Can One Add-in Access Another Through the Developer Studio Object Model?
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No, an add-in cannot access another add-in through the Developer Studio object model. An add-in can call the ExecuteCommand method to carry out another add-in's commands, but the add-in cannot control or get information about the other add-in.
Add-ins that have compile-time knowledge of each other can expose custom interfaces through which they can communicate with each other.