解决“请求被中止: 未能创建 SSL/TLS 安全通道”的问题

public Stream GetResponseStreamByHttpGet(string url)
{
    try
    {
        //创建请求地址之前添加代码
        System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);
        request.Method = "Get";
        request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8";
        request.UserAgent = "User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.181 Safari/537.36";
        request.Proxy = new WebProxy();
        return request.GetResponse().GetResponseStream();
    }
    catch (Exception ex)
    {
        throw ex;
    }
    return null;
}

public string GetResponseStringByHttpGet(string url, Encoding encoding)
{
    string strResponse = string.Empty;
    using (StreamReader reqStream = new StreamReader(GetResponseStreamByHttpGet(url) ?? new MemoryStream(), encoding))
    {
        strResponse = reqStream.ReadToEnd();
    }
    return strResponse;
}