IOSTX : IUnknown

Office 2013 and later

Last modified: July 23, 2011

Applies to: Outlook

Provides synchronization methods. This interface retrieves the necessary information to replicate local changes to the server and server changes to the local store.

Provided by:


Interface identifier:



Gets extended information about the last error.


Informs the local store that synchronization is about to start.


Prepares the local store for synchronization in a particular state and retrieves the necessary information to replicate.


Ends synchronization in the current state and exits that state.


Starts synchronization for a message header.


Ends synchronization for a message header.


Sets the result of the synchronization.

Placeholder member

Not supported or documented.

When a client uploads or synchronizes folders and folder contents on a local store, it moves the local store from one state to another as depicted in the state transition diagram in About the Replication State Machine. The following is the order of events for the client to move the local store from one state to another:

  1. The client calls IOSTX::InitSync to inform the local store that replication is about to start.

  2. Depending on the direction of replication and the objects to replicate, the client calls IOSTX::SyncBeg to begin replication in the appropriate state. Outlook provides the client the necessary information, and the client performs the replication.

  3. The client calls IOSTX::SetSyncResult to return the result of the replication.

  4. The client calls IOSTX::SyncEnd to end the replication, providing Outlook the necessary information for subsequent replication.

In particular, when downloading message items, the client uses IOSTX::SyncHdrBeg and IOSTX::SyncHdrEnd to update a full message item with the message header on the local store:

  1. Upon IOSTX::SyncHdrBeg, the local store transitions into the download message header state. Outlook initially provides the client with the current message header on the local store.

  2. The client downloads a full message item together with the message header.

  3. Outlook updates the item on the local store with the full message item.