TypeError. Expect type conversion not allowed from “uint256” to “contract MyToken”

Question

I followed the video, NFT marketplace on ICE, and could not compile because of the following TypeErrors. Expect type conversion not allowed from “uint256” to “contract MyToken”. Expect type conversion not allowed from “contract marketPlace” to “library address”; Type uint256 is not implicitly convertible expected type address. Invalid type for argument in function call. Invalid implicit conversion from library Address to address requested. Invalid type for argument in function call. Invalid implicit conversion from luint256 to address requested.

full code for MarketPlace.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import "./MyToken.sol";

contract marketPlace 
{
    struct Item {
        address tokenAddress;
        uint256 index;
        uint256 tokenID;
        uint256 price;
        address payable owner;
        string url;
    }
    // Types
    Item[] public items;

    mapping(address => mapping(uint256 => bool)) listedTokens;

    function listForSale(
        uint256 _tokenID,
        uint256 _price,
        uint256 _tokenAddress,
        string memory _url
    ) public {
        uint256 index = items.length;
        MyToken ERC721 = MyToken(_tokenAddress);
        require(
            msg.sender == ERC721.ownerOf(_tokenID),
            "MarketPlace:  Sender is not the owner of the token."
        );
        require(
            !listedTokens[_tokenAddress][_tokenID],
            "MarketPlace:  The token is already listed."
        );
        require(
            address(this) == ERC721.getApproved(_tokenID) ||
            ERC721.isApprovedForAll(msg.sender, Address(this)),
            "MarketPlace:  NFTMarketPlace was not provided approval to transfer."
        );
        items.push(
            Item(
                index,
                _tokenAddress,
                _tokenID,
                _price,
                payable(msg.sender),
                _url
            )
        );

        listedTokens[_tokenAddress][_tokenID] = true;
}
    function buyNFT() public payable returns (uint256 _index) 
    {
        require(
            msg.value == items[_index].price,
            "MarketPlace:  Please pay the amount equal to the price."
        );

   MyToken ERC721 = MyToken(items[_index].tokenAddress);
   ERC721.safeTransferFrom(
       items[_index].owner,
       msg.sender,
       items[_index].tokenID
   );
    items[_index].owner.transfer(msg.value);
    listedTokens[items[_index].tokenAddress][items[_index].tokenID] = false;
    for (uint256 i = _index; i < items.length - 1; i++) {
        items[i] = items[i + 1];
        items[i].index -= 1;
    }
    items.pop();
    }
    function getItemsList() external view returns (Item[] memory) {
        return items;
    }
    function totalItems() external view returns (uint256) {
        return items.length;
    }

}

Answer 1

In this statement:

MyToken ERC721 = MyToken(_tokenAddress);

you're passing to MyToken smart contract instance an uint256, and this is the cause of your issue. When you instance a smart contract, you must pass its address. This address must be address datatype. Example:

MyToken ERC721 = MyToken([addressMyTokensmartcontract]);

I adjusted your smart contract in this way:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import "./MyToken.sol";

contract marketPlace 
{
    struct Item {
        address tokenAddress;
        uint256 index;
        uint256 tokenID;
        uint256 price;
        address payable owner;
        string url;
    }
    // Types
    Item[] public items;

    mapping(address => mapping(uint256 => bool)) listedTokens;

    function listForSale(
        uint256 _tokenID,
        uint256 _price,
        address _tokenAddress,
        string memory _url
    ) public {
        uint256 index = items.length;
        MyToken ERC721 = MyToken(_tokenAddress);
        require(
            msg.sender == ERC721.ownerOf(_tokenID),
            "MarketPlace:  Sender is not the owner of the token."
        );
        require(
            !listedTokens[_tokenAddress][_tokenID],
            "MarketPlace:  The token is already listed."
        );
        require(
            address(this) == ERC721.getApproved(_tokenID) ||
            ERC721.isApprovedForAll(msg.sender, address(this)),
            "MarketPlace:  NFTMarketPlace was not provided approval to transfer."
        );
        items.push(
            Item(
                index,
                _tokenAddress,
                _tokenID,
                _price,
                payable(msg.sender),
                _url
            )
        );

        listedTokens[_tokenAddress][_tokenID] = true;
}
    function buyNFT() public payable returns (uint256 _index) 
    {
        require(
            msg.value == items[_index].price,
            "MarketPlace:  Please pay the amount equal to the price."
        );

   MyToken ERC721 = MyToken(items[_index].tokenAddress);
   ERC721.safeTransferFrom(
       items[_index].owner,
       msg.sender,
       items[_index].tokenID
   );
    items[_index].owner.transfer(msg.value);
    listedTokens[items[_index].tokenAddress][items[_index].tokenID] = false;
    for (uint256 i = _index; i < items.length - 1; i++) {
        items[i] = items[i + 1];
        items[i].index -= 1;
    }
    items.pop();
    }
    function getItemsList() external view returns (Item[] memory) {
        return items;
    }
    function totalItems() external view returns (uint256) {
        return items.length;
    }
}