Request.ClientCertificate属性为空

Question

我正在尝试将客户端证书传递给POST请求，但 Request.ClientCertificate 属性为空（无证书）

这是我的代码（Program.cs）

 class Program
{
    static void Main(string[] args)
    {       

        var cert = GetCertificate();

        DoclinkWebClient client = new DoclinkWebClient(cert);
        string urlToSend = "http://localhost:47635/Home/PostData";

        ServicePointManager.ServerCertificateValidationCallback += ValidateRemoteCertificate;

        try
        {
            byte[] content = File.ReadAllBytes(@"D:\2528fac5e1734839b5068c225b493e7d.cms");               

            client.UploadData(urlToSend, "POST", content);

        }
        catch (WebException webException)
        {
            string additionalText = "";
            if (webException.Response != null)
            {
                Stream stream = webException.Response.GetResponseStream();
                if (stream != null)
                {
                    additionalText = new StreamReader(stream).ReadToEnd();
                }
            }

            throw new WebException(webException.Message + "." + additionalText);
        }
    }
}

DoclinkWebClient.cs

public class DoclinkWebClient : WebClient
{
    private X509Certificate2 _certificate;

    public DoclinkWebClient(X509Certificate2 certificate)
    {
        _certificate = certificate;           
    }

    protected override WebRequest GetWebRequest(Uri address)
    {            
        HttpWebRequest request = (HttpWebRequest)base.GetWebRequest(address);
        request.ClientCertificates.Add(_certificate);
        return request;        
    }
}

证书有私钥（可以签名）

为什么Web应用程序端的Request.ClientCertificate为空？

  [HttpPost]
    public ActionResult PostData(FormCollection formCollection)
    {
        var clientCertificate = Request.ClientCertificate;

        var raw = clientCertificate.Certificate;
        //raw.Length = 0;
        return null;
    }

我错过了什么？