If you've worked with ASP.NET before then you've almost certainly seen an application-relative URL like ~/SomeFolder/SomePage.aspx.  The tilde at the beginning is a stand in for the application path, and it can easily be resolved using the Page object's ResolveUrl method:

string url = Page.ResolveUrl("~/SomeFolder/SomePage.aspx");

There are times, however, when you don't have a page object available and you need to resolve an application relative URL.  Assuming you have an HttpContext object available, the following method will accomplish just that:

public static string ResolveAppRelativeUrl(string url)
{
     return url.Replace("~", System.Web.HttpContext.Current.Request.ApplicationPath);
}

It just replaces the tilde with the application path, which is essentially all the ResolveUrl method does.