Changing the colour of the contents of an AutoCAD block using .NET

Here's a question that came in to us, recently:

How can I show the AutoCAD color dialog from .NET? I need to allow the user to select a block, show the AutoCAD color dialog and apply the selected color to the contents of the selected block.

A new member of DevTech Americas - Augusto Gonçalves, who's based in our São Paulo office - answered with the following code (which I've modified slightly, mostly to follow this blog's coding conventions). Thanks, Augusto!

By the way, these previous posts may also be useful to those interested in this topic.

Here's the C# code:

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.Windows;
using Autodesk.AutoCAD.Colors;

namespace ColorPicking
{
  public class Commands
  {
    [CommandMethod("CB")]
    public void ColorBlock()
    {
      Document doc =
          Application.DocumentManager.MdiActiveDocument;
      Database db = doc.Database;
      Editor ed = doc.Editor;

      // Ask the user to select a block

      PromptEntityOptions peo =
        new PromptEntityOptions("\nSelect a block:");      
      peo.AllowNone = false;
      peo.SetRejectMessage("\nMust select a block.");
      peo.AddAllowedClass(typeof(BlockReference), false);

      PromptEntityResult per =
        ed.GetEntity(peo);

      if (per.Status != PromptStatus.OK)
        return;

      // Open the entity using a transaction

      Transaction tr =
        db.TransactionManager.StartTransaction();
      using (tr)
      {
        try
        {
          Entity ent =
            (Entity)tr.GetObject(
              per.ObjectId,
              OpenMode.ForRead
            );

          // Should always be a block reference,
          // but let's make sure

          BlockReference br = ent as BlockReference;
          if (br != null)
          {
            // Select the new color

            ColorDialog cd = new ColorDialog();
            cd.IncludeByBlockByLayer = true;
            cd.ShowDialog();

            // Change the color of the block itself

            br.UpgradeOpen();
            br.Color = cd.Color;

            // Change every entity to be of color ByBlock

            BlockTableRecord btr =
              (BlockTableRecord)tr.GetObject(
                br.BlockTableRecord,
                OpenMode.ForRead
              );

            // Iterate through the BlockTableRecord contents

            foreach (ObjectId id in btr)
            {
              // Open the entity

              Entity ent2 =
                (Entity)tr.GetObject(id, OpenMode.ForWrite);

              // Change each entity's color to ByBlock

              ent2.Color =
                Color.FromColorIndex(ColorMethod.ByBlock, 0);
            }
          }

          // Commit if there hasn't been a problem
          // (even if no objects changed: it's just quicker)

          tr.Commit();
        }
        catch (Autodesk.AutoCAD.Runtime.Exception e)
        {
          // Something went wrong

          ed.WriteMessage(e.ToString());
        }
      }
    }
  }
}

Here's a quick example of running the CB command on a block inserted from the "Architectural - Imperial" sample block library that ships with AutoCAD.

After launching the CB command selecting our sports car, we can see the colour selection dialog, which allows us to select an AutoCAD colour index, a true color or a standard colour from a color book:

We can then see our block is changed to this colour (well, in fact the block is changed to be this colour and all its contents are all changed to be coloured ByBlock):

This block happens to be a dynamic block, so if we change it to its truck representation, we see the colour has propagated there, also (as the geometry for this view also resides in the block table record, of course):