I have to calculate the volume enclosed by CAD lines, but it doesn't come out correctly.

The problem with your DXF is that your walls are vertical. Breaklines wouldn't cross or be in the same positions. Just draw a new DXF where the top polyline is slightly longer than the lower polyline, also it mustn't have any intersections with the lower one. If they intersect only one will be used.

Thank you, it worked great!