@thermobio.com
"Software Engineering is a race between the programmers, trying to make bigger and better fool-proof software, and the universe trying to make bigger fools. So far the Universe in winning."
--anonymous
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