AutoCAD .NET API gotcha: dynamic blocks don't play by the rules you expect.

I spent some time hunting down a frustrating bug in an AutoCAD plugin. The goal was simple — find all inserted references to a named block called TITLE_BLOCK. Standard stuff.

Except BlockTable.Has("TITLE_BLOCK") wasn't reliably working. The lookup would fail or miss references entirely.

The culprit? When AutoCAD inserts a dynamic block and its parameters get modified, it creates an anonymous copy of the block definition — named something like *U47 — and the BlockReference now points to that, not to your named definition.

The fix: stop looking up blocks by name in the BlockTable. Instead, iterate over every entity in every space record, and for each BlockReference, resolve it through DynamicBlockTableRecord — which always gives you back the original named definition, regardless of anonymization.

That one property call is the difference between a lookup that silently misses half your blocks and one that reliably finds every insert.

The broader lesson: the AutoCAD object model has a lot of these subtle indirections. When something that should be simple stops working, there's usually an abstraction layer in between that you haven't accounted for yet.

#AutoCAD #dotnet #CSharp #CAD #SoftwareEngineering #AutoDeskAPI