Instance diagrams

[GregA]

Hi,

This may be as much a UML question as an EA question. I'm relatively inexperienced in UML though I have significant OOP experience.  All of the following is in the context of a Domain model.

Say I'm building a class diagram and want to illustrate the use of the classes with an instance diagram.  It seems to me it would be helpful to be able to constrain the instance diagram to only allow those things that are permitted by the classes -- but I can't see how to do this in EA.

Right now, it looks like instance diagrams are essentially free form.  I can add attributes wherever I want, whether they exist on the purported class of the instance, or not.  Similarly I can establish relationships that aren't part of the class model.  This is convenient when doodling but maybe not desirable if one wants to create formal illustrations.

Is there any way to declare that an instance diagram must be a legitimate instantiation of the classes in a package?

Motivation:  I'm finding our domain folks find the illustrations helpful to understand the class diagrams. However, I find creating instance diagrams a bit of a run-around in EA.  For instance, I would have expected that the class of a new instance would be the very first thing of interest ... yet it is not readily available via the Properties of the instance ... but rather through "Advanced".  This makes me wonder ... is making instance diagrams not common in UML?  

Thanks,
Greg