Add Functionality Code and Build the Assembly

Civil 3D plugin sample code

using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.ApplicationServices.Core;
  
using Autodesk.Civil.ApplicationServices;
[assembly: ExtensionApplication(C3D_Plugin.C3D_Plugin)]
[assembly: CommandClass(C3D_Plugin.C3D_Plugin)] 
namespace C3D_Plugin
{
    public class C3D_Plugin : IExtensionApplication
    {
        public void Initialize()
        {
        }
  
        public void Terminate()
        {
        }
  
        [CommandMethod("HelloCivil3D")]
        public void HelloCivil3D()
        {           
            Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage("\nHello Civil 3D! It's a Civil 3D plugin.\n");
  
            CivilDocument doc = CivilApplication.ActiveDocument;
            ObjectIdCollection alignments = doc.GetAlignmentIds();
            ObjectIdCollection sites = doc.GetSiteIds();
            string docInfo = string.Format("\nThis document has {0} alignments and {1} sites.\n", alignments.Count, sites.Count);
            Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage(docInfo);
        }
    }
}