无法使用TLSv1.2协议连接WebService？C#调用报错求助

此前用C#访问某WebService一切正常，对方将安全协议改为TLSv1.2后，无法访问服务，返回错误：

Unhandled Exception: System.Net.WebException: The underlying
connection was closed: Unexpected error on a send. --->
System.IO.IOException: Unable to read data from transport connection:
An existing connection was forced to be canceled by the remote host.
---> System.Net.Sockets.SocketException: An existing connection was forced to be canceled by the remote host

相关代码如下：

class Program
{
    public static string arquivopfx;
    public static string senha;
    
    public static void Main(String[] args)
    {
        int counter = 0;  
        string line;  

        System.IO.StreamReader file = new System.IO.StreamReader(@"assina.config");  
        while((line = file.ReadLine()) != null)  
        {  
            if (counter == 3)  
            {
                break;
            }
            if (counter == 0)  
            {
                arquivopfx = line;
            }
            if (counter == 1)  
            {
                senha = line.Trim();
            }
            counter++;  
        }  
        file.Close();
        Execute();
    }

    /// <summary>
    /// Execute a Soap WebService call
    /// </summary>
    /// doc.Save("c:\\try.xml");
    
    public static void Execute()
    {
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Ssl3;            

        HttpWebRequest request = CreateWebRequest();
        XmlDocument soapEnvelopeXml = new XmlDocument();
        soapEnvelopeXml.Load(@"SOAP_request.xml");
        soapEnvelopeXml.PreserveWhitespace = true;            
        request.ServicePoint.Expect100Continue = false;
        
        try {
            File.Delete(@"SOAP_response.xml");

            using (Stream stream = request.GetRequestStream())
            {
                soapEnvelopeXml.Save(stream);
            }
            
            using (WebResponse response = request.GetResponse())
            {
                using (StreamReader rd = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding("UTF-8")))
                {
                    // ljê a resposta para uma string
                    string soapResult = rd.ReadToEnd();
                    Console.WriteLine(soapResult);
                    // grava a string no disco
                    using (StreamWriter writer = new StreamWriter("SOAP_response.xml", false, Encoding.Default))
                    {
                        writer.WriteLine(soapResult);
                    }
                }
            }
        }
        catch (WebException ex)
        {
            //string message = new StreamReader(ex.Response.GetResponseStream()).ReadToEnd();
            throw ex;    
        }
    }
    /// <summary>
    /// Create a soap webrequest to [Url]
    /// </summary>
    /// <returns></returns>
    public static HttpWebRequest CreateWebRequest()
    {
        string url = "https://webservices.envio.esocial.gov.br/servicos/empregador/enviarloteeventos/WsEnviarLoteEventos.svc";
        HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
        webRequest.Headers.Add("SOAPAction","http://www.esocial.gov.br/servicos/empregador/lote/eventos/envio/v1_1_0/ServicoEnviarLoteEventos/EnviarLoteEventos");
        webRequest.ContentType = "text/xml;charset=\"utf-8\"";
        webRequest.Accept = "text/xml";
        webRequest.KeepAlive = false;            
        webRequest.Method = "POST";
        webRequest.Timeout = 30000;
        webRequest.ClientCertificates.Add(new X509Certificate2(arquivopfx, senha));            
        return webRequest;
    }
}

问题根源是代码指定的安全协议未包含TLSv1.2，对方服务已强制使用该协议，导致握手失败，连接被远程主机关闭。

针对.NET 4.5及以上版本 #

修改Execute方法中的ServicePointManager.SecurityProtocol配置，添加Tls12并移除已废弃的Ssl3：

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls;

针对.NET 4.0版本 #

由于.NET 4.0原生枚举无Tls12，需手动定义枚举值后再配置：

const SecurityProtocolType Tls12 = (SecurityProtocolType)3072;
ServicePointManager.SecurityProtocol = Tls12 | SecurityProtocolType.Tls;

修改后重新调用WebService，即可正常建立TLSv1.2连接。

内容的提问来源于stack exchange，提问作者DQL SISTEMAS