这是我一直以来的方法来这个小滋扰。注意，这里使用的VirtualPathUtility.ToAbsolute(relativeUrl)允许该方法被声明为静态类的扩展。

  ///＆LT;总结＆gt;
///所提供的应用程序相对路径转换为包含绝对URL
///完整主机名
///＆LT; /总结＆gt;
///＆LT; PARAM NAME =relativeUrl＆GT;应用程序相对路径和LT; /参数＆GT;
///＆LT;返回＆GT;提供relativeUrl参数作为完整的URL和LT; /回报＆GT;
///＆LT;示例＆gt;〜/路径/要/富到http://www.web.com/path/to/foo</example>
公共静态字符串ToAbsoluteUrl（此字符串relativeUrl）{
    如果（string.IsNullOrEmpty（relativeUrl））
        返回relativeUrl;

    如果（HttpContext.Current == NULL）
        返回relativeUrl;

    如果（relativeUrl.StartsWith（/））
        relativeUrl = relativeUrl.Insert（0，〜）;
    如果（！relativeUrl.StartsWith（〜/））
        relativeUrl = relativeUrl.Insert（0，〜/）;

    VAR URL = HttpContext.Current.Request.Url;
    VAR端口= url.Port！= 80？ （：+ url.Port）：的String.Empty;

    返回的String.Format（{0}：// {1} {2} {3}，
        url.Scheme，url.Host，港口，VirtualPathUtility.ToAbsolute（relativeUrl））;
}
 

I am really surprised that there is no native .NET method to get an absolute url from a relative url. I know this has been discussed many times, but never have come across a satisfactory method that handles this well. Can you help fine tune the method below?

I think all I need left is to auto choose the protocol instead of hard coding it (http/https). Anything else I am missing (caveats, performance, etc)?

public static string GetAbsoluteUrl(string url)
    {
        //VALIDATE INPUT FOR ALREADY ABSOLUTE URL
        if (url.StartsWith("http://", StringComparison.OrdinalIgnoreCase) 
           || url.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
        { 
            return url;
        }

        //GET PAGE REFERENCE FOR CONTEXT PROCESSING
        Page page = HttpContext.Current.Handler as Page;

        //RESOLVE PATH FOR APPLICATION BEFORE PROCESSING
        if (url.StartsWith("~/"))
        {
            url = page.ResolveUrl(url);
        }

        //BUILD AND RETURN ABSOLUTE URL
        return "http://" + page.Request.ServerVariables["SERVER_NAME"] + "/" 
                         + url.TrimStart('/');
    }

This has always been my approach to this little nuisance. Note the use of VirtualPathUtility.ToAbsolute(relativeUrl) allows the method to be declared as an extension in a static class.

/// <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))
        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 String.Format("{0}://{1}{2}{3}",
        url.Scheme, url.Host, port, VirtualPathUtility.ToAbsolute(relativeUrl));
}