In the case when we need to get the absolute url for any item, we can make use of the instance method SPSite.MakeFullUrl.

Example use,-

using(SPSite site = new SPSite(mysiteurl))
{     using(SPWeb web = site.OpenWeb())
    {
      //.. Other lines of code
      string absoluteURL = site.MakeFullUrl(serverRelativeURL);
    }
}