Solidity Interview Workbook

A teaching workbook for engineers preparing for Solidity / smart-contract interviews — especially those coming from adjacent depth (protocol engineering, backend systems, other ledger stacks) who need the contract layer and its idioms, not a from-zero course.

Ten exercises, from a native-ETH escrow to a permissioned tokenized bond, each in three parts:

• exercises/NN_*.sol — stubs with TODOs. Work here.
• solutions/NN_*.sol — annotated reference implementations, each headed by an INTERVIEW NOTES block: the design decisions, security patterns, and production gaps an interviewer probes.
• test-specs/NN_*.md — behavior checklists; test/NN_*.t.sol implements each checklist one test per line (69 tests total).

Beyond the contracts:

• NOTES.md — the conceptual backbone: native ETH mechanics (CALL, receive/fallback, forced ETH, Cancun/Pectra updates), WETH and wrap-vs-swap, diamond inheritance, EIP-712, delegatecall and every proxy upgrade pattern, and a map of what's EVM protocol vs. Solidity compiler fiction.
• DEFENSE.md — a one-page design-defense sheet for architect-level interviews: per contract, the decision / the credible alternative / the why.

How to use it

1. Implement one contract at a time in exercises/; keep solutions/ closed until you've attempted it.
2. Write tests against the matching test-specs/ checklist (or run the provided suite against your implementation by flipping the import in test/NN_*.t.sol from ../solutions/... to ../exercises/...).
3. Explain your design out loud — the INTERVIEW NOTES blocks and DEFENSE.md show the level of reasoning to aim for.
4. Compare with the reference solution and refactor rather than copy.

Suggested order

Escrow → ERC20 → Vesting → MultiSig → Voting → ERC721 → Oracle consumer → Upgradeable (UUPS) → Treasury → Tokenized bond

The escrow is deliberately first: it teaches the native-value layer (payable, msg.value, checks-effects-interactions, reentrancy, push-vs-pull) that everything else builds on.

Environment

Foundry project. OpenZeppelin v5.6 (and Contracts Upgradeable) vendored as git submodules — clone with --recurse-submodules or run git submodule update --init after cloning.

forge test                                   # full suite (69 tests)
forge test --match-path test/01_Escrow.t.sol # one contract
./run.sh                                     # full driver, see below

./run.sh runs the test suite, starts a local anvil, deploys all ten solutions wired and funded (via script/DeployAll.s.sol), then performs a live UUPS V1→V2 upgrade drill with cast — printing the EIP-1967 slot before and after, the new version(), and the preserved counter state. Anvil runs on port 9545 (not 8545) to avoid colliding with other local nodes.

The contracts are intentionally compact: interview exercises, not production systems. Each solution's notes say explicitly what a production version would add.