Thread: Unstructured Data Model

I'd like to write an application whose data model needs to be flexible and unstructured (similar to, say, XML). The model itself can contain fields and the field values can also be an array of the same structure (forming a hierarchy). Ideally, I'd like to be able to have a dictionary of field identifiers which are coupled to their field types. In that sense, it would be similar to web services but targeted at intra-application (within the application) method calls.

That would be the main functionality requirement. Nice to haves would be a way to dump the structure to a String, create a structure from a formatted String, identify whether a structure contains a certain field type, etc. and be easy and natural to code to.

Does a library or programming API like that exist and is open and freely available?