How to create multipart/related request in ASP .NET

SOAP server requires request with multipart/related content type. Code

using System;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
public class MyController : Controller
{
    public async Task<IActionResult> SendSoapRequest()
    {
        try
        {
            using var client = new HttpClient();
            using var content = new MultipartFormDataContent();
            // Add the SOAP XML
            var soapXml = @"<?xml version=""1.0"" encoding=""utf-8""?>
                            <soap:Envelope xmlns:soap=""http://schemas.xmlsoap.org/soap/envelope/"">
                                <!-- Your SOAP content here -->
                            </soap:Envelope>";
            var soapContent = new StringContent(soapXml, Encoding.UTF8, "text/xml");
            content.Add(soapContent, "file", "soap.xml");
            // Add any additional files (if needed)
            // var fileBytes = ... // Read your file content
            // var fileContent = new ByteArrayContent(fileBytes);
            // content.Add(fileContent, "file", "filename.ext");
            // Send the request
            var response = await client.PostAsync("https://example.com/your-soap-endpoint", content);
            if (response.IsSuccessStatusCode)
            {
                // Process the success response
                return Ok("SOAP request sent successfully!");
            }
            else
            {
                // Handle the error response
                return BadRequest("Error sending SOAP request.");
            }
        }
        catch (Exception ex)
        {
            // Handle exceptions
            return StatusCode(500, $"An error occurred: {ex.Message}");
        }
    }
}

creates request with content type multipart/form-data . How to create multipart/realated request?

Using ASP .NET 8 MVC controller.

Update

Created code using sample from https://www.codeproject.com/Articles/1166668/Sending-MIME-Messages-to-a-Web-Service-Endpoint-fr

    using var client = new HttpClient(httpClientHandler);
    const string filecontentid = "cid:filecontentid";
    var soapXml = @$"<?xml version=""1.0"" encoding=""utf-8""?>
<soapenv:Envelope xmlns:soapenv=""http://schemas.xmlsoap.org/soap/envelope/""
                  xmlns:xro=""http://x-road.eu/xsd/xroad.xsd""
                  xmlns:iden=""http://x-road.eu/xsd/identifiers"">
  <soapenv:Header>
    <xro:protocolVersion>4.0</xro:protocolVersion>
    <xro:userId>EE00000000000</xro:userId>
    <xro:id>5-57</xro:id>
    <xro:service iden:objectType=""SERVICE"">
  </soapenv:Header>
  <soapenv:Body>
  </soapenv:Body>
</soapenv:Envelope>
";

    const string _boundary = "=-glZ6jtnXmVAn68ZSF5H6Zw==";
    MultipartContent _multiPartcontent = new MultipartContent("Related", _boundary);

    HttpContent _xmlContent = new StringContent(soapXml);
    _xmlContent.Headers.ContentType = new MediaTypeHeaderValue("text/xml");
    _xmlContent.Headers.Add("Content-ID", "soapcontentid");
    _multiPartcontent.Add(_xmlContent);
    HttpContent fileContent = new StringContent("xml xml");
    fileContent.Headers.ContentType = new MediaTypeHeaderValue("text/xml");
    fileContent.Headers.Add("Content-ID", filecontentid);
    _multiPartcontent.Add(fileContent);
    var response = await client.PostAsync(url, _multiPartcontent);
    if (response.IsSuccessStatusCode)
    {
      // Process the success response
      return Ok("SOAP request sent successfully "            );
    }
    else
    {
      // Handle the error response
      return BadRequest("Error sending SOAP request. ");
    }

Will it generate proper request?