C # HttpClient Existing connection was forcibly closed by remote host

Question

I am working on integration with Alternative payments using their page integration on the page . Their C # SDK does not currently have this integration, but as you can see it is quite simple, and I created a small class to send a publish request and receive a JSON response.

I tested the json object that I send to PostMan and cURL and both work, as well as the authentication header, so I think they are not a problem. Here is the constructor of my class:

public AlternativePaymentsCli(string apiSecretKey)
{
    this._apiSecretKey = apiSecretKey;

    _httpClient = new HttpClient();
    _httpClient.DefaultRequestHeaders.Accept
        .Add(new MediaTypeWithQualityHeaderValue("application/json"));

    var authInfo = _apiSecretKey;
    authInfo = Convert.ToBase64String(System.Text.ASCIIEncoding.ASCII.GetBytes(string.Format("{0}:", _apiSecretKey)));

    // The two line below because I saw in an answer on stackoverflow.
    _httpClient.DefaultRequestHeaders.Add("Connection", "Keep-Alive"); 
    _httpClient.DefaultRequestHeaders.Add("Keep-Alive", "3600");

    _httpClient.DefaultRequestHeaders.UserAgent.ParseAdd("Anything.com custom client v1.0");
    _httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", authInfo);

}

And the method by which I publish the data:

public string CreateHostedPageTransaction(HostedPageRequest req) 
{
    var settings = new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore };

    // I send this same json content on PostMan and it works. The json is not the problem
    var content = new StringContent(JsonConvert.SerializeObject(req, settings), Encoding.UTF8, "application/json");
    var response = _httpClient.PostAsync(this._baseUrl + "/transactions/hosted", content).Result;
    var responseText = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();

    if (response.IsSuccessStatusCode)
        return responseText;

    return "";
}

Then I get this error: An existing connection was forcibly closed by the remote hostin the PostAsync line. These are the error details:

[SocketException (0x2746): An existing connection was forcibly closed by the remote host]
   System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult) +8192811
   System.Net.Sockets.NetworkStream.EndRead(IAsyncResult asyncResult) +47

[IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.]
   System.Net.TlsStream.EndWrite(IAsyncResult asyncResult) +294
   System.Net.ConnectStream.WriteHeadersCallback(IAsyncResult ar) +149

[WebException: The underlying connection was closed: An unexpected error occurred on a send.]
   System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context) +324
   System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar) +137

[HttpRequestException: An error occurred while sending the request.]

# 4.5, Asp.Net MVC. , . ?

Answer 1

, _baseUrl, , -. , , , URL- - HTTPS. TLS 1.0 TLS 1.0, . , # 4.6 TLS 1.0/1.1/1.2 , , # 4.6 - SSL3/TLS 1.0, TLS 1.1 1.2. , TLS 1.1 1.2 , .

System.Net.ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

Answer 2

.Net 4.0, SecurityProtocolType.Tls11 SecurityProtocolType.Tls2 , .

ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;

Answer 3

- , :

- .Net 4. 6+, web.config :

<system.web>
  <compilation targetFramework="4.6" /> 
  <httpRuntime targetFramework="4.6" /> 
</system.web>