ExtKey on the other hand, only contains the key information.
You can get the ExtKey from BitcoinExtKey by using the property BitcoinExtKey.Key.
However, to do the reverse, ExtKey need information about which Network to use to generate your base58 string.
So here is how your do to go back and forth :
Quote
String keyStr = "xprv9s21ZrQH143K4XUJvv1fQcoDGhLdtQUsfqX7zX4p5XvMtbh8bQxjjMRSTdeVakjmMtg52no13wj g16eSgKA32i1WRxSNYpDaRkF2GkG2NZc";
BitcoinExtKey base58key = new BitcoinExtKey(keyStr, Network.Main); //Would throw FormatException if the base58 is for Network.TestNet
ExtKey key = base58key.Key;
ExtPubKey pubkey = key.Neuter();
//Or, if you want NBitcoin to deduce the network from the base58 directly, this does exactly the same thing
base58key = Network.CreateFromBase58Data(keyStr);
key = base58key.Key;
pubkey = key.Neuter();
//Now, if you want to transform back in BitcoinExtKey
base58key = new BitcoinExtKey(key, Network.Main); //Or Network.Main.CreateBitcoinExtKey(key)
string base58Str = base58key.ToString(); //base58Str == key
//If you want the PubKey in base58 :
var base58PubKey = new BitcoinExtPubKey(pubkey, Network.Main); //Or Network.Main.CreateBitcoinExtPubKey(key)
base58Str = base58PubKey.ToString(); //xpub661MyMwAqRbcH1Yn2wYfmkjwpjB8HsCj34SinuURdsTLmQ2H8xGzH9jvJxMYRRKchqEYF3xEB1sXsnQTRKU8oNYDChLuYPgyBneJvdzhc1J
//BitcoinExtPubKey.PubKey to get back the ExtPubKey
BitcoinExtKey base58key = new BitcoinExtKey(keyStr, Network.Main); //Would throw FormatException if the base58 is for Network.TestNet
ExtKey key = base58key.Key;
ExtPubKey pubkey = key.Neuter();
//Or, if you want NBitcoin to deduce the network from the base58 directly, this does exactly the same thing
base58key = Network.CreateFromBase58Data
key = base58key.Key;
pubkey = key.Neuter();
//Now, if you want to transform back in BitcoinExtKey
base58key = new BitcoinExtKey(key, Network.Main); //Or Network.Main.CreateBitcoinExtKey(key)
string base58Str = base58key.ToString(); //base58Str == key
//If you want the PubKey in base58 :
var base58PubKey = new BitcoinExtPubKey(pubkey, Network.Main); //Or Network.Main.CreateBitcoinExtPubKey(key)
base58Str = base58PubKey.ToString(); //xpub661MyMwAqRbcH1Yn2wYfmkjwpjB8HsCj34SinuURdsTLmQ2H8xGzH9jvJxMYRRKchqEYF3xEB1sXsnQTRKU8oNYDChLuYPgyBneJvdzhc1J
//BitcoinExtPubKey.PubKey to get back the ExtPubKey
So to respond to your question, the reason why there is both BitcoinExtKey and ExtKey is because BitcoinExtKey represents a base58 data, which contains more information that simply the ExtKey. (that has no Network information attached to ExtKey, just the strict cryptographic data)
This works the same for all base58Data. (Key vs BitcoinSecret for example)
