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Exception while making SSL connection from within Windows Service
class MyWebClient : WebClient
{
    public X509Certificate cert { set; get; }
    protected override WebRequest GetWebRequest(Uri address)
    {
        HttpWebRequest req = (HttpWebRequest)base.GetWebRequest(address);
        req.ClientCertificates.Clear();
        req.ClientCertificates.Add(cert);
        return req;
    }
}

try {
//...
string qry = "Some xml query...";
    System.Net.ServicePointManager.ServerCertificateValidationCallback = delegate(object sender,
    X509Certificate certificate,
    X509Chain chain,
    SslPolicyErrors sslPolicyErrors) { return true; };

X509Certificate cert = X509Certificate.CreateFromCertFile(appPath + @"datacert.der");
MyWebClient cl = new MyWebClient();
cl.cert = cert;
string xmlReq = qry;
cl.Headers[HttpRequestHeader.ContentType] = "text/xml";
var data = Encoding.UTF8.GetBytes(xmlReq);
byte[] res = cl.UploadData(apiUrl, data);
cl.Dispose();

string result = Encoding.UTF8.GetString(res);
} catch (Exception ex) {
//...
}
//...

X509Certificate cert = X509Certificate.CreateFromCertFile(appPath + @"datacert.der");