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Q189498: INFO: Using DDE in VB to Communicate with Other Windows Programs

Article: Q189498
Product(s): Microsoft Visual Basic for Windows
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbenv kbtool kbGrpDSVB
Last Modified: 06-AUG-2002

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SUMMARY
=======

This article describes how to use Dynamic Data Exchange (DDE) from a Visual
Basic program. Although Visual Basic offers more powerful, robust and reliable
intercommunication between programs by using OLE, it still supports the older
DDE technology. You can use DDE to link to and share data between programs.

Using OLE to communicate with other programs is better than using DDE with Visual
Basic. OLE offers you better performance than DDE does. If a program supports
OLE automation, it exposes an "Object Model" you can use to manipulate the
program (through code) to complete actions for you. DDE is more useful in
communicating with older programs that might not support OLE but do support DDE,
and many 16-bit programs fall into this category.

Microsoft recommends that you use OLE rather than DDE to communicate with another
program when you are developing your Visual Basic program. However, because
there are times when you may need or want to use DDE, this article describes how
to use DDE from a Visual Basic program.

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

Visual Basic can exchange data, such as information, commands or keystrokes,
with any program that supports DDE. A DDE link requires the following three
things:

- Program name

- Topic

- Item

The program name is the executable (.exe) file name of the source program minus
the .exe file extension. The topic describes the type of data that is being
exchanged. The topic is specific to the DDE source program. However, "System" is
the one universal topic that you can use to query what topics are available in a
particular program. The item parameter is the actual data that is being
transferred. In Visual Basic, a topic could be a form with a textbox as an item.
In Microsoft Excel, a topic could be a spreadsheet with a cell as an item. The
program name and topic cannot be modified once the DDE link has been established
because any modifications break the connection. The item parameter can be
modified to any valid item during the DDE conversation without breaking the DDE
link.

Specific to Visual Basic, the LinkTopic property supplies a combination of the
program name and topic parameters. The LinkTopic property is available for
Label, PictureBox, or TextBox controls which can share data through a DDE
connection. The program name and topic parameters are combined by using the pipe
character (|) as the separator. For example, to create a DDE link with Microsoft
Word as the source program and by using the System topic. the LinkTopic property
of a textbox would be:

  Text1.LinkTopic="WinWord|System"

When Visual Basic is the source program for the DDE link, the LinkTopic of the
form which contains the control to be shared is set to a known name. This known
name is used in the LinkTopic of the Visual Basic destination program as the
topic information. For example, to access data from a form with a LinkTopic that
is set to "DDELink", a Visual Basic destination program would have a matching
control with a LinkTopic set to "program name|DDELink".

The LinkItem property corresponds to the item parameter in a DDE connection, and
this property is set to a corresponding item in the source program, for example,
the "Total" cell in an Excel spreadsheet. The LinkItem property is set for the
control that is used as in a destination program. When a Visual Basic form is
the source in a DDE connection, the name of the control on the form can be the
item argument in the program|topic|item string that is used by the destination
program. When Visual Basic is used as both the source and destination program,
the LinkItem property of the destination program would be set to the name of the
control in the Visual Basic source program. For example, with Visual Basic used
as both the source and destination program and a text box that is named
"txtSource" as the item in the source program, a corresponding "txtData" text
box in the destination program would have its LinkItem property set to
txtSource:

  txtData.LinkItem = "txtSource"

Before a connection can be established, the mechanism to update the information
needs to be set. In a DDE connection, there are three types of links:

- Automatic

- Manual

- Notify

An automatic link updates the destination information whenever the source is
modified. A manual link requires that the data be requested by the destination
program, and notify informs the destination program that the source data has
been changed but doesn't actually update the information in the destination
program. The one exception to an automatic update is the Picturebox control.
Visual Basic does not automatically notify a DDE destination program when the
Picture property setting of a PictureBox on a source Form changes. Because the
amount of data in a graphic can be very large and because it seldom makes sense
to update a destination program as each pixel in the picture changes, Visual
Basic requires that you use the LinkSend method to explicitly notify DDE
destination programs when the contents of a PictureBox change.

Once a DDE link has been established, there are several methods and events in
Visual Basic that can be used to control how data is sent between the source and
the destination program.

Methods
-------

LinkRequest: Asks the source program to update the contents of a destination
item.
LinkExecute: Sends a command string to the source program.
LinkPoke: Transfers the contents of the item control to the source program.
Typically, information in a DDE conversation flows from the source to the
destination. However, LinkPoke provides a destination object with the ability to
supply data to the source.
LinkSend: Transfers the contents of a PictureBox control to the destination
program in a DDE conversation.

Events
------

LinkOpen: Occurs when a DDE link is being initiated.
LinkClose: Occurs when a DDE link terminates. Either program in a DDE
conversation may terminate a link at any time.
LinkError: Occurs when there is an error during a DDE conversation. This event is
recognized only as the result of a DDE-related error that occurs when no Visual
Basic code is being executed. The error number is passed as an argument.
LinkExecute: Occurs when a command string is sent by a destination program in a
DDE conversation. The destination program expects the source program to perform
the operation described by the string.
LinkNotify: Occurs when the source has changed the data that is defined by the
DDE link if the LinkMode property of the destination control is set to Notify.

REFERENCES
==========

For additional information about how to create DDE connections from Visual Basic
to other applications like Visual Basic, Word or Excel, click the article
numbers below to view the articles in the Microsoft Knowledge Base:

  Q74861 Example of Client-Server DDE Between Visual Basic Applications

  Q74862 DDE Example Between Visual Basic and Word for Windows

  Q75089 DDE from Visual Basic for Windows to Excel for Windows

MSDN contains documentation about DDE methods, properties, and events that are
supported in Visual Basic 6.0. In MSDN, set the "Active Subset" to "Visual Basic
Documentation," click the Index tab, and then type "DDE" (without the quotation
marks). The topics that are available are applicable to Visual Basic 6.0.

In Visual Basic 5.0, click Books Online on the Help menu. When Books Online
appears, click Index on the View menu. When the Index search browser appears,
type "DDE" (without the quotation marks).

Additional query words: not grayed out greyed applications

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Keywords          : kbenv kbtool kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version           : :5.0,6.0
Issue type        : kbhowto

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