Dynamic Data Exchange

Dynamic Data Exchange (DDE) is a technology for interprocess communication under Microsoft Windows or OS/2.

Overview

Dynamic Data Exchange was first introduced in 1987 with the release of Windows 2.0 as a method of interprocess communication so that one program can communicate with or control another program, somewhat like Sun's RPC (Remote Procedure Call).^[1] It used the "Windows Messaging Layer" functionality within Windows. Therefore, DDE continues to work even in modern versions of Windows. DDE has been superseded by newer technologies. Windows for Workgroups introduced a remoting version called NetDDE. OLE and OLE Automation were more advanced, but proved to be bulky and difficult to code. OLE was GUI intensive, but when stripped down to reveal COM. Its remoting version that works between networked machines is DCOM (Distributed COM). .NET Remoting provides a layered architecture for interprocess communication in the .NET Framework. However, legacy DDE is still used in several places inside Windows, e.g. for Shell file associations and for the copy, cut and paste functions.

The primary function of DDE is to allow Windows applications to share data. For example, a cell in Microsoft Excel could be linked to a value in another application and when the value changed, it would be automatically updated in the Excel spreadsheet. The data communication was established by a simple, three-segment model. Each program was known to DDE by its "application" name. Each application could further organize information by groups known as "topic" and each topic could serve up individual pieces of data as an "item". For example, if a user wanted to pull a value from Microsoft Excel which was contained in a spreadsheet called "Book1.xls" in the cell in the first row and first column, the application would be "Excel", the topic "Book1.xls" and the item "r1c1".

A common use of DDE is for custom-developed applications to control off-the-shelf software. For example, a custom in-house application might use DDE to open a Microsoft Excel spreadsheet and fill it with data, by opening a DDE conversation with Excel and sending it DDE commands. Today, however, one could also use the Excel object model with OLE Automation (part of COM). The technique is, however, still in use, particularly for distribution of financial data.^[2]

While newer technologies like COM offer features DDE doesn't have, there are also issues with regard to configuration that can make COM more difficult to use than DDE. Also, DDE is a generic protocol that allows any application to monitor changing data provided by any other application, while to achieve similar results in COM one would generally need to know details of the application that is either to produce or consume the data. For example, a single DDE financial data distribution application can provide live prices to either Excel or a financial charting application without needing to know which it is doing, while to achieve the same results with COM would usually require the distribution application's authors to write custom code for each use scenario.^{[citation needed]}

NetDDE

California-based company Wonderware developed an extension for DDE called NetDDE that could be used to initiate and maintain the network connections needed for DDE conversations between DDE-aware applications running on different computers in a network and transparently exchange data. A DDE conversation is an interaction between client and server applications. NetDDE could be used along with DDE and the DDE management library (DDEML) in applications.

/Windows/SYSTEM32
DDESHARE.EXE (DDE Share Manager)
NDDEAPIR.EXE (NDDEAPI Server Side)
NDDENB32.DLL (Network DDE NetBIOS Interface)
NETDDE.EXE (Network DDE - DDE Communication)

Microsoft licensed a basic (NetBIOS Frames protocol only) version of the product for inclusion in various versions of Windows from Windows for Workgroups to Windows XP. In addition, Wonderware also sold an enhanced version of NetDDE to their own customers that included support for TCP/IP. The technology is extensively used in the SCADA field. Basic Windows applications using NetDDE are Clipbook Viewer, WinChat and Microsoft Hearts.

NetDDE was still included with Windows Server 2003 and Windows XP Service Pack 2, although it was disabled by default. It has been removed entirely in Windows Vista.^[3] However, this will not prevent existing versions of NetDDE from being installed and functioning on later versions of Windows.

See also

• ActiveX
• Component Object Model (COM)
• Distributed Component Object Model (DCOM)
• Microsoft .NET
• Object Linking and Embedding (OLE)

References

1. ^ Net DDE to DOT NET Remoting 12 Jan, 2006
2. ^ Joel Spolsky - comment on Fog Creek Software discussion forum
3. ^ NetDDE has been removed in Windows Vista.

External links

• MSDN: About Dynamic Data Exchange (DDE)
• NetDDE
• Dynamic Data Exchange (DDE) and NetDDE FAQ