C#调用HttpClient.SendAsync报错：System.Net.Http.HttpRequestException:
发送请求时出错。

var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken);

问题出在SSL/TLS，Windows Server 2012不支持 TLS 1.2 ，需要手动设置

    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;