The diagram can work with .xml files out of the box only if they are serialized and formatted using its serialization
service. For example, the control exposes two methods that can be used to serialize/deserialize its shapes - Save() and Load(). The Save() method get all items defined in the RadDiagram and serialize them to an xml string. On the other hand the Load() method gets an xml string and loads items from it. Custom xml (custom tags, attributes, etc.) is not supported by the Load() method.
To display the information from the xml that contains the classes' packages you will need to manually parse the xml and create RadDiagramShapes (or whatever diagram item you need) based on the file's content.
About the last requirement in your post, I am not sure that I understand what you mean by that you want to populate the diagram automatically in the specific circumstances. Can you please give me more details on this. This will help me in better understanding your scenario.
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