Disclaimer:
This code is intended for experimental/demonstration purposes only,and as-provided, is not necessarily fit or suitable for any specific purpose.
AutoNumberAttributeOverrule:
An ObjectOverrule that automates the assignment of unique, incremental numeric values to all newly-created instances of an attribute with a given tag, owned by references to a given block.
With the code loaded and running, it should not be possible for a user to create multiple instances of the block reference, having identical values in the attribute that's managed by the Overrule, including by way of operations like COPY, INSERT, DXFIN, etc.
To use the sample, you need a block with at least one attribute. With at least one insertion of the block in the current drawing, issue the AUTONUM command, and select the attribute whose value is to be managed.
The demonstration uses block attributes mainly for the purpose of making the behavior easily observable, but the same concepts and approach used can be applied to data stored in Xdata or Xrecords as well.
When using this approach for ensuring unique values in xdata or in an extension dictionary, you would use an Overrule with an XData filter or Dictionary filter, to minimize overhead, which may be the only thing that may make the use of an ObjectOverrule feasable, considering the high overhead of an unconstrained ObjectOverrule.