C#调用webapi HTTPS报错：基础连接已经关闭: 未能为 SSL/TLS 安全通道建立信任关系--安全证书问题

1、首先加入命名空间：
using System.Net.Security;
using System.Security.Authentication;
using System.Security.Cryptography.X509Certificates;

SSL网站，连接时需要提供证书，对于非必须提供客户端证书的情况，只要返回一个安全确认即可。我的是.NET FrameWork4.0

2、加入以下代码：

 public bool CheckValidationResult(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors)
    {
        //直接确认，否则打不开
        return true;
    }

3、接收证书进行身份验证ssl,在调用api接口前调用此方法：
ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);

以下是完整案例：

 public string HttpPost(string url, string body)
    {
        Encoding encoding = Encoding.UTF8;
        string jsonText = string.Empty;
        string dataText1 = string.Empty;
        if (string.IsNullOrEmpty(url.Trim()))
        {
            return "";
        }
        //接收证书进行身份验证
        ServicePointManager.ServerCertificateValidationCallback =
            new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);
        //ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "POST";
        request.Accept = "text/plain, */*; q=0.01";
        request.ContentType = "application/json;charset=utf-8";
        byte[] buffer = encoding.GetBytes(body);
        request.ContentLength = buffer.Length;
        request.GetRequestStream().Write(buffer, 0, buffer.Length);
        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.UTF8))
        {
            jsonText = reader.ReadToEnd();
            dataText1 = Regex.Replace(jsonText, @"\", "");
        }
        return dataText1;
    }

调用：
dlbzUrl是调用地址 https://…/api/tddbzzljcxt/zybrxx/vnoentry-query
model.FCYRQSTART 开始时间
model.FCYRQEND 结束时间

  string DaliBaiHospitalJson = HttpPost(dlbzUrl, "{"FIDATES":"" + model.FCYRQSTART + " 00:00:00" + "","FIDATEE":"" + model.FCYRQEND + " 23:59:59" + ""}");