COleDropSource - How to implement

Posted on 1998-07-06
Last Modified: 2008-02-01
I have written a control derived from CWnd and I'm trying to implement drag and drop between two instances of it.

I have tried adding a COleDropTarget derived member variable and calling member.Register(this) in the control's OnCreate handler. For some reason the Register function returns 0 (failure) and the overidden OnDrag functions don't get called!

Any ideas? According to the documentation, I can use a COleDropTarget in a non-CView derived class. Why is failing? What must I do?

Ignore that question - I figured it out - It was the AfxOleInit that was missing.

One more question...

How do you implement the COleDataSource part.
My data will be a text string for now - Max length about 250-500 characters.

I only need CF_TEXT format to start, but I'd like to know how to use my own data format too.

Question by:richweed
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
  • 4
  • 2

Accepted Solution

galkin earned 100 total points
ID: 1167241
I don't understand why you need to implement COleDropSource class. It wraps IDropInterface that allows you get feedback during drag and drop operation. You need to declare on stack where you want to implement drag and drop COleDataObject class instance. Then you need to create data object. Allocate memory block large enogh for your text data with GlobalAlloc. Lock it with GlobalLock. Copy text to allocated memory. Then call COleDataObject::CachGlobalData passing handle to global memro you allocated and locked. Next step is call to COleDataObject::DoDragDrop. Last default parameter is pointer to COleDropSource class. you can derive you own class from COleDropSource and override its virtual functions which will be called during drag and drop. DoDragDrop enters modal loop until drop occurs or drag and drop operation is cancelled.
Finally you need to unlock and free memory with GlobalUnlock and GlobalFree. If you want to use your own format you need first to register it with COleDataObject::RegisterClipboardFormat

Author Comment

ID: 1167242
I'm not using a text string anymore, but my FVInfo data structure instead. (IMAGEBARINFO FVInfo)
Is this code correct?

      COleDataSource MyDataSource;
      HGLOBAL hMyBlock = GlobalAlloc(GMEM_FIXED, sizeof (IMAGEBARINFO));
      IMAGEBARINFO* pMyBlock = (IMAGEBARINFO*) GlobalLock(hMyBlock);

      CopyMemory(pMyBlock,&FVInfo,sizeof(FVInfo)); //Is this correct?


Do I use the same code for normal copy/paste operations? If not, how is it different? (besides not calling dodragdrop)

Expert Comment

ID: 1167243
Yes, you allocate global memory with GlobalAlloc for your structure, lock it with GlobalLock. given a pointer to allocated memory you copy your structure. Then you register custom clipboard format with RegisterClipboardFormat. After that you call CacheGlobalData and DoDragDrop.
Industry Leaders: 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!


Expert Comment

ID: 1167244
You code looks correct

Author Comment

ID: 1167245
Here is my drop code:

      HGLOBAL hMyBlock = pDataObject->GetGlobalData(m_cfIB_CBFormat);
      LPVOID pMyBlock = GlobalLock(hMyBlock);

Is thist correct? Do I have to free the memory block myself or is it done by the DataObject?

What about Copy/Paste Clipboard operations?

      COleDataSource MyDataSource;
      HGLOBAL hMyBlock = GlobalAlloc(GMEM_FIXED, sizeof (x));
      LPVOID pMyBlock = GlobalLock(hMyBlock);


      GlobalFree(hMyBlock);   Is it correct to free the memory block now?


Expert Comment

ID: 1167246
your code is correct and you have to unlock and free global memory

Featured Post

Free Tool: Port Scanner

Check which ports are open to the outside world. Helps make sure that your firewall rules are working as intended.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

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

Suggested Solutions

Title # Comments Views Activity
Building cUrl in Windows v7.43.0 4 45
How do i run a c++ file? 15 58
C++ Code Issue 4 48
How to add columns to a RAD Studio FMX grid at design time 1 60
Often, when implementing a feature, you won't know how certain events should be handled at the point where they occur and you'd rather defer to the user of your function or class. For example, a XML parser will extract a tag from the source code, wh…
IntroductionThis article is the second in a three part article series on the Visual Studio 2008 Debugger.  It provides tips in setting and using breakpoints. If not familiar with this debugger, you can find a basic introduction in the EE article loc…
The goal of the video will be to teach the user the concept of local variables and scope. An example of a locally defined variable will be given as well as an explanation of what scope is in C++. The local variable and concept of scope will be relat…
The viewer will learn how to user default arguments when defining functions. This method of defining functions will be contrasted with the non-default-argument of defining functions.

739 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