?
Solved

Calling a COM written in VB from a C program

Posted on 2000-03-12
10
Medium Priority
?
196 Views
Last Modified: 2010-04-15
Hi
 
   I have a COM object in VB, and I need
   to call it from a C program.
   Does anyone know how to do that?
     -= Gadi =-

0
Comment
Question by:lgadi
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 3
  • 2
  • +1
10 Comments
 
LVL 12

Expert Comment

by:pjknibbs
ID: 2611346
If the COM object was written to use Automation standards (likely, given it's a VB COM object), you can call it the same as you would any other COM interface.
0
 

Author Comment

by:lgadi
ID: 2611519
Okay, but that's what I can't seem to get done. I'm not sure which APIs to
use (I was told about the CComPtr MFC object, but I need to use only C).
A short example of how it might be done could be great!
0
 
LVL 12

Expert Comment

by:pjknibbs
ID: 2611579
Have you ever used COM before? I just ask to find out what level of detail you need!
0
What does it mean to be "Always On"?

Is your cloud always on? With an Always On cloud you won't have to worry about downtime for maintenance or software application code updates, ensuring that your bottom line isn't affected.

 
LVL 32

Accepted Solution

by:
jhance earned 300 total points
ID: 2612808
Basically the process is as follows:

1) Call OleInitialize() or CoInitialize() from your program.

2) Use CLSIDFromProgID() (usually) to get the CLSID for the object from its "friendly" name.

3) Use CoCreateInstance() to get the object's IUnknown interface.

4) Use IUnknown::QueryInterface() using the pointer returned in #3 to get a pointer to the object's IDispatch interface.  This _IS_ the AUTOMATION interface pointer and is the key to the rest of this.

5) Use IDispatch::GetIDsOfNames() to get the dispIDs for all of the methods and properties on the Automation interface you'll be using from your C program.

6)Use IDispatch::Invoke() to actually call methods or properties on the interface.

7) when you're done, don't forget to call IDispatch::Release(), IUnknown::Release(), and CoUninitialize().

 
0
 

Author Comment

by:lgadi
ID: 2613644
jhance: I haven't tried your solution yet, as soon as I will, I'll let you know the results.
pjknibbs: Well, I've created a COM object using C++, but I've only invoked it using VB, and the whole thing is pretty much encapsulated there...So, I never did go through the entire process in C.
0
 
LVL 5

Expert Comment

by:Wyn
ID: 2615979
That easy in C ?
0
 
LVL 5

Expert Comment

by:Wyn
ID: 2615982
The class stuff and so on maybe hard to around or create it manually by struct.
0
 

Author Comment

by:lgadi
ID: 2619033
jhance: Okay, I started following your directions, but as I got to step 3, I typed CoCreateInstance, and the 3rd parameter there is REFIID, and the compiler says it must be initialized. The problem is I don't know how to initialize it... I think a wokring example (if possible) will prevent alot of problems, if none is available, I will happily settle for an explenation.
0
 
LVL 12

Expert Comment

by:pjknibbs
ID: 2619059
Actually the REFIID is the FOURTH parameter of CoCreateInstance, and it's the GUID that uniquely identifies your Visual BASIC COM object. To clarify, all COM objects must be identified by a Globally Unique Identifier, a 128-bit number assigned when the control was created in the first place. You need to know this value to instantiate a COM object. If you don't already know this value for your COM object I'm not sure how you'd find it out without searching the registry.
0
 
LVL 32

Expert Comment

by:jhance
ID: 2619134
lgadi,

In almost all cases, refiid for CoCreateInstance will be IID_IUnknown.

Use it like this:

CLSID myClsid;
LPUNKNOWN pUnk;

if(!SUCCEEDED(CLSIDFromProgID(L"ObjectName.ControlName.1", &myClsid))){

 // Error
}

if(CoCreateInstance(myClsid, NULL, CLSCTX_ALL, IID_IUnknown,  (LPVOID *)&pUnk) != S_OK){

  // Error
}
else{
  // OK
}
0

Featured Post

Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

This is a short and sweet, but (hopefully) to the point article. There seems to be some fundamental misunderstanding about the function prototype for the "main" function in C and C++, more specifically what type this function should return. I see so…
Examines three attack vectors, specifically, the different types of malware used in malicious attacks, web application attacks, and finally, network based attacks.  Concludes by examining the means of securing and protecting critical systems and inf…
The goal of this video is to provide viewers with basic examples to understand recursion in the C programming language.
The goal of this video is to provide viewers with basic examples to understand and use conditional statements in the C programming language.
Suggested Courses

752 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question