Solved

Calling a COM written in VB from a C program

Posted on 2000-03-12
10
193 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
  • 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
Optimizing Cloud Backup for Low Bandwidth

With cloud storage prices going down a growing number of SMBs start to use it for backup storage. Unfortunately, business data volume rarely fits the average Internet speed. This article provides an overview of main Internet speed challenges and reveals backup best practices.

 
LVL 32

Accepted Solution

by:
jhance earned 100 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

ScreenConnect 6.0 Free Trial

At ScreenConnect, partner feedback doesn't fall on deaf ears. We collected partner suggestions off of their virtual wish list and transformed them into one game-changing release: ScreenConnect 6.0. Explore all of the extras and enhancements for yourself!

Question has a verified solution.

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

Have you thought about creating an iPhone application (app), but didn't even know where to get started? Here's how: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Important pre-programming comments: I’ve never tri…
Preface I don't like visual development tools that are supposed to write a program for me. Even if it is Xcode and I can use Interface Builder. Yes, it is a perfect tool and has helped me a lot, mainly, in the beginning, when my programs were small…
The goal of this video is to provide viewers with basic examples to understand how to use strings and some functions related to them in the C programming language.
Video by: Grant
The goal of this video is to provide viewers with basic examples to understand and use while-loops in the C programming language.

790 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