//CommandFlags.Session Session states that the command
        //will run on all documents in the session - instead of
        //just the calling doc.
        [CommandMethod("cdb",CommandFlags.Session)]
        public void cdb()
        {
            //create new database

            DocumentCollection docs
                = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager;
            //I think you should be able to add the new database to the current session
            //via the overloads for Database(...) but I didn't get it to work. Didn't try
            //to hard though. So I just open it below.
            Database ndb = new Database();
            ndb.SaveAs(@"c:\temp.dwg", DwgVersion.Current);
            ndb.Dispose();

            //open doc making it the active document
            Document newDoc = Application.DocumentManager.Open(@"C:\temp.dwg", false);
            //Before any editing can be done on a doc it needs to be locked. This is
            //done automatically on the calling doc. Any others need to be locked
            //manually. Comment it out and you'll see the boom!
            newDoc.LockDocument();
            Database db = Application.DocumentManager.MdiActiveDocument.Database;
            Editor ed =
             Application.DocumentManager.MdiActiveDocument.Editor;

            Point3d startpt = new Point3d(4.0, 2.0, 0.0);
            Point3d endpt = new Point3d(10.0, 7.0, 0.0);
            Line line = new Line(startpt, endpt);

            using (Transaction tr = ed.Document.TransactionManager.StartTransaction())
            {
                BlockTable bt = (BlockTable)tr.GetObject(db.BlockTableId, OpenMode.ForRead);
                ObjectId blkRecId = bt[BlockTableRecord.ModelSpace];
                BlockTableRecord btr = (BlockTableRecord)tr.GetObject(blkRecId, OpenMode.ForWrite);

                btr.AppendEntity(line);
                tr.AddNewlyCreatedDBObject(line, true);
                tr.Commit();
            }
        }