问c#中的服务器变量中缺少HTTP_X_FORWARDED_FOR

EN

public string GetIPAddress()
{
    System.Web.HttpContext context = System.Web.HttpContext.Current;
    string ipAddress = context.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];

    if (!string.IsNullOrEmpty(ipAddress))
    {
        string[] addresses = ipAddress.Split(',');
        if (addresses.Length != 0)
        {
            return addresses[0];
        }
    }

    return context.Request.ServerVariables["REMOTE_ADDR"];
}

public static string GetUserIP() {   
    var ip = ( HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] != null
          && HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] != "" )
         ? HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"]
         : HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
    if (ip.Contains( "," ))
        ip = ip.Split( ',' ).First();
    return ip.Trim();
}