var lines = new List<Line>();

using (doc.LockDocument())
{
    using (var tr = db.TransactionManager.StartTransaction())
    {
        BlockTable bt = (BlockTable)tr.GetObject(db.BlockTableId, OpenMode.ForRead, false);
        BlockTableRecord btr = (BlockTableRecord)tr.GetObject(bt[BlockTableRecord.ModelSpace], OpenMode.ForRead, false);

        foreach (ObjectId id in btr)
        {
            var entity = tr.GetObject(id, OpenMode.ForRead);

            if (entity.GetType() != typeof(Line))
                continue;

            lines.Add(entity as Line);
        }

        var csv = new StringBuilder();

        foreach (var line in lines)
        {
            var start = line.StartPoint;
            var end = line.EndPoint;

            csv.AppendLine($"{start.X};{start.Y};{start.Z};{end.X};{end.Y};{end.Z};");
        }

        File.WriteAllText(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), "output.csv"), csv.ToString());

        tr.Commit();
    }
}