Exploding AutoCAD objects using .NET

mccad

Time to go back to basics. I realised recently – on receiving this comment – that I hadn’t specifically covered the nature of Entity.Explode() in a post (even if it’s been used in a few of them, over the years).

Entity.Explode() is one of those tricky methods: it’s actually a
faux-ami with the AutoCAD command of the same name, in a very similar way to Database.Purge(). The way in which Explode() and Purge() differ from their “equivalent” commands is that they’re non-destructive: they don’t actually result in a change to the AutoCAD drawing database. Explode() populates a DBObjectCollection with the results of the explode operation, and it’s then up to the caller of the function to add them to the appropriate location (typically the current – i.e. model- or paper- – space). Purge() checks a list of objects for ones that can safely be erased, and then it’s up to the caller of the function to do so.

So there is some occasional – and understandable – confusion around Explode() or Purge() not working as expected.
Here’s a simple approach to mimicking the behaviour of the EXPLODE command programmatically: after calling Entity.Explode() on each selected entity, we optionally erase the original entities and add the results of the Explode() to the drawing.
There’s a lot this code doesn’t do – such as check for entities that did not actually generate any exploded results (and so probably shouldn’t be erased) or output some feedback on what is happening to the command-line – but the principle should be clear enough.
Here’s the C# code:

using Autodesk.AutoCAD.ApplicationServices;

using Autodesk.AutoCAD.DatabaseServices;

using Autodesk.AutoCAD.EditorInput;

using Autodesk.AutoCAD.Runtime;

namespace Explosion

{

public
class
Commands

  {

    [CommandMethod("EXP", CommandFlags.UsePickSet)]

public
void ExplodeEntities()

    {

Document doc =

Application.DocumentManager.MdiActiveDocument;

Database db = doc.Database;

Editor ed = doc.Editor;

// Ask user to select entities

PromptSelectionOptions pso =

new
PromptSelectionOptions();

      pso.MessageForAdding = "\nSelect objects to explode: ";

      pso.AllowDuplicates = false;

      pso.AllowSubSelections = true;

      pso.RejectObjectsFromNonCurrentSpace = true;

      pso.RejectObjectsOnLockedLayers = false;

PromptSelectionResult psr = ed.GetSelection(pso);

if (psr.Status != PromptStatus.OK)

return;

// Check whether to erase the original(s)

bool eraseOrig = false;

if (psr.Value.Count > 0)

      {

PromptKeywordOptions pko =

new
PromptKeywordOptions("\nErase original objects?");

        pko.AllowNone = true;

        pko.Keywords.Add("Yes");

        pko.Keywords.Add("No");

        pko.Keywords.Default = "No";

PromptResult pkr = ed.GetKeywords(pko);

if (pkr.Status != PromptStatus.OK)

return;

        eraseOrig = (pkr.StringResult == "Yes");

      }

Transaction tr =

        db.TransactionManager.StartTransaction();

using (tr)

      {

// Collect our exploded objects in a single collection

DBObjectCollection objs = new
DBObjectCollection();

// Loop through the selected objects

foreach (SelectedObject so in psr.Value)

        {

// Open one at a time

Entity ent =

            (Entity)tr.GetObject(

              so.ObjectId,

OpenMode.ForRead

            );

// Explode the object into our collection

          ent.Explode(objs);

// Erase the original, if requested

if (eraseOrig)

          {

            ent.UpgradeOpen();

            ent.Erase();

          }

        }

// Now open the current space in order to

// add our resultant objects

BlockTableRecord btr =

          (BlockTableRecord)tr.GetObject(

            db.CurrentSpaceId,

OpenMode.ForWrite

          );

// Add each one of them to the current space

// and to the transaction

foreach (DBObject obj in objs)

        {

Entity ent = (Entity)obj;

          btr.AppendEntity(ent);

          tr.AddNewlyCreatedDBObject(ent, true);

        }

// And then we commit

        tr.Commit();

      }

    }

  }

}

The results of calling the EXP command should be obvious enough – and do not result in anything very visually stimulating – so I’ll finish the post here.