X509Certificate2 private key to PEM

x509topem.cs

public static string ExportPrivateKey(X509Certificate2 certificateWithPrivateKey)
{
    var parameterList = RsaParametersToList(certificateWithPrivateKey.GetRSAPrivateKey().ExportParameters(true));
    var parameterBytes = SerializeList(parameterList);
    var base64 = Convert.ToBase64String(parameterBytes);

    var builder = new StringBuilder();
    builder.Append("-----BEGIN RSA PRIVATE KEY-----\n");

    for (int i = 0; i < base64.Length; i += 64)
        builder.Append($"{ base64.Substring(i, Math.Min(64, base64.Length - i)) }\n");

    builder.Append("-----END RSA PRIVATE KEY-----\n");

    return builder.ToString();
}

private static List<byte[]> RsaParametersToList(System.Security.Cryptography.RSAParameters parameters)
{
    return new List<byte[]> {
        new byte[] {0x00},
            parameters.Modulus,
            parameters.Exponent,
            parameters.D,
            parameters.P,
            parameters.Q,
            parameters.DP,
            parameters.DQ,
            parameters.InverseQ
        };
}

private static byte[] SerializeList(List<byte[]> list)
{
    var keyBytes = list.Select(e => Encode(e)).SelectMany(e => e).ToArray();

    using (var ms = new MemoryStream())
    using (var binaryWriter = new BinaryWriter(ms))
    {
        binaryWriter.Write((byte)0x30); // SEQUENCE
        binaryWriter.Write(Encode(keyBytes, false));
        binaryWriter.Flush();
        return ms.ToArray();
    }
}

private static byte[] Encode(byte[] inBytes, bool useTypeOctet = true)
{
    var length = inBytes.Length;
    var bytes = new List<byte>();

    if (useTypeOctet == true)
        bytes.Add(0x02); // Integer, 2 bytes

    bytes.Add(0x84); // Long, 4 bytes
    bytes.AddRange(BitConverter.GetBytes(length).Reverse());
    bytes.AddRange(inBytes);

    return bytes.ToArray();
}