test.cs

static X509Certificate2 RetrieveCertificate() {
         
   // get and open certificate store for current user
   X509Store store = new X509Store(StoreLocation.CurrentUser);
   store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);

   // retrieve the certificate using the integrated Windows UI
   X509Certificate2Collection selectedCertificates =
           X509Certificate2UI.SelectFromCollection(
              store.Certificates, 
              "Choose your certificate",
              "Please select a certificate that is used to sign the PDF.",
              X509SelectionFlag.SingleSelection);

   // return the first selected certificate with a private key
   foreach (var certificate in selectedCertificates) {
      if (certificate.HasPrivateKey)
         return certificate;
   }

   return null;
}