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PidTagAttachNumber Canonical Property
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Contains a number that uniquely identifies the attachment within its parent message.
Message stores generate and maintain this property. The attachment number is the secondary sort key, after the rendering position, in the attachment table.
PR_ATTACH_NUM is used to open the attachment with the IMessage::OpenAttach method. Within a client application's session, the PR_ATTACH_NUM property of a message attachment remains constant as long as the attachment table is open.
The message store propagates changes to the table using the IMessage::CreateAttach and IMessage::DeleteAttach methods. At its option the message store can generate table notifications on open attachment tables so that clients can resynchronize to those changes.