English Auction (Solidity Code Example)

Code example of an English auction for an NFT.

Auction

1. Seller of NFT deploys this contract.
2. Auction lasts for 7 days.
3. Participants can bid by depositing ETH greater than the current highest bidder.
4. All bidders can withdraw their bid if it is not the current highest bid.

After the auction

1. Highest bidder becomes the new owner of NFT.
2. The seller receives the highest bid of ETH.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

interface IERC721 {
    function safeTransferFrom(address from, address to, uint256 tokenId)
        external;
    function transferFrom(address, address, uint256) external;
}

contract EnglishAuction {
    event Start();
    event Bid(address indexed sender, uint256 amount);
    event Withdraw(address indexed bidder, uint256 amount);
    event End(address winner, uint256 amount);

    IERC721 public nft;
    uint256 public nftId;

    address payable public seller;
    uint256 public endAt;
    bool public started;
    bool public ended;

    address public highestBidder;
    uint256 public highestBid;
    mapping(address => uint256) public bids;

    constructor(address _nft, uint256 _nftId, uint256 _startingBid) {
        nft = IERC721(_nft);
        nftId = _nftId;

        seller = payable(msg.sender);
        highestBid = _startingBid;
    }

    function start() external {
        require(!started, "started");
        require(msg.sender == seller, "not seller");

        nft.transferFrom(msg.sender, address(this), nftId);
        started = true;
        endAt = block.timestamp + 7 days;

        emit Start();
    }

    function bid() external payable {
        require(started, "not started");
        require(block.timestamp < endAt, "ended");
        require(msg.value > highestBid, "value < highest");

        if (highestBidder != address(0)) {
            bids[highestBidder] += highestBid;
        }

        highestBidder = msg.sender;
        highestBid = msg.value;

        emit Bid(msg.sender, msg.value);
    }

    function withdraw() external {
        uint256 bal = bids[msg.sender];
        bids[msg.sender] = 0;
        payable(msg.sender).transfer(bal);

        emit Withdraw(msg.sender, bal);
    }

    function end() external {
        require(started, "not started");
        require(block.timestamp >= endAt, "not ended");
        require(!ended, "ended");

        ended = true;
        if (highestBidder != address(0)) {
            nft.safeTransferFrom(address(this), highestBidder, nftId);
            seller.transfer(highestBid);
        } else {
            nft.safeTransferFrom(address(this), seller, nftId);
        }

        emit End(highestBidder, highestBid);
    }
}