StringExtensions.cs

public static class StringExtensions
{
    /// <summary>
    /// Converts the provided app-relative path into an absolute Url containing the
    /// full host name
    /// </summary>
    /// <param name="relativeUrl">App-Relative path</param>
    /// <returns>Provided relativeUrl parameter as fully qualified Url</returns>
    /// <example>~/path/to/foo to http://www.web.com/path/to/foo</example>
    public static string ToAbsoluteUrl(this string relativeUrl)
    {
        if (string.IsNullOrEmpty(relativeUrl) || relativeUrl.StartsWith("http"))
            return relativeUrl;

        if (HttpContext.Current == null)
            return relativeUrl;

        if (relativeUrl.StartsWith("/"))
            relativeUrl = relativeUrl.Insert(0, "~");
        if (!relativeUrl.StartsWith("~/"))
            relativeUrl = relativeUrl.Insert(0, "~/");

        var url = HttpContext.Current.Request.Url;
        var port = url.Port != 80 ? (":" + url.Port) : string.Empty;

        return $"{url.Scheme}://{url.Host}{port}{VirtualPathUtility.ToAbsolute(relativeUrl)}";
    }
}