protected override Task AuthenticateClient(string targetHost, X509CertificateCollection clientCertificates, bool checkCertificateRevocation)
{
return Task.Factory.FromAsync(
(callback, state) => _clientStream.BeginAuthenticateAsClient(targetHost, clientCertificates, checkCertificateRevocation, callback, state),
_clientStream.EndAuthenticateAsClient,
state:null);
}