This should take care of most (all?) situations:

private string GetExternalUrl(ContentReference contentLink, CultureInfo language)
{
    var result = this.urlResolver.GetUrl(
        contentLink,
        language.Name,
        new VirtualPathArguments { ContextMode = ContextMode.Default, ForceCanonical = true });

    // HACK: Temprorary fix until GetUrl and ForceCanonical works as expected,
    // i.e returning an absolute URL even if there is a HTTP context that matches the page's
    // site definition and host.
    if (Uri.TryCreate(result, UriKind.RelativeOrAbsolute, out var relativeUri) &&
        relativeUri.IsAbsoluteUri == false)
    {
        var siteDefinition = this.siteDefinitionResolver.GetByContent(contentLink, true, true);
        var baseUri = siteDefinition.GetBaseUriForCulture(language);
        var absoluteUri = new Uri(baseUri, relativeUri);

        return absoluteUri.AbsoluteUri;
    }

    return result;
}

Hi Johan.

Thanks for your reply. After posting my message yesterday I came across your blog post, "How to get the external URL to content" from Jan 2016. I think it is working for me, I just need to test by setting up an additional mapping in IIS to check it on different URLs. 

My project's version of Episerver (9.5) does not contain some of the methods from the above, but the code below, which is a combination of your two samples, seems to be do what I need.

Thanks,

Peter

public static string GetExternalUrl(ContentReference contentLink, CultureInfo language = null)
        {
            if (contentLink == null)
            {
                return null;
            }

            if (language == null)
            {
                language = CultureInfo.CurrentCulture;
            }

            var result = ServiceLocator.Current.GetInstance<UrlResolver>().GetUrl(
                contentLink,
                language.Name,
                new VirtualPathArguments
                {
                    ContextMode = ContextMode.Default,
                    ForceCanonical = true
                });

            if (!Uri.TryCreate(result, UriKind.RelativeOrAbsolute, out var relativeUri) && !relativeUri.IsAbsoluteUri)
            {
                return result;
            }

            var siteDefinitionResolver = ServiceLocator.Current.GetInstance<SiteDefinitionResolver>();
            var siteDefinition = siteDefinitionResolver.GetDefinitionForContent(contentLink, true, true);

            var hosts = siteDefinition.GetHosts(language, true).ToList();
            var host = hosts.FirstOrDefault(h => h.Type == HostDefinitionType.Primary) ?? hosts.FirstOrDefault(h => h.Type == HostDefinitionType.Undefined);
            var baseUri = siteDefinition.SiteUrl;

            if (host != null && host.Name.Equals("*") == false)
            {
                // Try to create a new base URI from the host with the site's URI scheme. Name should be a valid
                // authority, i.e. have a port number if it differs from the URI scheme's default port number.
                Uri.TryCreate(siteDefinition.SiteUrl.Scheme + "://" + host.Name, UriKind.Absolute, out baseUri);
            }

            var absoluteUri = new Uri(baseUri, relativeUri);

            return absoluteUri.AbsoluteUri;
        }

Actually, it is not working for me after all. It still returns the SiteDefinition.SiteUrl every time, even when accessing in IIS using mysiteurl.local

In the end, I went with a simpler approach that works for me. Instead of using the Episerver cms values, I simply get the HttpContext.Current.Request and combine it with the Episerver ContentReference friendlyUrl retreived via the UrlResolver.GetUrl method. Full code below

public static string GetBaseUrl()
        {
            var request = HttpContext.Current.Request;
            
            return string.Format(
                "{0}://{1}",
                request.Url.Scheme,
                request.Url.Authority
            );
        }

        public static string GetExternalUrl(ContentReference contentLink)
        {
            var baseUrl = GetBaseUrl();

            if (baseUrl.EndsWith("/"))
            {
                baseUrl = baseUrl.TrimEnd('/');
            }

            var contentPath = ServiceLocator.Current.GetInstance<UrlResolver>().GetUrl(contentLink);

            if (!contentPath.StartsWith("/"))
            {
                contentPath += '/';
            }

            return string.Concat(baseUrl, contentPath);
        }