External URL Helpers

UrlExtensions.cs

using System;
using System.Web;
using System.Web.Mvc;

public static class UrlExtensions
{
    public static string ExternalAction(this UrlHelper url, string actionName, string controllerName, object routeValues = null)
    {
        var requestUrl = url.Action(actionName, controllerName, routeValues);
        return ExternalUrl(url, requestUrl);
    }

    public static string ExternalUrl(this UrlHelper url, string relativeUrl)
    {
        return ExternalUrl(url.RequestContext.HttpContext.Request, relativeUrl);
    }

    public static string ExternalUrl(this HtmlHelper html, string relativeUrl)
    {
        return ExternalUrl(html.ViewContext.HttpContext.Request, relativeUrl);
    }

    public static string ExternalUrl(this HttpRequestBase request, string relativeUrl)
    {
        return UrlHelpers.ExternalUrl(request.Url, relativeUrl);
    }

}

UrlHelpers.cs

using System;
using System.Web;

public static class UrlHelpers
{
    public static string ExternalUrl(string requestUrl, string relativeUrl, bool forcePort80 = false)
    {
        return ExternalUrl(new Uri(requestUrl), relativeUrl, forcePort80);
    }

    public static string ExternalUrl(Uri requestUrl, string relativeUrl, bool forcePort80 = false)
    {
        var absoluteUrl = relativeUrl;

        if(relativeUrl.StartsWith("~/"))
            absoluteUrl = VirtualPathUtility.ToAbsolute(relativeUrl);

        var hostPath = new UriBuilder
        {
            Host = requestUrl.Host,
            Path = "/",
            Port = requestUrl.Port,
            Scheme = requestUrl.Scheme,
        };

        if(forcePort80)
            hostPath.Port = 80;

        return new Uri(hostPath.Uri, absoluteUrl).AbsoluteUri;
    }
}