Verify and Publish Contract on Etherscan with Imported OpenZeppelin file

Question

I'm currently building a ERC721 compliant contract and have published the contract here: https://ropsten.etherscan.io/address/0xa513bc0a0d3af384fefcd8bbc1cc0c9763307c39 - I'm now attempting to verify and publish the contract source code

The start of my file looks like so:

// SPDX-License-Identifier: MIT

// We will be using Solidity version 0.8.4
pragma solidity 0.8.4;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract ViperToken is ERC721 {

However, when attempting to verify and publish with a Solidity single file I have the following error appear:

ParserError: Source "@openzeppelin/contracts/token/ERC721/ERC721.sol" not found: File import callback not supported
 --> myc:6:1:
  |
6 | import "@openzeppelin/contracts/token/ERC721/ERC721.sol"
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Can anyone point me in the direction to either 1. Solve this problem or 2. Documentation on how to appropriately write a contract that has dependencies imported that can be verified with Etherscan. Right now this is just a single file contract.

Answer 1

Simply put I had to go down a rabbit hole to work this out as I'm pretty new to Solidity.

I had to do the following;

1. Learn and use https://www.trufflesuite.com/ to setup a project and put my contract there (Using Ganache helped a lot with testing for anyone new to Solidity too)
2. Use HD Wallet provider package and follow tutorial here to get it on ropsten Etherscan https://medium.com/coinmonks/5-minute-guide-to-deploying-smart-contracts-with-truffle-and-ropsten-b3e30d5ee1e
3. Finally, use truffle-plugin-verify https://github.com/rkalis/truffle-plugin-verify to verify the contract on Etherscan

All in all, I am pretty sure there is no way within the Etherscan web app to verify a contract that contains an imported file.

The final product is here if anyone is interested in seeing how I structured it all https://github.com/lukecurtis93/viper-nft (it's just a CryptoKitties clone I found online as a base and updated it all)

Answer 2

I used npx hardhat flatten to compile all the code into one page, then copy and paste the code into Etherscan's single file verification. I think it is fine if you are just learning to get a feel for verifying your smart contract in Etherscan. But when it comes to production level code, I think OP's solution is better.

Answer 3

npx hardhat flatten gives license identifiers error when trying to verify in etherscan.

Solution is to add the below in hardhat.config.js

 task("flat", "Flattens and prints contracts and their dependencies (Resolves licenses)")
  .addOptionalVariadicPositionalParam("files", "The files to flatten", undefined, types.inputFile)
  .setAction(async ({ files }, hre) => {
    let flattened = await hre.run("flatten:get-flattened-sources", { files });
    
    // Remove every line started with "// SPDX-License-Identifier:"
    flattened = flattened.replace(/SPDX-License-Identifier:/gm, "License-Identifier:");
    flattened = `// SPDX-License-Identifier: MIXED\n\n${flattened}`;

    // Remove every line started with "pragma experimental ABIEncoderV2;" except the first one
    flattened = flattened.replace(/pragma experimental ABIEncoderV2;\n/gm, ((i) => (m) => (!i++ ? m : ""))(0));
    console.log(flattened);
  });

An then run npx hardhat flat contracts/ContractToFlatten.sol > Flattened.sol

Answer 4

If you are compiling into REMIX IDE

1. From REMIX IDE

2. Search for "Flattener" pluging

3. Flatten yourcontract.sol

4. Copy/Paste on Etherscan