If the API you're using was implemented correctly, then PropertyDescriptors for properties of type System.Double will have a UnitTypeAttribute attached to them, which has a UnitType property that tells you if the property represents a distance, angle, area, scale, etc.
The UnitType enum:
public enum UnitType
{
Angle = 2,
AngleNotTransformed = 0x10000,
Area = 4,
Currency = 0x10,
Distance = 1,
Percentage = 0x20,
Unitless = 0,
Volume = 8
}
As far as how you're getting the properties, that's the way it should be done, but you need to be careful because Autodesk has internalized the use of System.ComponentModel, and allows the property representation of DBObjects to be mangled by AutoCAD Data Extraction. In other words, after data extraction is used in a session, you will find all sorts of additional properties that correspond to the fields that can appear in a data extraction table (like for example, file properties like Title, LastSavedBy, and so on, which you will get on every single entity type).
Had it been done correctly (no it certainly was not), they would have used TypeDescriptor.AddProvider() and RemoveProvider() to add/remove the data extraction properties so they will only be returned when they're being requested by data extraction, rather than by other components. Unfortunately, it wasn't done that way, and so once Data Extraction has been run in the session, those additional properties will remain for the duration of the session.
You can probably eliminate them by filtering on attributes and/or the runtime type of the PropertyDescriptor, but you'll still have a real mess on your hands.
What I usually do is write my own custom typedescriptors and have them bypass the Autodesk-provided ones to avoid having to contend with all that junk that's added by data extraction.
