https, 在WIN7 下如果服务器断没有启用tls1.0, 那webClient 请求就会失败，

问题解决方案 https://stackoverflow.com/questions/2859790/the-request-was-aborted-could-not-create-ssl-tls-secure-channel

try
{
ServicePointManager.Expect100Continue = true;
//ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls;
ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;

string testUrl =  "https://www.baidu.com/";

testUrl = "https://www.thoughtco.com/swimming-4132739";//不用上面代码就会失败，error:The request was aborted: Could not create SSL/TLS secure channel.

WebClient wc = new WebClient();
string s = wc.DownloadString(testUrl);
richTextBox1.Text = s;
}
catch(Exception ex) {

richTextBox1.Text = "error:" + ex.Message;
}

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The solution to this, in .NET 4.5 is

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
If you don’t have .NET 4.5 then use

ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;

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很遗憾的是这个在XP下还是不行。

。