如下图：根据提示选择若干图形要素，空格或右键结束选择，返回图元的objectid，以便进一步操作图元实体。

 代码如下：

using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
using System.Collections.Generic;
using Autodesk.AutoCAD.ApplicationServices;
namespace sc
{
    public class Class1
    {
        public static ObjectId GetEntity(string message)
        {
            Editor ed = Application.DocumentManager.MdiActiveDocument.Editor;
            PromptEntityOptions po = new PromptEntityOptions(message);
            PromptEntityResult pr = ed.GetEntity(po);
            return pr.ObjectId;
        }
        public static List<ObjectId> SelectEntities(string message)
        {
            Editor ed = Application.DocumentManager.MdiActiveDocument.Editor;
            List<ObjectId> objectIds = new List<ObjectId>();

            PromptSelectionOptions pso = new PromptSelectionOptions();
            pso.AllowDuplicates  = true; // 允许用户选择多个实体  
            //pso.UsePickset = true;    // 使用Pickset模式，这样可以返回用户选择的所有实体  

            PromptSelectionResult psr = ed.GetSelection(pso);
            
            if (psr.Status == PromptStatus.OK)
            {
                SelectionSet selectionSet = psr.Value;
                foreach (SelectedObject selectedObj in selectionSet)
                {
                    objectIds.Add(selectedObj.ObjectId);
                }
            }

            return objectIds;
        }
        [CommandMethod("xx")]
        public void Msc()
        {
              List<ObjectId> objectIds = SelectEntities("请选择若干实体：");
            Editor ed = Application.DocumentManager.MdiActiveDocument.Editor;
            try
            {
                foreach (ObjectId id in objectIds)
                {
                    ed.WriteMessage($"\nObjectId: {id}");
                }
            }
            catch (System.Exception ex)
            {
                ed.WriteMessage($"\n错误: {ex.Message}");
            }
        }
    }
}