In the .NET program create a new RSACryptoServiceProvider. Export the public key as RSAParameters and write the Modulus and Exponent values to disk. Like this:
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(4096); //4096 bit key
RSAParameters par = rsa.ExportParameters(false); // export the public key
File.WriteAllBytes(@"C:\modulus.bin", par.Modulus); // write the modulus and the exponent to disk
File.WriteAllBytes(@"C:\exponent.bin", par.Exponent);
On the C++ side you'll need to read the modulus and exponent values from disk convert them into BIGNUM values. These values will be loaded into an RSA key and then you can encrypt the plain text and write the cipher text to disk. Like this:
RSA * key;
unsigned char *modulus;
unsigned char *exp;
FILE * fp = fopen("c:\\modulus.bin", "rb"); // Read the modulus from disk
modulus = new unsigned char[512];
memset(modulus, 0, 512);
fread(modulus, 512, 1, fp);
fclose(fp);
fp = fopen("c:\\exponent.bin", "rb"); // Read the exponent from disk
exp = new unsigned char[3];
memset(exp, 0, 3);
fread(exp, 3, 1, fp);
fclose(fp);
BIGNUM * bn_mod = NULL;
BIGNUM * bn_exp = NULL;
bn_mod = BN_bin2bn(modulus, 512, NULL); // Convert both values to BIGNUM
bn_exp = BN_bin2bn(exp, 3, NULL);
key = RSA_new(); // Create a new RSA key
key->n = bn_mod; // Assign in the values
key->e = bn_exp;
key->d = NULL;
key->p = NULL;
key->q = NULL;
int maxSize = RSA_size(key); // Find the length of the cipher text
cipher = new char[valid];
memset(cipher, 0, valid);
RSA_public_encrypt(strlen(plain), plain, cipher, key, RSA_PKCS1_PADDING); // Encrypt plaintext
fp = fopen("C:\\cipher.bin", "wb"); // write ciphertext to disk
fwrite(cipher, 512, 1, fp);
fclose(fp);
Finally you can take the ciphertext and decrypt it in C# without any difficulty.
byte[] cipher = File.ReadAllBytes(@"c:\cipher.bin"); // Read ciphertext from file
byte[] plain = rsa.Decrypt(cipher, false); // Decrypt ciphertext
Console.WriteLine(ASCIIEncoding.ASCII.GetString(plain)); // Decode and display plain text