I need to get a list of all objects separately of a application , the application contains around 3500 objects , any suggestion how can i get this
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What do you need that list for? (there may be alternate options if you explain the context, what it is you're actually trying to achieve).
In direct answer to your question: nothing in the Appian Designer environment that would help you with this other than Designer itself which by definition lists of of the objects contained in an Application.
If you need that list exported for use elsewhere you could export your Application as a zip file and use your favourite file tool(s) to unzip and extract the information you're looking for.
Actually, there is a catch here. an Appian object can be a part of more than one application so there isn't a concept like an Application's objects. It can be a part of multiple apps at once. Or as a matter of fact, it can be just a standalone object without being a part of any application.
This is true, but not typical. But it does really depend on what the author here is actually trying to achieve
This is something the OP might be looking for - community.appian.com/.../get-application-s-content-objects