How to get Master Chain Code I'm using NBitcoin and I created an ExtKey object. Now I need to get the Master Chain Code (see page 89 of Mastering Bitcoin for a good explanation) for this HD node. How do I do this?

According to Mastering Bitcoin a root seed is used with HMAC-SHA512 to create a 512-bits output. The left 256 bits are used for the master private key, the right 256 bits are used for the master chain code. When I call GetBytes() on my ExtKey object, however, it returns a byte[74] array. This corresponds to 592 bits. I don't know what the extra 80 bits are so I can't just take the right 256 bits.
I solved this with a pull request that adds a property to ExtKey and ExtPubKey that allows you to call the ChainCode property. See pull request 156.

It has been merged into NBitcoin 3.0.0.28 which is available on NuGet.