UrlHelper.ToAbsolute

gistfile1.cs

public static string ToAbsolute(this UrlHelper urlHelper, string appRelativeUrl,
    string relativeHostName = null, bool? useSecureConnection = null) {
    var httpContext = urlHelper.RequestContext.HttpContext;

    if(string.IsNullOrWhiteSpace(relativeHostName) && httpContext.Request == null) {
        throw new ArgumentException("The provided host-name cannot be null or empty if there is no current HttpContext.", "relativeHostName");
    }

    relativeHostName = (relativeHostName ?? string.Empty).Trim(new[] { '/' });
    if(!Uri.IsWellFormedUriString(relativeHostName, UriKind.Relative)) {
        throw new ArgumentException("The provided host-name must be a well-formed relative URI.", "relativeHostName");
    }            

    bool isSecure = (useSecureConnection.HasValue) ? useSecureConnection.Value
        : ((httpContext != null) ?  httpContext.Request.IsSecureConnection : false);
    string protocol = isSecure ? "https://" : "http://";


    string domain = (!string.IsNullOrWhiteSpace(relativeHostName))
        ? relativeHostName : httpContext.Request.Url.Host;

    string relativeUrl = urlHelper.Content(appRelativeUrl);

    string absoluteUrl = string.Format("{0}{1}/{2}", protocol, domain, relativeUrl);
    return absoluteUrl;
}