C#

Duplicate openssl pkey -in private.pem -pubout -out pubkey.pem

See more OpenSSL Examples

How to output the public part of a private key: Demonstrates how to duplicate this OpenSSL command:

openssl pkey -in private.pem -pubout -out pubkey.pem

C#

bool success = false;

Chilkat.PrivateKey pkey = new Chilkat.PrivateKey();

// Load the private key from an PEM file:
success = pkey.LoadPemFile("private.pem");
if (success == false) {
    Debug.WriteLine(pkey.LastErrorText);
    return;
}

Chilkat.PublicKey pubKey = new Chilkat.PublicKey();
pkey.ToPublicKey(pubKey);

success = pubKey.SavePemFile(false,"pubKey.pem");
if (success != true) {
    Debug.WriteLine(pubKey.LastErrorText);

    return;
}

Debug.WriteLine("Success.");