我想在CAD中拾取一个实体，得到该实体的颜色，然后再选择同颜色的物体，请给我一段C#示例代码，实现上述功能

好的，以下是一段C#示例代码，实现了您的需求：

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.Colors;
using System.Collections.Generic;

namespace PickColorExample
{
    public class Commands
    {
        [CommandMethod("PickColor")]
        public void PickColor()
        {
            Document doc = Application.DocumentManager.MdiActiveDocument;
            Editor ed = doc.Editor;

            // 选择一个实体
            PromptEntityOptions peo = new PromptEntityOptions("\n请选择一个实体：");
            peo.SetRejectMessage("\n只能选择一个实体！");
            peo.AddAllowedClass(typeof(Entity), true);
            PromptEntityResult per = ed.GetEntity(peo);

            if (per.Status == PromptStatus.OK)
            {
                using (Transaction tr = doc.TransactionManager.StartTransaction())
                {
                    // 获取实体的颜色
                    Entity ent = (Entity)tr.GetObject(per.ObjectId, OpenMode.ForRead);
                    Color color = ent.Color.ColorValue;

                    // 找到与该颜色相同的所有实体
                    List<ObjectId> ids = new List<ObjectId>();
                    BlockTable bt = (BlockTable)tr.GetObject(doc.Database.BlockTableId, OpenMode.ForRead);
                    foreach (ObjectId id in bt)
                    {
                        BlockTableRecord btr = (BlockTableRecord)tr.GetObject(id, OpenMode.ForRead);
                        foreach (ObjectId entId in btr)
                        {
                            Entity e = (Entity)tr.GetObject(entId, OpenMode.ForRead);
                            if (e.Color.ColorValue == color)
                            {
                                ids.Add(entId);
                            }
                        }
                    }

                    // 输出结果
                    ed.WriteMessage("\n选择的实体颜色为：" + color.ToString());
                    ed.WriteMessage("\n与该颜色相同的实体有：" + ids.Count.ToString() + " 个");
                    foreach (ObjectId id in ids)
                    {
                        Entity e = (Entity)tr.GetObject(id, OpenMode.ForRead);
                        ed.WriteMessage("\n" + e.GetType().ToString() + "，颜色为：" + e.Color.ColorValue.ToString());
                    }

                    tr.Commit();
                }
            }
        }
    }
}

您可以将以上代码保存为一个C#文件，然后在AutoCAD中使用NETLOAD命令加载该程序集。在命令行输入PickColor，按回车键即可执行该命令。