Comparing DDEML to the OLE Libraries (74806)

The functionality of the Dynamic Data Exchange Management Library (DDEML) is largely encompassed by that of the OLE libraries. However, there is some functionality that the DDEML supports that the OLE libraries do not.

Multiple Items Per Conversation

The DDEML supports multiple, simultaneous, item advise loops per conversation. A single pair of window handles serves as the source and destination for DDE messages that may create advise loops for any number of items. In contrast, the OLE libraries support only one item (object) per conversation. If a client wants to establish a link to several items (objects) in a server document, it must establish multiple conversations with the server, one for each item. Although this is not difficult to program in the client application, it does incur the memory and performance overhead of multiple DDE windows, as opposed to a single DDE window.

It is possible to use the OLE libraries to simulate the multiple items-per-conversation capability of the DDEML. The identification of an item can be attached to all data packets for the item. However this technique works only for clients and servers that agree upon a particular convention for creating such data packets. This technique is not be backward-compatible with existing DDE applications.

Generalized Support for the DDE "System" Topic

The OLESVR library responds to requests for these standard DDE "System" topic items: EditItems, protocols, topics, formats, and status. (EditItems is a new standard DDE "System" topic item that supports the OLE StdFileEditing command.) An application using OLESVR to implement DDE has no opportunity to influence the response of OLESVR to requests for these "System" topic items. Further, the application has no opportunity to respond to requests for other "System" topic items that it might define, or to requests for the SysItems "System" topic item.

In contrast, the DDEML provides generalized support for any "System" topic.