Many times I've looked for a way to convert an app relative url (such as one using the ~/ app root notation) to an absolute path without ResolveUrl. If the code is executing outside a Control, for example in an IHttpHandler or business layer code somewhere that has no reference to a Control, you can't call Control.ResolveUrl.
Today, I discovered a way to do this without a Control reference. The System.Web.VirtualPathUtility class has some very useful methods for manipulating paths, including one that converts from an app relative path to an absolute path. Instead of Control.ResolveUrl, just call VirtualPathUtility.ToAbsolute:
string url =...