x509证书的检查签名

我有:

1. x509证书(Base64);

可以检查签名吗?

我的代码:

  bool valid = false;

  var signature = Convert.FromBase64String(base64Signature);
  var data = Encoding.UTF8.GetBytes(stringData);

  var x509 = new X509Certificate2(Convert.FromBase64String(certificate));
  var dsa = x509.PublicKey.Key as DSACryptoServiceProvider;
  if (dsa!=null)
    valid = dsa.VerifySignature(data, signature);
  else {
    var rsa = x509.PublicKey.Key as RSACryptoServiceProvider;
    if (rsa!=null)
      valid = rsa.VerifyHash(data, ???, signature);
  }