Through the Interface: Purging registered application names in the current AutoCAD drawing using .NET

Kean Walmsley

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August 13, 2007

Purging registered application names in the current AutoCAD drawing using .NET

Purging can seriously reduce the size of AutoCAD drawings by removing unnecessary symbol table and dictionary entries. The PURGE command in AutoCAD allows you to safely purge these non-graphical objects (layers, linetypes, block definitions, etc.).

Since AutoCAD 2005 (if I recall correctly), PURGE also supports the removal of Registered Application names. Applications that make use of Extended Entity Data (XData) must register unique application names in drawings. A few applications have, in the past, created many more names than they required, and as these RegApp names get copied from drawing to drawing (when they get XRefed, for instance), they ended spreading from DWG file to DWG file (one comparison I remember hearing was to a virus, although that was perhaps a little extreme). To the best of my knowledge the applications that mistakenly caused this problem have now been fixed (and shall remain nameless), but there are still drawings out there with a lot of these records, which is ultimately why we extended PURGE to address the problem from within AutoCAD.

Anyway, I've chosen to implement a command to purge these RegApp names - even though it's now there in the product - really just an example of how to code this type of functionality. It could very easily be extended to handle the specific data you feel needs purging in your company's (or customers') DWG files.

The Database.Purge() function available in .NET wraps AcDbDatabase::purge(). These functions are both misleadingly named, as neither of them actually performs a purge on the database. What they do - and this is actually more useful, as it gives you more control - is filter a list of object IDs you pass in, removing any that cannot safely be purged by your application. So they would more accurately be named TellMeWhatCanSafelyBePurged(), or something like that. Typically objects get removed from the list because a reference exists somewhere in the drawing to that object - such as from an entity to a layer (making the layer dangerous to purge). The code calling the Purge function will usually then erase the objects that have been returned. And that's how the PURGE command is implemented.

Here's some C# code that purges the Registered Application names in the current document:

using Autodesk.AutoCAD.ApplicationServices;

using Autodesk.AutoCAD.DatabaseServices;

using Autodesk.AutoCAD.EditorInput;

using Autodesk.AutoCAD.Runtime;

using System.IO;

using System;

namespace Purger


  public class Commands



    public void PurgeCurrentDocument()


      Document doc =


      Database db = doc.Database;

      Editor ed = doc.Editor;

      int count =



        "\nPurged {0} object{1} from " +

        "the current database.",


        count == 1 ? "" : "s"



    private static int PurgeDatabase(Database db)


      int idCount = 0;

      Transaction tr =


      using (tr)


        // Create the list of objects to "purge"

        ObjectIdCollection idsToPurge =

          new ObjectIdCollection();

        // Add all the Registered Application names

        RegAppTable rat =





        foreach (ObjectId raId in rat)


          if (raId.IsValid)





        // Call the Purge function to filter the list


        Document doc =


        Editor ed = doc.Editor;


          "\nRegistered applications being purged: "


        // Erase each of the objects we've been

        // allowed to

        foreach (ObjectId id in idsToPurge)


          DBObject obj =

            tr.GetObject(id, OpenMode.ForWrite);

          // Let's just add to me "debug" code

          // to list the registered applications

          // we're erasing

          RegAppTableRecord ratr =

            obj as RegAppTableRecord;

          if (ratr != null)



              "\"{0}\" ",






        // Return the number of objects erased

        // (i.e. purged)

        idCount = idsToPurge.Count;



      return idCount;




Here's what happens when you run the PC command:

Command: PC

Registered applications being purged: "AcadAnnoPO" "PE_URL" "AcadAnnoAV"


Purged 4 objects from the current database.

In the next post we'll look at extending this to - once again - work on a folder of drawings.


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