Extract private key bytes in C #

This is what worked for me. Should also work for you:

using System;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Security;

namespace SO6258771
{
    class Program
    {
        static void Main()
        {
            // Load your certificate from file
            X509Certificate2 certificate = new X509Certificate2("filename.pfx", "password", X509KeyStorageFlags.Exportable | X509KeyStorageFlags.PersistKeySet);

            // Now you have your private key in binary form as you wanted
            // You can use rsa.ExportParameters() or rsa.ExportCspBlob() to get you bytes
            // depending on format you need them in
            RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)certificate.PrivateKey;

            // Just for lulz, let write out the PEM representation of the private key
            // using Bouncy Castle, so that we are 100% sure that the result is exaclty the same as:
            // openssl pkcs12 -in filename.pfx -nocerts -out privateKey.pem
            // openssl.exe rsa -in privateKey.pem -out private.pem

            // You should of course dispose of / close the streams properly. I'm skipping this part for brevity
            MemoryStream memoryStream = new MemoryStream();
            TextWriter streamWriter = new StreamWriter(memoryStream);
            PemWriter pemWriter = new PemWriter(streamWriter);

            AsymmetricCipherKeyPair keyPair = DotNetUtilities.GetRsaKeyPair(rsa);
            pemWriter.WriteObject(keyPair.Private);
            streamWriter.Flush();

            // Here is the output with ---BEGIN RSA PRIVATE KEY---
            // that should be exactly the same as in private.pem
            Console.Write(Encoding.ASCII.GetString(memoryStream.GetBuffer()));
        }
    }
}