CertPinning.cs

public class CertPinning
{
    public static bool CertCheck(object sender, X509Certificate certificate, 
                                      X509Chain chain, SslPolicyErrors sslPolicyErrors)
    {
        String certSha256FingerprintHex = "xxxxxxxxxxxxxxxxxx";

        if (certificate == null){
            return false;
        }

        var request = sender as HttpWebRequest;
        if (request == null){
            return false;
        }

        // if the request is for the target domain, perform certificate pinning
        if (Const.WhiteListDomainNames.Contains(request.Address.Authority))
        {
            String sha256FingerprintHex;
            using (SHA256 sha256 = SHA256.Create())
            {
                byte[] bytes = sha256.ComputeHash(certificate.GetPublicKey());
                sha256FingerprintHex = Conversion.HexToString(bytes);
            }
            return certSha256FingerprintHex.Equals(sha256FingerprintHex);
        }

        // Check whether there were any policy errors for any other domain
        return sslPolicyErrors == SslPolicyErrors.None;
    }
}