CertificateExpireChecker.cs

public class CertificateExpireChecker
{
    private TimeSpan TimeOut => TimeSpan.FromSeconds(5);

    public string Url { get; }
    public DateTime LimitDate => DateTime.Now;
    public bool IsExists => Certificate != null;
    public X509Certificate2 Certificate { get; private set; }
    public string CommonName => Certificate?.SubjectName.Name.Split(',').FirstOrDefault(x => x.StartsWith("CN="));
    public TimeSpan? DaysLeft => ExpiredDate - LimitDate;
    public bool IsExpired => DateTime.Now > Certificate?.NotAfter;
    public DateTime? ExpiredDate => Certificate?.NotAfter;
    public string Issuer => Certificate?.Issuer;
    public string Subject => Certificate?.Subject;
    public string FriendlyName => Certificate?.FriendlyName;
    public string ThumbPrint => Certificate?.Thumbprint;
    public string SignatureAlgorithm => Certificate?.SignatureAlgorithm.FriendlyName;

    public CertificateExpireChecker(string url)
    {
        Url = url;
    }

    public void ReadX509Certificate2()
    {
        var request = WebRequest.Create(Url) as HttpWebRequest;
        request.Timeout = (int)TimeOut.TotalMilliseconds;
        try
        {
            using (var response = request.GetResponse()) { }
        }
        catch (Exception)
        {
            return;
        }
        var cert2 = new X509Certificate2(request.ServicePoint.Certificate);
        Certificate = cert2;
    }
}