How can i use sharepoint class in asp.net web application

Hi.

I have a file search application which I made with asp.net.

And I must　modify this application use the sharepoint search engine

i wanted know if i can use sharepoint class in Asp.net project

New Project -> ASP.net WEB application -> add web form.aspx -> write source aspx.cs file(SPSite site = new SPSite("http://myserverpc23:2222/");) -> build

errors submit is can`t find http://myserverpc23:2222/ (sorry i use Japaness version so i don`t know english error message)

http://myserverpc23:2222/ is Sharepoint Site

I can Access http://myserverpc23:2222/ And No problem if i tried SPSite site = new SPSite("http://myserverpc23:2222/"); in Sharepoint Web part project .

It is an errorif I usetheSPSiteinASP.netproject.

please teach to me