What's the difference between request made by RestClient and HttpClient on .NET Core

Question

I'm working on a bank integration in a project and stumbled upon a weird error.

I've a pretty simple JSON which looks like this:

{
    "MbrId": "*",
    "MerchantID": "0*********04",
    "UserCode": "Q******3DPAY",
    "UserPass": "U****",
    "OrderId": "T********3",
    "SecureType": "N****e",
    "InstallmentCount": "**",
    "TxnType": "****",
    "PurchAmount": "****",
    "Currency": "***",
    "CardHolderName": "*****e",
    "Pan": "4*********11",
    "Expiry": "****",
    "Cvv2": "****",
    "Lang": "**"
}

Sending this from POSTMAN works and it returns a success response. I also installed RestClient nuGet package. Copying and pasting the code on Postman also worked:

var client = new RestSharp.RestClient(" BANK'S URI ");
client.Timeout = -1;
var request = new RestRequest(Method.POST);

request.AddHeader("Content-Type", "application/json");

// I take the info from the object
request.AddParameter("application/json", $"{{\"MbrId\":\"{_paymentInfo.MbrId}\",\"MerchantID\":\"{_paymentInfo.MerchantID}\",\"UserCode\":\"{_paymentInfo.UserCode}\"," +
            $"\"UserPass\":\"{_paymentInfo.UserPass}\",\"OrderId\":\"{_paymentInfo.OrderId}\",\"SecureType\":\"{_paymentInfo.SecureType}\",\"TxnType\":\"{_paymentInfo.TxnType}\"," +
            $"\"PurchAmount\":\"{_paymentInfo.PurchAmount}\",\"Currency\":\"{_paymentInfo.Currency}\",\"CardHolderName\":\"{_paymentInfo.CardHolderName}\",\"Pan\":\"{_paymentInfo.Pan}\"," +
            $"\"Expiry\":\"{_paymentInfo.Expiry}\",\"Cvv2\":\"{_paymentInfo.Cvv2}\",\"Lang\":\"{_paymentInfo.Lang}\"}}",
ParameterType.RequestBody);

IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);

The problem is I cannot do the same with HttpClient:

• I tried to use httpClient.PostAsJsonAsync() method which resulted in 403 response.

• Tried to send request with PostAsync like this:

      var response = await _httpClient.PostAsync("URI", new StringContent(System.Text.Json.JsonSerializer.Serialize(_paymentInfo), Encoding.UTF8, "application/json;"));
      var resultString = await response.Content.ReadAsStringAsync();
  

  This resulted in 200 OK but bank's respond said plugin not found and request couldn't be handled.

• I also tried this:

   var x = $"{{\"MbrId\":\"{_paymentInfo.MbrId}\",\"MerchantID\":\"{_paymentInfo.MerchantID}\",\"UserCode\":\"{_paymentInfo.UserCode}\"," +
              $"\"UserPass\":\"{_paymentInfo.UserPass}\",\"OrderId\":\"{_paymentInfo.OrderId}\",\"SecureType\":\"{_paymentInfo.SecureType}\",\"TxnType\":\"{_paymentInfo.TxnType}\"," +
              $"\"PurchAmount\":\"{_paymentInfo.PurchAmount}\",\"Currency\":\"{_paymentInfo.Currency}\",\"CardHolderName\":\"{_paymentInfo.CardHolderName}\",\"Pan\":\"{_paymentInfo.Pan}\"," +
              $"\"Expiry\":\"{_paymentInfo.Expiry}\",\"Cvv2\":\"{_paymentInfo.Cvv2}\",\"Lang\":\"{_paymentInfo.Lang}\"}}";
  
          _httpClient.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
  
          HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, "BANK'S URI");
  
          req.Content = new StringContent(x, Encoding.UTF8, "application/json");
  
          var response = _httpClient.SendAsync(req).Result;
  
  
          var resultString = await response.Content.ReadAsStringAsync();
  

What's the difference between the HttpClient and RestClient? I tried to send same content made in Postman with HttpClient and it resulted in same error: Plugin Not Found. Is there something that I miss? Both Method is POST, content type is "application/json", encoding is UTF8. There are no auth or anything since it's the test endpoint. But supposedly same request works on RestClient while standard HttpClient does not.

---

Here is the bank's response on error:

{
  "PaymentRequest": {
    "RequestGuid": 0,
    "InsertDatetime": "0001-01-01T00:00:00",
    "MbrId": 101,
    "RequestIp": "my IP",
    "RequestStat": "1",
    "Currency": 0,
    "BonusAmount": "",
    "InstallmentCount": 0,
    "Ecommerce": 1,
    "ErrMsg": "Plugin not found",
    "ProcReturnCode": "M042",
    "BatchNo": 0,
    "ReqId": 0,
    "UsedPoint": 0.0,
    "SrcType": "VPO",
    "RefundedAmount": 0.0,
    "RefundedPoint": 0.0,
    "ReqDate": 0,
    "SysDate": 0,
    "F11": 0,
    "VposElapsedTime": 0,
    "BankingElapsedTime": 0,
    "SocketElapsedTime": 0,
    "HsmElapsedTime": 0,
    "MpiElapsedTime": 0,
    "hasOrderId": false,
    "TemplateType": 0,
    "HasAddressCount": false,
    "IsPaymentFacilitator": false,
    "F11_ORG": 0,
    "F12_ORG": 0,
    "F22_ORG": 0,
    "F25_ORG": 0,
    "MTI_ORG": 0,
    "IntervalType": 0,
    "IntervalDuration": 0,
    "RepeatCount": 0,
    "RequestClientIp": "my Ip",
    "VoidTime": 0,
    "PaymentLinkId": 0
  },
  "PaymentAddress": {
    "RequestGuid": 0
  },
  "ExtraParameters": [
    [
      "INCOMING_DATA",
      "<INCOMING_DATA></INCOMING_DATA>"
    ]
  ]
}

and here is a expected response: (I masked some info)

{
    "PaymentRequest": {
        "RequestGuid": 1000000060391232,
        "InsertDatetime": "0001-01-01T00:00:00",
        "MbrId": **,
        "MerchantID": "0********04",
        "OrderId": "T*******13",
        "RequestIp": "MY Ip",
        "RequestStat": "1,10",
        "SecureType": "NonSecure",
        "PurchAmount": "0.1",
        "Exponent": "2",
        "Currency": 949,
        "TerminalID": "******",
        "TxnType": "Auth",
        "CardType": "V",
        "Lang": "**",
        "BonusAmount": "",
        "InstallmentCount": 0,
        "AlphaCode": "**",
        "Ecommerce": 1,
        "MrcCountryCode": "****",
        "MrcName": "*****",
        "MerchantHomeUrl": " BANK'S URI",
        "CardHolderName": "B********e",
        "TxnStatus": "Y",
        "ErrMsg": "Success",
        "TxnResult": "Success",
        "AuthCode": "S66172",
        "ProcReturnCode": "00",
        "BatchNo": 2285,
        "CardMask": "415565******6111",
        "ReqId": 6947449,
        "UsedPoint": 0.0,
        "SrcType": "VPO",
        "RefundedAmount": 0.0,
        "RefundedPoint": 0.0,
        "ReqDate": 20210614,
        "SysDate": 20210614,
        "F11": 262812,
        "F37": "116513262812",
        "VposElapsedTime": 17,
        "BankingElapsedTime": 0,
        "SocketElapsedTime": 0,
        "HsmElapsedTime": 4,
        "MpiElapsedTime": 0,
        "hasOrderId": true,
        "TemplateType": 0,
        "HasAddressCount": false,
        "IsPaymentFacilitator": false,
        "MerchantCountryCode": "**",
        "F11_ORG": 0,
        "F12_ORG": 0,
        "F22_ORG": 0,
        "F25_ORG": 0,
        "MTI_ORG": 0,
        "IntervalType": 0,
        "IntervalDuration": 0,
        "RepeatCount": 0,
        "RequestClientIp": "my IP",
        "VoidTime": 0,
        "PaymentLinkId": 0
    },
    "PaymentAddress": {
        "RequestGuid": 0
    },
    "ExtraParameters": [
        [
            "INCOMING_DATA",
            "<INCOMING_DATA><MbrId>5</MbrId><MerchantID>********</MerchantID><UserCode>Q*******Y</UserCode><UserPass_masked>*****</UserPass_masked><OrderId>T*******3</OrderId><SecureType>NonSecure</SecureType><InstallmentCount>0</InstallmentCount><TxnType>Auth</TxnType><PurchAmount>0.1</PurchAmount><Currency>***</Currency><CardHolderName>B*******e</CardHolderName><Pan_masked>41*********6111</Pan_masked><Expiry>****</Expiry><Cvv2_masked>***</Cvv2_masked><Lang>TR</Lang></INCOMING_DATA>"
        ],
        [
            "PAYFORFROMJSONREQUEST",
            "1"
        ],
        [
            "SESSION_MRC_CODE",
            "085300000009704"
        ],
        [
            "SESSION_SYSTEM_USER",
            "0"
        ]
    ]
}

and this is how HttpClient is created at Startup.cs

services.AddScoped(sp =>
            {
                var client = new HttpClient
                {
                    BaseAddress = new Uri("BANK's URI")
                };
                return client;
            });