Recommended Articles:
（1）https://coinswitch.co/news/ethereum-hard-fork-jan-19-know-everything-about-3-upcoming-eth-hard-forks
There are 3 hard forks lined up for this month of January.
- Classic Vision Hard Fork
All the Ethereum hodlers will get 3 ETCV (Ethereum Classic Vision) coins for every ETH in their private wallets. This hard fork is about to happen on 11th January 2019 - Ethereum hard fork date.
- Ethereum Nowa Hard Fork
Just a day after the Classic vision hard fork, on 12th of January, 2019, Ethereum Nowa Fork will happen, where the ETH owners will get 1 ETN (Ethereum Nowa) coin for every ETH in their private wallets.
- Constantinople Ethereum Hard Fork (Major one)
This is the major and the most significant ETH hard fork happening between before 14th to 18th January 2019. The Constantinople Ethereum hard fork, which is the 2nd phase of the Metropolis project, is expected to be built on 7080000 blocks, improving efficiency and performance. This is undertaken as a step to transit from Proof of Work (PoW) protocol to Proof of Stake (PoS) protocol (Casper update). This is expected to make the ETH network faster and less costly to operate. It might be controversial as it would decrease the miners' reward from 3 ETH to 2 ETH as well.
（2）https://cryptonomist.ch/en/2018/12/11/ethereum-constantinople-hard-fork-2/
In fact, the five improvements that will be introduced with this hard fork will permanently change the Ethereum network because they are not backwards compatible with the current protocol.
This means that all the nodes in the Ethereum network will need to be updated and this will happen with the 7,080,000th block.
In theory, some nodes and some miners could even continue to use the current protocol, and this would fork the blockchain.
However, to date, it does not appear that this is a very likely possibility because no major actor in the Ethereum network has stated that they will not update the software in use.
If this were to happen, there would be a doubling resulting in the birth of a new cryptocurrency, but this is an unlikely possibility to date.
Geth:
https://github.com/ethereum/go-ethereum
https://github.com/ethereum/go-ethereum/releases
Geth v1.8.20 is a bit of a special release. On one hand it's the release that finally enables the Constantinople hard fork on mainnet at block 7080000 (and Rinkeby at block 3660663). It's also our last planned release of the 1.8 family (we'll still do hotfixes if need be), meaning that we'll start merging backwards incompatible changes onto master in preparation of Geth 1.9.0 (we don't have an ETA for it, but January the earliest).
C# sdk:
https://github.com/Nethereum/Nethereum
https://github.com/Nethereum/Nethereum/issues/489