IIS/ASP/ASP.net: How to structure web-site to expose mobile version

Pretend i have an existing web-site, e.g.:

www.stackoverflow.com

i now want to expose a mobile version of this web-site:

m.stackoverflow.com

IIS, with its host-header name resolution, would normally require two web-sites to be created:

www.stackoverflow.com
m.stackoverflow.com

Except now i have two web-sites in IIS. This means i have to duplicate code/files between them. i don't need to (nor do i want to) duplicate all the "model" and "controller" code between two web-sites. i would much rather have one web-site that exposes a mobile version.

i could have the default page in m.stackoverflow.com simply perform a redirect to a mobile landing page on the "real" web-site:

m.stackoverflow.com\default.asp:

 <% Response.Redirect "www.stackoverflow.com/mobile" %>

Then the client will end up at (for example) www.stackoverflow.com/mobile/default.aspx.

This isn't what i want. i want it to appear to the browser that they are visiting m.stackoverflow.com.

So what i could do in IIS is have two host-header names for one IIS web-site:

stackoverflow.com
m.stackoverflow.com

and inspect the http-request HOST header:

GET https://stackoverflow.com/questions/ViewQuestion.aspx?qid=3623844
Host: stackoverflow.com

verses

GET https://stackoverflow.com/questions/ViewQuestion.aspx?qid=3623844
Host: m.stackoverflow.com

Except now my all my web-site pages must first inspect Host attribute, and then change rendering behavior depending on which it finds. This works somewhat well in classic ASP:

ViewQuestion.asp

<% Dim mobileVersion 

    ...

If MobileVersion Then
%>
<html>
...
</html>
<% Else %>
<html>
...
</html>
<% End If %>

But making dual view's in one page is very painful. i would prefer to have a ViewQuestion view, that is dedicated to showing a regular or mobile view.

i also know that i can't use the media=handheld media type.

not only do i not want to have "full" page content being sent to mobile clients (wasting their bandwidth and download speed) and then hide content based on CSS
not all handhelds (e.g. iPhone) honor the media=handheld media type

So what is the accepted combination of source-code and IIS configuration that best supports mobile versions of a web-site?

Edit One: Removed title reference to MVC. Don't want people to assume the use of some particular MVC framework, since i'm not using any MVC framework. i'm using ASP/ASP.net.

IIS/ASP/ASP.net: How to structure web-site to expose mobile version

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