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Q152092: DRAGD95.EXE OLE Drag/Drop in Windows 95 Common Controls

Article: Q152092
Product(s): Microsoft C Compiler
Version(s): 4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbfile kbole kbSample kbDragDrop kbMFC kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbGrpDSM
Last Modified: 26-JUN-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
   - Microsoft Visual C++.NET (2002) 
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NOTE: Microsoft Visual C++ .NET (2002) supports both the managed code model that is provided by the Microsoft .NET Framework and the unmanaged native Microsoft Windows code model. The information in this article applies to unmanaged Visual C++ code only.

SUMMARY
=======

This sample demonstrates implementing OLE drag and drop in Windows 95 Common
controls. It also demonstrates how the drag image functionality can be preserved
while dragging within the application that is the source of the data.

MORE INFORMATION
================

The following files are available for download from the Microsoft Download
Center:

Visual C++ 6.0:

The following file is available for download from the Microsoft Download Center:

  DownloadDownload Dragd95.exe now
  (http://download.microsoft.com/download/vc60pro/Sample10/1/NT4/EN-US/Dragd95.exe)

For additional information about how to download Microsoft Support files, click
the following article number to view the article in the Microsoft Knowledge
Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft scanned this file for viruses. Microsoft used the most current
virus-detection software that was available on the date that the file was
posted. The file is stored on secure servers that prevent any unauthorized
changes to the file.

Visual C++ .NET:

The following file is available for download from the Microsoft Download Center:

  DownloadDownload Dragd95vcnet.exe now
  (http://download.microsoft.com/download/visualstudionet/sample/1.3/win98mexp/en-us/Dragd95vcnet.exe)

Release Date: June 24, 2002

For additional information about how to download Microsoft Support files, click
the following article number to view the article in the Microsoft Knowledge
Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft scanned this file for viruses. Microsoft used the most current
virus-detection software that was available on the date that the file was
posted. The file is stored on secure servers that prevent any unauthorized
changes to the file.


NOTE: Use the -d option when running dragd95.exe to decompress the file and
recreate the proper directory structure. If you are using Visual C++ 5.0 you may
get a message:

  "This project was generated by a previous version of Developer Studio.
  Continuing will convert it to the new format. Do you want to convert the
  project?"

Accept this by clicking "Yes."

Please Note the Following Problem
---------------------------------

Dragd95.exe reports a memory leak when the steps below are taken:

1. Build the debug version of the sample. Then, run the .exe.

2. Drag one item from left pane to right pane.

3. Close the .exe. You should see the memory leak in the output window of VC++.

Resolution
----------

The following code needs to be at the bottom of OnHandleDrag() (that is, before
the last return statement):

     delete m_pDragImage;
     m_pDragImage = NULL;


More Information
----------------

Windows 95 Common controls implement their own style of drag and drop. However,
this style of drag and drop does not support dragging to other applications. To
enable this functionality, it is possible to implement OLE drag and drop using
the Common controls. Unfortunately, when OLE drag and drop is implemented, the
Common control drag image is lost.

It is possible, when implementing OLE drag and drop, to preserve the Common
control drag image. However, this image will only be displayed when the pointer
is over the application that is the source of the data. This is because the
image is not a system-wide resource and belongs to the application that is the
source of the data. This behavior is consistent with that of the shell in
Windows 95. In Windows 95, you will notice that the drag image is lost over
applications other than the Explorer or the Windows shell. The shell and any
instances of the Explorer that are running are a single instance of the same
application.

OLE drag and drop as implemented by this sample is straight forward. When a drag
is started, a COleDataSource object is loaded with data and
COleDataSource::DoDragDrop() is called. The data source is loaded with a CF_TEXT
format and a private clipboard format that has been registered. Applications
that understand CF_TEXT, such as Microsoft Word, can be a drop target for the
data. You can also implement a COleDropTarget object so that you can be a drop
target for your own custom clipboard format.

Common controls normally begin a drag operation in response to the LVN_BEGINDRAG
message. You can also take advantage of this message to begin the drag
operation. This is where the similarity with Common control drag and drop ends.
Common control drag and drop uses mouse messages to control tracking the drag
image and processing the drop. You will not be able to use mouse messages
because, after you begin the drag operation, control is passed to OLE's
DoDragDrop() function.

To control the tracking and display of the common control drag image, you will
implement a COleDropSource object and pass it to the DoDragDrop() function.
COleDropSource implements a GiveFeedback() function that is called to give
feedback about the effect of a drop at the current mouse position as the mouse
is moved over a drop target. Overriding the GiveFeedback() function and
obtaining the position of the mouse gives you a chance to control tracking and
display of the drag image.

REFERENCES
==========

Programming with MFC: Encyclopedia - Drag-and-Drop (OLE)

Additional query words: Dragd95 Dragd95vcnet

======================================================================
Keywords          : kbfile kbole kbSample kbDragDrop kbMFC kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : :4.0,4.1,4.2,5.0,6.0

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