Tracing a boundary defined by AutoCAD geometry using .NET

As alluded to in this previous post, today we’re going to see some code that makes use of a very cool new API feature in AutoCAD 2011: the Editor.TraceBoundary() function. This little gem performs something developers have struggled with – and have asked us for – for many years. Previously you had to jump through significant hoops (or should that be loops? <groan>) to get this to work: one common approach would be to drive the BOUNDARY command programmatically and check the results appended to the database. All very messy.

Anyway, we got there eventually. This function takes a “seed” point and returns a collection of newly-created entities that define our boundary. We can then either pump these through the transient graphics sub-system or – and this is the approach we’re going to take – add them to the drawing to make them persistent.

You need to bear in mind that this function ultimately has similar constraints to the BOUNDARY and BHATCH commands: it also works off AutoCAD’s display list and so the user will need to be appropriately ZOOMed into the geometry that is to be used for boundary detection.

Here’s some C# code to create boundaries – with a lineweight applied and of an automatically incremented colour index – for points the user selects:

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Runtime;
 
namespace TraceBoundary
{
  public class Commands
  {
    static int _index = 1;
 
    [CommandMethod("TB")]
    public void TraceBoundary()
    {
      Document doc =
        Application.DocumentManager.MdiActiveDocument;
      Database db = doc.Database;
      Editor ed = doc.Editor;
 
      // Select a seed point for our boundary
 
      PromptPointResult ppr =
        ed.GetPoint("\nSelect internal point: ");
 
      if (ppr.Status != PromptStatus.OK)
        return;
 
      // Get the objects making up our boundary
 
      DBObjectCollection objs =
        ed.TraceBoundary(ppr.Value, true);
 
      if (objs.Count > 0)
      {
        Transaction tr =
          doc.TransactionManager.StartTransaction();
        using (tr)
        {
          // We'll add the objects to the model space
 
          BlockTable bt =
            (BlockTable)tr.GetObject(
              doc.Database.BlockTableId,
              OpenMode.ForRead
            );
 
          BlockTableRecord btr =
            (BlockTableRecord)tr.GetObject(
              bt[BlockTableRecord.ModelSpace],
              OpenMode.ForWrite
            );
 
          // Add our boundary objects to the drawing and
          // collect their ObjectIds for later use
 
          ObjectIdCollection ids = new ObjectIdCollection();
          foreach (DBObject obj in objs)
          {
            Entity ent = obj as Entity;
            if (ent != null)
            {
              // Set our boundary objects to be of
              // our auto-incremented colour index
 
              ent.ColorIndex = _index;
 
              // Set the lineweight of our object
 
              ent.LineWeight = LineWeight.LineWeight050;
 
              // Add each boundary object to the modelspace
              // and add its ID to a collection
 
              ids.Add(btr.AppendEntity(ent));
              tr.AddNewlyCreatedDBObject(ent, true);
            }
          }
 
          // Increment our colour index
 
          _index++;
 
          // Commit the transaction
 
          tr.Commit();
        }
      }
    }
  }
}