Solidity

Synopsis

An e-auction lets bidders compete for an item online, but participants usually have to trust the auction house to record bids honestly and to declare the true highest bidder. In this project the bidding rules run in a smart contract, so every registration and bid is recorded on the blockchain and the winner is determined by public code.

The deploying account is the Auction Manager, who puts an item up for auction with a starting value. Other accounts register as bidders and place open, increasing bids – each bid must be higher than the current highest. When the manager declares the result, the last (highest) bid wins, and the winner pays the bid amount in Ether through the contract to the manager.

Contract Architecture

How It Works

Bid values are entered in whole ETH. checkTransactionAmount converts them to wei (bidValue * 10**18) and requires msg.value to match exactly.

Key Requirements (Concepts Used)

• Structs imported from a separate file (Chapters 2, 13).
• Mappings with counters used as keys (Chapter 12).
• Modifiers, including a modifier with an argument (Chapter 22).
• Payable function and transfer to an address payable (Chapters 3, 24).
• Private helper functions such as gotoNextBidder() (Chapter 15).

How to Run in Remix

1. Create contracts/BlockMaster.sol and contracts/AuctionContract.sol from the resource files; compile and deploy AuctionContract from the first account (manager).
2. Manager: call setAuctionItem("Painting", 1).
3. From two or three other accounts call BidderRegisterMaster, then MyBidding with rising values (e.g. 2, 3, 5).
4. Manager: call declareAuctionResult() and read winner.
5. Winner: set the VALUE field to the bid in Ether (e.g. 5 ether) and call transferBidAmount(); the manager’s balance increases.

Limitations and Extension Exercises

Exercise solution (escrow auction with refunds)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract EscrowAuction {
    address payable public immutable seller = payable(msg.sender);
    uint public immutable endTime;
    address public highestBidder;
    uint public highestBid;
    bool public ended;
    mapping(address => uint) public refunds;      // out-bid amounts to withdraw

    constructor(uint durationSeconds) {
        endTime = block.timestamp + durationSeconds;
    }

    function bid() external payable {
        require(block.timestamp < endTime, "Auction over");
        require(msg.value > highestBid, "Bid too low");
        if (highestBidder != address(0)) {
            refunds[highestBidder] += highestBid;  // previous leader can withdraw
        }
        highestBidder = msg.sender;
        highestBid = msg.value;
    }

    function withdrawRefund() external {
        uint amount = refunds[msg.sender];
        require(amount > 0, "Nothing to withdraw");
        refunds[msg.sender] = 0;                   // effects before interaction
        (bool ok, ) = payable(msg.sender).call{value: amount}("");
        require(ok, "Refund failed");
    }

    function endAuction() external {
        require(block.timestamp >= endTime, "Not yet ended");
        require(!ended, "Already ended");
        ended = true;
        (bool ok, ) = seller.call{value: highestBid}("");
        require(ok, "Payment failed");
    }
}

Key Takeaways

Video : Project 3 : Smart Contract for Bidding / e-Auction System

Resource Files

• BlockMaster.sol File
• AuctionContract.sol File

Question / Answer (15 questions)

MCQ Test : Check Your Understanding (5 questions)

Select an option and press Check Answer, or answer all questions and press Submit Test to see your score. Each answer comes with an explanation.

Bibliography/References

The notes, questions and MCQs on this page are original explanations written for this course. The works listed below were consulted and are cited for attribution and further reading. Product names, logos and trademarks belong to their respective owners. Short quotations, where used, are marked and attributed.

1. Solidity Team. (n.d.). Contracts (creating contracts, visibility and getters, function modifiers, constant and immutable, functions, events, custom errors, inheritance, abstract contracts, interfaces, libraries). Solidity Documentation. https://docs.soliditylang.org/en/latest/contracts.html
2. Solidity Team. (n.d.). Units and Globally Available Variables. Solidity Documentation. https://docs.soliditylang.org/en/latest/units-and-global-variables.html
3. Solidity Team. (n.d.). Security Considerations. Solidity Documentation. https://docs.soliditylang.org/en/latest/security-considerations.html
4. ethereum.org. (n.d.). Smart contract security. Ethereum Developer Documentation. https://ethereum.org/en/developers/docs/smart-contracts/security/
5. Remix Project. (n.d.). Remix IDE Documentation. https://remix-ide.readthedocs.io/en/latest/
6. Modi, R. (2018). Solidity Programming Essentials: A beginner’s guide to build smart contracts for Ethereum and blockchain. Packt Publishing. ISBN 978-1-78883-138-3.