It appears one has to add a "Property Data Format" to the drawing in Style Manager. Once you do that, use the Format column on the Property Set Definition tab to select the desired format and then the unit drop-down will then become available with the set units available for the unit type specified in the Property Data Formats. I changed Standard to Area and then the area options was then available.
I uploaded an attempt to draw an image using transient graphics to github.
https://github.com/C3DReminders/DFWBIUG_C3D_Data_Shortcuts_2020
It doesn't work well, but it's a start.
Civil 3D comes with sample code. It is a great place to start when writing code. For Civil 3D 2023, the code is located here: C:\ProgramData\Autodesk\C3D 2023\enu\Data\Reports\Net\Source
"C:\Program Files\Autodesk\AutoCAD 2023\C3D\Sample\Civil 3D API\DotNet\C3DDotNetSamples.sln"
This video shows how to get a substring using a DIESEL expression: https://youtu.be/vvMnV7T9_Vc
One can use Diesel expressions to convert data in AutoCAD. In this case, I want to convert the layout tab name to a fuller name. So if I have CIV I want the field to show "Civil" and if the layout tab name is "INF" it should be "Infrastructure".
$(if,$(eq,$(getvar, "ctab"),"CIV"),"Civil",$(if,$(eq,$(getvar, "ctab"),"INF"),"Infrastructure","Error"))
The help topic for Diesel expressions may be found here:
Note that the link will probably die in 3 to 4 years.
Here is a video showing it in action: https://youtu.be/zUpX04RfnNA
