in my c # project, I put this piece of code :

initially, I recovered from the certificate that I created with bouncy castle, the key pair, then I extract the private key, my goal is that , it is in a format. pem,

 AsymmetricKeyParameter private_RSA = keyPair2.Private;


            PrivateKeyInfo k_RSA = PrivateKeyInfoFactory.CreatePrivateKeyInfo(private_RSA);
            byte[] serializedKey_priv_RSA = k_RSA.ToAsn1Object().GetDerEncoded();


            //  byte[] clé = Org.BouncyCastle.Utilities.Encoders.Hex.Decode(serializedKey);

            string data_priv_RSA = Convert.ToBase64String(serializedKey_priv_RSA);

using (fluxInfos2 = new StreamWriter("myprivatekey.pem"))
        {
            string ligne = " -----BEGIN RSA PRIVATE KEY-----  ";
            fluxInfos2.WriteLine(ligne);   
            fluxInfos2.WriteLine(data_priv_RSA);
            string lige2 = "-----END RSA PRIVATE KEY----- ";

            fluxInfos2.WriteLine(lige2);


        }
        fluxInfos2.Close();

my key is created perfectly, that is, what the file contains;

-----BEGIN RSA PRIVATE KEY-----

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

-----END RSA PRIVATE KEY----- 

Now, I must do the opposite, ie, from a data key contained in the file, I must have a RsaPrivateCrtKeyParameters. to do this, I wrote this little code:

KeyFile string = @ "C: \ Users \ Me \ Documents \ Visual Studio 2010 \ Projects \ PEM \ PEM \ bin \ Debug \ myprivatekey.pem";

RsaPrivateCrtKeyParameters GetPrivateKey privat = (KeyFile);

such as GetPrivateKey, is as follows:

 public static RsaPrivateCrtKeyParameters GetPrivateKey(String pemFile)
    {
        if (string.IsNullOrEmpty(pemFile)) throw new ArgumentNullException("pemFile");

        string privateKey = File.Exists(pemFile) ? File.ReadAllText(pemFile) : pemFile;

        var reader = new PemReader(new StringReader(privateKey));
        RsaPrivateCrtKeyParameters privkey = null;
        Object obj = reader.ReadObject();
        if (obj is AsymmetricCipherKeyPair)
        {
            privkey = (RsaPrivateCrtKeyParameters)((AsymmetricCipherKeyPair)obj).Private;
        } return privkey;
    } 

the problem is that I receive an exception , in : Object obj = reader.ReadObject(); :

 Creating RSA private key problem: System.InvalidCastException:

 Unable to cast object of type 'Org.BouncyCastle.Asn1.DerSequence' to type 'Org.BouncyCastle.Asn1.DerInteger'.
    to Org.BouncyCastle.Asn1.Pkcs.RsaPrivateKeyStructure .. ctor (Asn1Sequence seq)
    to Org.BouncyCastle.OpenSsl.PemReader.ReadKeyPair (String type, String endMarker)

could you help me please, thank you for any help