Handle Object Events (.NET)

Object events are used to respond to the opening, adding, modifying, and erasing of objects from a drawing database. There are two types of object related events: object and database level. Object level events are defined to respond to a specific object in a database, whereas Database level events respond to all objects in a database.

You define an object level event by registering an event handler with the event of a database object. Database level object events are defined by registering an event handler with one of the events of an open Database object.

The following events are available for DBObjects:

Cancelled

Triggered when the opening of the object is cancelled text.

Copied

Triggered after the object is cloned.

Erased

Triggered when the object is flagged to be erased or is unerased.

Goodbye

Triggered when the object is about to be deleted from memory because its associated database is being destroyed.

Modified

Triggered when the object is modified.

ModifiedXData

Triggered when the XData attached to the object is modified.

ModifyUndone

Triggered when previous changes to the object are being undone.

ObjectClosed

Triggered when the object is closed.

OpenedForModify

Triggered before the object is modified.

Reappended

Triggered when the object is removed from the database after an Undo operation and then re-appended with a Redo operation.

SubObjectModified

Triggered when a subobject of the object is modified.

Unappended

Triggered when the object is removed from the database after an Undo operation.

The following are some of the events used to respond to object changes at the Database level:

ObjectAppended

Triggered when an object is added to a database.

ObjectErased

Triggered when an object is erased or unerased from a database.

ObjectModified

Triggered when an object has been modified.

ObjectOpenedForModify

Triggered before an object is modified.

ObjectReappended

Triggered when an object is removed from a database after an Undo operation and then re-appended with a Redo operation.

ObjectUnappended

Triggered when an object is removed from a database after an Undo operation.

Enable an Object event

This example creates a lightweight polyline with events. The event handler for the polyline then displays the new area whenever the polyline is changed. To trigger the event, simply change the size of the polyline in AutoCAD. Remember that you must run the CreatePLineWithEvents subroutine before the event handler is activated.

C# Example

using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Geometry;
 
// Global variable for polyline object
Polyline acPoly = null;
 
[CommandMethod("AddPlObjEvent")]
public void AddPlObjEvent()
{
  // Get the current document and database, and start a transaction
  Document acDoc = Application.DocumentManager.MdiActiveDocument;
  Database acCurDb = acDoc.Database;
 
  using (Transaction acTrans = acCurDb.TransactionManager.StartTransaction())
  {
      // Open the Block table record for read
      BlockTable acBlkTbl;
      acBlkTbl = acTrans.GetObject(acCurDb.BlockTableId,
                                   OpenMode.ForRead) as BlockTable;
 
      // Open the Block table record Model space for write
      BlockTableRecord acBlkTblRec;
      acBlkTblRec = acTrans.GetObject(acBlkTbl[BlockTableRecord.ModelSpace],
                                      OpenMode.ForWrite) as BlockTableRecord;
 
      // Create a closed polyline
      acPoly = new Polyline();
      acPoly.AddVertexAt(0, new Point2d(1, 1), 0, 0, 0);
      acPoly.AddVertexAt(1, new Point2d(1, 2), 0, 0, 0);
      acPoly.AddVertexAt(2, new Point2d(2, 2), 0, 0, 0);
      acPoly.AddVertexAt(3, new Point2d(3, 3), 0, 0, 0);
      acPoly.AddVertexAt(4, new Point2d(3, 2), 0, 0, 0);
      acPoly.Closed = true;
 
      // Add the new object to the block table record and the transaction
      acBlkTblRec.AppendEntity(acPoly);
      acTrans.AddNewlyCreatedDBObject(acPoly, true);
 
      acPoly.Modified += new EventHandler(acPolyMod);
 
      // Save the new object to the database
      acTrans.Commit();
  }
}
 
[CommandMethod("RemovePlObjEvent")]
public void RemovePlObjEvent()
{
  if (acPoly != null)
  {
      // Get the current document and database, and start a transaction
      Document acDoc = Application.DocumentManager.MdiActiveDocument;
      Database acCurDb = acDoc.Database;
 
      using (Transaction acTrans = acCurDb.TransactionManager.StartTransaction())
      {
          // Open the polyline for read
          acPoly = acTrans.GetObject(acPoly.ObjectId,
                                     OpenMode.ForRead) as Polyline;
 
          if (acPoly.IsWriteEnabled == false)
          {
              acTrans.GetObject(acPoly.ObjectId, OpenMode.ForWrite);
          }
 
          acPoly.Modified -= new EventHandler(acPolyMod);
          acPoly = null;
      }
  }
}
 
public void acPolyMod(object senderObj,
                      EventArgs evtArgs)
{
  Application.ShowAlertDialog("The area of " +
                              acPoly.ToString() + " is: " +
                              acPoly.Area);
}