Access to private keys of keychain on Mac OS X - macos

Question

I'm implementing an application for Windows/Mac OS X on C# that digitally signs files with a certificate. To do that I'm using BouncyCastle and iText libraries. On windows works perfectly without any special code. I can read the stored certificates on the machine using this code.
X509Store store = new X509Store(StoreLocation.CurrentUser);
store.Open(OpenFlags.ReadOnly);
foreach (X509Certificate2 certificate in store.Certificates)
{
if (certificate.HasPrivateKey && certificate.NotAfter >= DateTime.Now)
{
// USE CERTIFICATE
}
}
The problem that I'm facing is the access to the certificates stored in the Keychain. Because I can get the information of the certificates, but not their private keys. I suppose that there should be a way to access that information (after a confirmation from the user to allow the access), but I can't see how.
My current implementation to get the information of the certificates is:
var query = new SecRecord(SecKind.Certificate)
{
MatchValidOnDate = DatetimeToNSDate(DateTime.Now),
CanSign = true,
};
var certList = Security.SecKeyChain.QueryAsRecord(query, 100, out var result);
foreach(var cert in certLis)
{
SecCertificate tempCertificate = new SecCertificate(cert);
X509Certificate2 certificateObj = tempCertificate.ToX509Certificate2();
}
This certificateObj is a valid X509 certificate but its privateKey is null.