Solidity & Smart Contract Development

Solidity is a statically typed, curly-brace, contract-oriented language for writing smart contracts on the EVM (Ethereum and EVM-compatible chains: Arbitrum, Optimism, Base, Polygon, BNB Chain, Avalanche...). Syntax is influenced by C++, Python and JavaScript. Created by Gavin Wood and others; developed by the Ethereum Foundation's Solidity team (first released 2015).

A contract is like a class: state variables (stored permanently on-chain), functions, modifiers, events, errors, and can inherit from other contracts. Code is compiled with solc into EVM bytecode plus an ABI.

Smart contract development is high-stakes: deployed code is public, usually immutable, and often holds money — bugs are exploited quickly. Interviews focus heavily on security, gas costs and testing.

Always pin the compiler: pragma solidity 0.8.28; (or ^0.8.24) and use a recent 0.8.x release. Library versions: OpenZeppelin Contracts 4.x (2021–2023) → 5.x (2023+, custom errors, Ownable(initialOwner), namespaced storage).

Development lifecycle:

1. Specify — requirements, roles, invariants ("total supply equals sum of balances"), threat model.
2. Write — Solidity using audited building blocks (OpenZeppelin).
3. Compile — forge build / npx hardhat compile → bytecode + ABI.
4. Test — unit tests, fuzz tests, invariant tests, fork tests against mainnet state; coverage.
5. Static analysis — Slither, compiler warnings, linters (solhint).
6. Audit — internal review + external audit; bug bounty.
7. Deploy — testnet (Sepolia) → mainnet with a script; record addresses; deploy through a multisig where possible.
8. Verify — publish source on Etherscan/Blockscout.
9. Operate — monitoring (events, Defender/Forta), admin actions via multisig + timelock, incident response (pause).
10. Upgrade or migrate — through a proxy (if designed) or by deploying a new contract and migrating state.

Contract execution lifecycle (per call): transaction/call reaches the contract → EVM loads bytecode → function selector (first 4 bytes of calldata = keccak256("transfer(address,uint256)")[0:4]) dispatches to the function → modifiers run → body executes reading/writing storage, using memory/stack, possibly calling other contracts → emits events → returns data, or reverts (all state changes undone, remaining gas returned except for what was used).

Contract lifecycle on-chain: creation transaction runs the constructor (init code) → runtime bytecode stored at the new address → used via calls → (legacy selfdestruct is effectively disabled since Dencun/EIP-6780 except within the same transaction as creation).