问如何在C#中发布具有两种不同文档类型和HMAC签名的Multipart/mixed:Boundary=“MIME-Boundary”[开放发票]

EN

X509Certificate2 cert = new X509Certificate2(certificateName, certPassword);
        
var handler = new WebRequestHandler();
handler.ClientCertificateOptions = ClientCertificateOption.Manual;
handler.ClientCertificates.Add(cert);
HttpClient client = new HttpClient(handler);
client.BaseAddress = new Uri(openInvoiceURL);

        //Starting from byte[] for both files
        var xmlString = System.Text.Encoding.UTF8.GetString(xmlBytes);
        string pdf64 = Convert.ToBase64String(pdfBytes);
        string url = "/docp/supply-chain/v1/invoices";
        
        var multiPartContent = new MultipartContent("mixed", "_MIME-Boundary");

        var xmlHttpContent = new StringContent(xmlString);
        xmlHttpContent.Headers.ContentType = new MediaTypeHeaderValue("application/xml");
        multiPartContent.Add(xmlHttpContent);

        var pdfHttpContent = new StringContent(pdf64);
        pdfHttpContent.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");
        pdfHttpContent.Headers.Add("content-transfer-encoding", "base64");
        pdfHttpContent.Headers.Add("Content-ID", pdfName);
        multiPartContent.Add(pdfHttpContent);
        
        var request = new HttpRequestMessage(HttpMethod.Post, url);
        request.Content = multiPartContent;
        request = AttachHMAC(request, secretKey);
        
        var response = client.SendAsync(request).Result;
        var responseString = response.Content.ReadAsStringAsync().Result;

    private HttpRequestMessage AttachHMAC(HttpRequestMessage request, string secretKey)
    {
        var payload = string.Empty;
        if(request != null && request.Content != null)
        {
            payload = request.Content.ReadAsStringAsync().Result;
        }
        
        var hashMaker = new HMACSHA256(Encoding.UTF8.GetBytes(secretKey));
        var payloadByteArray = Encoding.UTF8.GetBytes(payload);
        
        byte[] hash = hashMaker.ComputeHash(payloadByteArray);
        var base64Hash = Convert.ToBase64String(hash);

        request.Headers.Add("mac", base64Hash);
        return request;
    }