这个代码对我有用。我必须添加TLS2，因为这是我感兴趣的URL正在使用的。

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
ServicePointManager.ServerCertificateValidationCallback +=
    (sender, cert, chain, sslPolicyErrors) => { return true; };
using (var client = new HttpClient())
{
    client.BaseAddress = new Uri(UserDataUrl);
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Accept.Add(new
      MediaTypeWithQualityHeaderValue("application/json"));
    Task<string> response = client.GetStringAsync(UserDataUrl);
    response.Wait();

    if (response.Exception != null)
    {
         return null;
    }

    return JsonConvert.DeserializeObject<UserData>(response.Result);
}

绕过SSL证书....

HttpClientHandler clientHandler = new HttpClientHandler();
clientHandler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) => { return true; };

// Pass the handler to httpclient(from you are calling api)
var client = new HttpClient(clientHandler)

这个代码对我有用。我必须添加TLS2，因为这是我感兴趣的URL正在使用的。

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
ServicePointManager.ServerCertificateValidationCallback +=
    (sender, cert, chain, sslPolicyErrors) => { return true; };
using (var client = new HttpClient())
{
    client.BaseAddress = new Uri(UserDataUrl);
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Accept.Add(new
      MediaTypeWithQualityHeaderValue("application/json"));
    Task<string> response = client.GetStringAsync(UserDataUrl);
    response.Wait();

    if (response.Exception != null)
    {
         return null;
    }

    return JsonConvert.DeserializeObject<UserData>(response.Result);
}

允许所有证书是非常强大的，但也可能是危险的。如果您希望只允许有效的证书加上某些特定的证书，可以这样做。

net核心:

using (var httpClientHandler = new HttpClientHandler())
{
    httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, sslPolicyErrors) =>
    {
        if (sslPolicyErrors == SslPolicyErrors.None)
        {
            return true;   //Is valid
        }

        if (cert.GetCertHashString() == "99E92D8447AEF30483B1D7527812C9B7B3A915A7")
        {
            return true;
        }
        return false;
    };

    using (var httpClient = new HttpClient(httpClientHandler))
    {
        var httpResponse = httpClient.GetAsync("https://example.com").Result;
    }
}

net框架:

System.Net.ServicePointManager.ServerCertificateValidationCallback += delegate (
    object sender,
    X509Certificate cert,
    X509Chain chain,
    SslPolicyErrors sslPolicyErrors)
{
    if (sslPolicyErrors == SslPolicyErrors.None)
    {
        return true;   //Is valid
    }

    if (cert.GetCertHashString() == "99E92D8447AEF30483B1D7527812C9B7B3A915A7")
    {
        return true;
    }

    return false;
};

更新:

如何在Chrome中获取cert.GetCertHashString()值:

在地址栏中单击“安全”或“不安全”。

然后单击Certificate -> Details -> Thumbprint并复制该值。记得执行cert.GetCertHashString(). tolower()。

这适用于。net Core。 调用Soap客户端:

client.ClientCredentials.ServiceCertificate.SslCertificateAuthentication =
                new X509ServiceCertificateAuthentication()
                {
                    CertificateValidationMode = X509CertificateValidationMode.None,
                    RevocationMode = X509RevocationMode.NoCheck
                };  

如果直接使用套接字并作为客户端进行身份验证，则Service Point Manager回调方法将不起作用。以下是对我有效的方法。请仅用于测试目的。

var activeStream = new SslStream(networkStream, false, (a, b, c, d) => { return true; });
await activeStream.AuthenticateAsClientAsync("computer.local");

这里的关键是在SSL流的构造函数中提供远程证书验证回调。