Is it possible to write a VB / VBI code that exports flat pattern in dxf format.

Exported file should contain following information

1. All cut-lines for the laser should be closed polylines "no ends"
2. layer should be in ex. "layer 0"
3. in the end calculate how many polylines there are in the exported file and the length of each of them.

Where should I start?

Try export dxf in idw. You may need to write and run VB code in AutoCAD to get total line length. I don't understand how you'll get "open" profile from the flat pattern.

When I use "Contour Trace" in Acad it sometime find open ends. "below 0,01 mm."

It would be great if there was 1 closed polyline for each "cut".

It's been a long time since I did the autocad thing but...

I think if you run a VB (in autocad) polyline edit, select all, convert to polyline=yes, join, fuzz distance=0 you will get the closed profile with all "ends" trimmed off that you are after.
You can also include a command to select all & "list" which should give youa total # of polylines & their length after they have been joined.