I need to draw the diagonals in a collection of many 4-sided closed polylines. The polylines could be any mix of polyline types: lightweight, heavyweight etc. The obvious solution is to get the xyz coords of the 4 vertices and draw a line from v1 to v3 and v2 to v4.

I got it to work in 2d when z=0. However, the polygons are mostly in 3D, flipped and dipped and facing everywhich way. Even so, polyline.coordinates only returns the xy coords. Determining the z coord apparently requires some sort of calculation. Ironically, a simple list command will return exactly what I need:

Code:
 
				 LWPOLYLINE Layer: "Diagrid"
							Space: Model space
				 Handle = 59
			Closed
	Constant width 0.00000000
Extrusion direction relative to UCS:
				 X=-0.34529029 Y=-0.36162773 Z=0.86602540
			 area 16936.19977269
		 perimeter 520.69569424
		 at point X=-13.85050859 Y=647.56652579 Z=0.00000000
		 at point X=110.82245818 Y=638.65452535 Z=45.98657127
		 at point X=119.33183683 Y=520.40102410 Z=0.00000000
		 at point X=-5.34112994 Y=529.31302453 Z=-45.98657127
Is there a simple way to get the xy AND z of any polyline vertex?

The layer name gives away what the application is about.