微信退款 The remote server returned an error: (400) Bad Request. 微信支付退款接口：基础连接已经关闭: 连接被意外关闭

微信退款接口需要添加证书， 特别注意：有些情况下即使在post请求时添加了证书依然报同样的错误，在添加证书时注意，

如果使用： X509Certificate cer = new X509Certificate(path, configInfo.mch_id); 也报错的话

改用：X509Certificate2 cer = new X509Certificate2(path, configInfo.mch_id);

private string requestWxRefundApi(WeixinPromotionChannelConfigInfo configInfo, string param)
{
　　string path = System.AppDomain.CurrentDomain.SetupInformation.ApplicationBase + @"bin\" + configInfo.cert_path;

　　ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult);
　　X509Certificate2 cer = new X509Certificate2(path, configInfo.mch_id);
　　HttpWebRequest webrequest = (HttpWebRequest)HttpWebRequest.Create(configInfo.refund_url);
　　webrequest.ClientCertificates.Add(cer);
　　webrequest.Method = "post";
　　//webrequest.KeepAlive = true;
　　//webrequest.Timeout = 60 * 1000;

　　Encoding encoding = Encoding.UTF8;
　　byte[] data = encoding.GetBytes(param);
　　using (Stream inputStream = webrequest.GetRequestStream())
　　{
　　　　inputStream.Write(data, 0, data.Length);
　　}

　　string resp = string.Empty;
　　using (HttpWebResponse webreponse = (HttpWebResponse)webrequest.GetResponse())
　　{
　　　　Stream stream = webreponse.GetResponseStream();
　　　　using (StreamReader reader = new StreamReader(stream))
　　　　{
　　　　　　resp = reader.ReadToEnd();
　　　　}
　　}

　　return resp;
}

　　

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原文链接：https://blog.csdn.net/m0_37693130/article/details/100563820