You could just pass the url as a query string parameter to the page in the subsite. Wouldn't something like this work? http://myportal/mysite/mysubsite/default.aspx?refurl=http://myportal/mysite

In your code on the subsite page, you can read the querystring and create your site object using that. The query strings should be retained on postback.

You could just pass the url as a query string parameter to the page in the subsite. Wouldn't something like this work? http://myportal/mysite/mysubsite/default.aspx?refurl=http://myportal/mysite

In your code on the subsite page, you can read the querystring and create your site object using that. The query strings should be retained on postback.

UPDATE: Maybe I misunderstood the problem. As I understand it now, you are trying to get the url of the subsite in which the url resides... Try the following:

using (SPSite siteCollection = new SPSite("http://MyServer/MySite/MySubSite/MyDocLib/Page.aspx"))
{
SPWeb myWeb = siteCollection.OpenWeb();
string weburl = myWeb.Url;
}