Extension Ordering

Microsoft Exchange calls extensions in the order in which they are listed in the .INI file or registry. This can often affect the execution of the extension. For example, given two extensions that are registered within the same context, the first extension that is registered to participate in the context might handle an event or a command and not give the other extension a chance to respond. It is therefore important to plan how your extension will be entered into the registry or .INI file.

On Windows NT, extensions are ordered alphabetically within the registry. On Windows 95, the ordering of extensions is the order in which they are inserted.

    To register your extension so that it appears in the correct position whether it is installed on Windows NT or Windows 95

1. Identify the existing extension that you want your extension to be placed before and give your extension a name that alphabetically comes before that extension.
2. Read the registry entry for the existing extension you want your extension to be placed before, store it, and delete the entry for that extension.
3. Write the entry for your extension in place of the deleted extension entry.
4. Re-insert the entry for the extension that was deleted in Step 2.

Although it is always possible to insert your extension at the top of the list to guarantee that it is called, this might cause contention between extensions.