对于真正需要绝对路径并支持负载平衡系统的每个来到这里的人来说，这是我想出的：
//http://stackoverflow.com/questions/126242/how-do-i-turn-a-relative-url-into-a-full-url
public static string AbsoluteAction(this UrlHelper url, string actionName, string controllerName, object routeValues = null)
{
Uri publicFacingUrl = GetPublicFacingUrl(url.RequestContext.HttpContext.Request, url.RequestContext.HttpContext.Request.ServerVariables);
string relAction = url.Action(actionName, controllerName, routeValues);
//this will always have a / in front of it.
var newPort = publicFacingUrl.Port == 80 || publicFacingUrl.Port == 443 ? "" : ":"+publicFacingUrl.Port.ToString();
return publicFacingUrl.Scheme + Uri.SchemeDelimiter + publicFacingUrl.Host + newPort + relAction;
}
然后，从https://github.com/aarnott/dotnetopenid/blob/v3.4/src/DotNetOpenAuth/Messaging/HttpRequestInfo.cs 到http://go4answers.webhost4life.com/Example/azure-messing-port-numbers-creates-28516.aspx
/// <summary>
/// Gets the public facing URL for the given incoming HTTP request.
/// </summary>
/// <param name="request">The request.</param>
/// <param name="serverVariables">The server variables to consider part of the request.</param>
/// <returns>
/// The URI that the outside world used to create this request.
/// </returns>
/// <remarks>
/// Although the <paramref name="serverVariables"/> value can be obtained from
/// <see cref="HttpRequest.ServerVariables"/>, it's useful to be able to pass them
/// in so we can simulate injected values from our unit tests since the actual property
/// is a read-only kind of <see cref="NameValueCollection"/>.
/// </remarks>
internal static Uri GetPublicFacingUrl(HttpRequestBase request, NameValueCollection serverVariables)
{
//Contract.Requires<ArgumentNullException>(request != null);
//Contract.Requires<ArgumentNullException>(serverVariables != null);
// Due to URL rewriting, cloud computing (i.e. Azure)
// and web farms, etc., we have to be VERY careful about what
// we consider the incoming URL. We want to see the URL as it would
// appear on the public-facing side of the hosting web site.
// HttpRequest.Url gives us the internal URL in a cloud environment,
// So we use a variable that (at least from what I can tell) gives us
// the public URL:
if (serverVariables["HTTP_HOST"] != null)
{
//ErrorUtilities.VerifySupported(request.Url.Scheme == Uri.UriSchemeHttps || request.Url.Scheme == Uri.UriSchemeHttp, "Only HTTP and HTTPS are supported protocols.");
string scheme = serverVariables["HTTP_X_FORWARDED_PROTO"] ?? request.Url.Scheme;
Uri hostAndPort = new Uri(scheme + Uri.SchemeDelimiter + serverVariables["HTTP_HOST"]);
UriBuilder publicRequestUri = new UriBuilder(request.Url);
publicRequestUri.Scheme = scheme;
publicRequestUri.Host = hostAndPort.Host;
publicRequestUri.Port = hostAndPort.Port; // CC missing Uri.Port contract that's on UriBuilder.Port
return publicRequestUri.Uri;
}
// Failover to the method that works for non-web farm enviroments.
// We use Request.Url for the full path to the server, and modify it
// with Request.RawUrl to capture both the cookieless session "directory" if it exists
// and the original path in case URL rewriting is going on. We don't want to be
// fooled by URL rewriting because we're comparing the actual URL with what's in
// the return_to parameter in some cases.
// Response.ApplyAppPathModifier(builder.Path) would have worked for the cookieless
// session, but not the URL rewriting problem.
return new Uri(request.Url, request.RawUrl);
}