RsaKeyPairGenerator r = new RsaKeyPairGenerator();
            r.Init(new KeyGenerationParameters(new SecureRandom(), 1024));
            AsymmetricCipherKeyPair keys = r.GenerateKeyPair();
            AsymmetricKeyParameter private_key = keys.Private;
            AsymmetricKeyParameter public_key = keys.Public;
            TextWriter textWriter = new StringWriter();
            PemWriter pemWriter = new PemWriter(textWriter);
            pemWriter.WriteObject(keys.Private);
            pemWriter.Writer.Flush();
            string privateKey = textWriter.ToString();
            txtPri.Text = privateKey;
            TextWriter textpubWriter = new StringWriter();
            PemWriter pempubWriter = new PemWriter(textpubWriter);
            pempubWriter.WriteObject(keys.Public);
            pempubWriter.Writer.Flush();
            string pubKey = textpubWriter.ToString();
            txtPub.Text = pubKey;

using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Security;

--XML格式转PEM格式，引用：http://www.jianshu.com/p/faefcc58c79b

 public static void XMLConvertToPEM()//XML格式密钥转PEM
        var rsa2 = new RSACryptoServiceProvider();
        using (var sr = new StreamReader("e:\\PrivateKey.xml"))
            rsa2.FromXmlString(sr.ReadToEnd());
        var p = rsa2.ExportParameters(true);
        var key = new RsaPrivateCrtKeyParameters(
            new BigInteger(1, p.Modulus), new BigInteger(1, p.Exponent), new BigInteger(1, p.D),
            new BigInteger(1, p.P), new BigInteger(1, p.Q), new BigInteger(1, p.DP), new BigInteger(1, p.DQ),
            new BigInteger(1, p.InverseQ));
        using (var sw = new StreamWriter("e:\\PrivateKey.pem"))
            var pemWriter = new Org.BouncyCastle.OpenSsl.PemWriter(sw);
            pemWriter.WriteObject(key);
网上示例都是从文件读取出XML格式私钥，存入PEM文件中,PEM中就是转换好的格式。    
读取XML私钥的代码可以换成生成XML私钥，以下代码是转好格式存入字符串string 中。
public void xmlToPem()
            CspParameters parameters = new CspParameters();
            parameters.KeyContainerName = DateTime.Now.ToString();
            //可以选择导入XML格式的私钥还是生成。无论是存储在.XML文件中还是在数据库等地方。      这里是新生成一个C# XML格式私钥来转成PEM格式。
            var rsa2 = new RSACryptoServiceProvider(parameters);
            //using (var sr = new StreamReader("e:\\PrivateKey.xml"))
            //    rsa2.FromXmlString(sr.ReadToEnd());
            var p = rsa2.ExportParameters(true);
            var key = new RsaPrivateCrtKeyParameters(
                new BigInteger(1, p.Modulus), new BigInteger(1, p.Exponent), new BigInteger(1, p.D),
                new BigInteger(1, p.P), new BigInteger(1, p.Q), new BigInteger(1, p.DP), new BigInteger(1, p.DQ),
                new BigInteger(1, p.InverseQ));
            using (TextWriter sw = new StringWriter())
                var pemWriter = new Org.BouncyCastle.OpenSsl.PemWriter(sw);
                pemWriter.WriteObject(key);
                pemWriter.Writer.Flush();
                string priKey = sw.ToString();
XML公钥转PEM公钥的代码还未研究。