Generating preview images for all blocks in an AutoCAD drawing using .NET

This was a nice little solution I saw provided recently by Viru Aithal from DevTech India. It’s a simple command to open a drawing and generate preview icons for each of the blocks it contains. It forces an icon to be generated – when it doesn’t already exist – via Viru’s old trick of using InvokeMember() to call IAcadDocument::SendCommand() via COM (which is synchronous, where Document.SendStringToExecute() is asynchronous and only executes once the command has completed).

Viru’s trick allows him to call SendCommand() without creating a dependency on the AutoCAD Type Library (something developers often prefer to avoid, as it simplifies supporting multiple AutoCAD product versions). A similar effect can presumably be achieved using Type Embedding, but I like Viru’s approach for just the odd function call.

Here’s the C# code:

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.Runtime;
using System.IO;
 
namespace BlockPreviews
{
  publicclassCommands
  {
    [CommandMethod("GBP", CommandFlags.Session)]
    staticpublicvoid GenerateBlockPreviews()
    {
      Editor ed =
        Application.DocumentManager.MdiActiveDocument.Editor;
 
      PromptFileNameResult res =
        ed.GetFileNameForOpen(
          "Select file for which to generate previews"
        );
      if (res.Status != PromptStatus.OK)
        return;
 
      Document doc = null;
 
      try
      {
        doc =
          Application.DocumentManager.Open(res.StringResult, false);
      }
      catch
      {
        ed.WriteMessage("\nUnable to read drawing.");
        return;
      }
 
      Database db = doc.Database;
 
      string path = Path.GetDirectoryName(res.StringResult),
             name = Path.GetFileName(res.StringResult),
             iconPath = path + "\\" + name + " icons";
 
      int numIcons = 0;
 
      Transaction tr =
        doc.TransactionManager.StartTransaction();
      using (tr)
      {
        BlockTable table =
          (BlockTable)tr.GetObject(
            db.BlockTableId, OpenMode.ForRead
          );
 
        foreach (ObjectId blkId in table)
        {
          BlockTableRecord blk =
            (BlockTableRecord)tr.GetObject(
              blkId, OpenMode.ForRead
            );
 
          // Ignore layouts and anonymous blocks
 
          if (blk.IsLayout || blk.IsAnonymous)
            continue;
 
          // Attempt to generate an icon, where one doesn't exist
 
          if (blk.PreviewIcon == null)
          {
            object ActiveDocument = doc.AcadDocument;
            object[] data = { "_.BLOCKICON " + blk.Name + "\n" };
            ActiveDocument.GetType().InvokeMember(
              "SendCommand",
              System.Reflection.BindingFlags.InvokeMethod,
              null, ActiveDocument, data
            );
          }
 
          // Hopefully we now have an icon
 
          if (blk.PreviewIcon != null)
          {
            // Create the output directory, if it isn't yet there
 
            if (!Directory.Exists(iconPath))
              Directory.CreateDirectory(iconPath);
 
            // Save the icon to our out directory
 
            blk.PreviewIcon.Save(
               iconPath + "\\" + blk.Name + ".bmp"
            );
 
            // Increment our icon counter
 
            numIcons++;
          }
        }
        tr.Commit();
      }
      doc.CloseAndDiscard();
 
      ed.WriteMessage(
        "\n{0} block icons saved to \"{1}\".", numIcons, iconPath
      );
    }
  }
}

When we run the GBP command use it to select a number of AutoCAD’s sample files containing blocks, we see it generates a number of icons for each:

Command: GBP
13 block icons saved to "C:\Program Files\Autodesk\AutoCAD 2012 - 
English\Sample\Dynamic Blocks\Annotation - Imperial.dwg icons".
Command: GBP
10 block icons saved to "C:\Program Files\Autodesk\AutoCAD 2012 - 
English\Sample\Dynamic Blocks\Architectural - Imperial.dwg icons".
Command: GBP
6 block icons saved to "C:\Program Files\Autodesk\AutoCAD 2012 - 
English\Sample\Dynamic Blocks\Electrical - Imperial.dwg icons".
Command: GBP
11 block icons saved to "C:\Program Files\Autodesk\AutoCAD 2012 - 
English\Sample\Dynamic Blocks\Civil - Imperial.dwg icons".

While the code executes, you’ll see the BLOCKICON command displaying a dialog for each block it processes, but once completed the drawing is closed (without being saved to disk).

Here’s what we see in Windows Explorer: