Absolute URL given HttpContext, action, and controller

gistfile1.cs

// Assuming you have an HttpContext named httpContext
var urlHelper = new UrlHelper(httpContext.Request.RequestContext, RouteTable.Routes);
var url = urlHelper.Action("ActionName", "ControllerName");

// Or you can get really explicit and include the scheme and host
url = urlHelper.Action("ActionName", "ControllerName", null, httpContext.Request.Url.Scheme, httpContext.Request.Url.Host);