Solidity by Example
version 0.8.3

dark mode

English Auction

English auction for NFT.


  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 except the highest bidder can withdraw their 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 ^0.8.3;

interface IERC721 {
    function transfer(address, uint) external;

    function transferFrom(
    ) external;

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

    IERC721 public nft;
    uint public nftId;

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

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

        address _nft,
        uint _nftId,
        uint _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 {
        uint bal = bids[msg.sender];
        bids[msg.sender] = 0;

        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.transfer(highestBidder, nftId);
        } else {
            nft.transfer(seller, nftId);

        emit End(highestBidder, highestBid);

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