IgnoreBadCertificates方法:

//I use a method to ignore bad certs caused by misc errors
IgnoreBadCertificates();

// after the Ignore call i can do what ever i want...
HttpWebRequest request_data = System.Net.WebRequest.Create(urlquerystring) as HttpWebRequest;

/*
and below the Methods we are using...
*/

/// <summary>
/// Together with the AcceptAllCertifications method right
/// below this causes to bypass errors caused by SLL-Errors.
/// </summary>
public static void IgnoreBadCertificates()
{
    System.Net.ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(AcceptAllCertifications);
}  

/// <summary>
/// In Short: the Method solves the Problem of broken Certificates.
/// Sometime when requesting Data and the sending Webserverconnection
/// is based on a SSL Connection, an Error is caused by Servers whoes
/// Certificate(s) have Errors. Like when the Cert is out of date
/// and much more... So at this point when calling the method,
/// this behaviour is prevented
/// </summary>
/// <param name="sender"></param>
/// <param name="certification"></param>
/// <param name="chain"></param>
/// <param name="sslPolicyErrors"></param>
/// <returns>true</returns>
private static bool AcceptAllCertifications(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certification, System.Security.Cryptography.X509Certificates.X509Chain chain, System.Net.Security.SslPolicyErrors sslPolicyErrors)
{
    return true;
} 

添加证书验证处理程序。返回true将允许忽略验证错误:

ServicePointManager
    .ServerCertificateValidationCallback += 
    (sender, cert, chain, sslPolicyErrors) => true;

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);
}

这适用于。net Core。 调用Soap客户端:

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

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)

老了，但仍然有用……

实现相同行为的另一个好方法是通过配置文件(web.config)

 <system.net>
    <settings>
      <servicePointManager checkCertificateName="false" checkCertificateRevocationList="false" />
    </settings>
  </system.net>

注意:测试在。net full上。