Why am I getting TypeError: Member "balanceOf" not found or not visible after argument?

Question

On line 22 my code is -- require(tokenContract.balanceOf(this) >= _numberOfTokens);

This gives the error message

TypeError: Member "balanceOf" not found or not visible after argument-dependent lookup in string storage ref.require(tokenContract.balanceOf(this) >= numberOfTokens); ^---------------------^

My entire code is:

pragma solidity ^0.5.1;

contract ChampTokenSale {
    address admin;
    string public tokenContract;
    uint256 public tokenPrice;
    uint256 public tokensSold;

    event Sell(address _buyer, uint256 _amount);

    constructor (ChampTokenSale _tokenContract, uint256 _tokenPrice) public {
      admin = msg.sender;
      address expected = address(this);
      tokenPrice = _tokenPrice;
    }

    function multiply(uint x, uint y) internal pure returns (uint z) {
      require(y == 0 || (z = x * y) / y == x);
    }

    function buyTokens(uint256 _numberOfTokens) public payable {
      require(msg.value == multiply(_numberOfTokens, tokenPrice));
      require(tokenContract.balanceOf(this)>= _numberOfTokens);
      require(tokenContract.transfer(msg.sender, _numberOfTokens));

      tokensSold += _numberOfTokens;

      emit Sell(msg.sender, _numberOfTokens);
    }

    function endSale() public {
     require(msg.sender == admin);
     require(tokenContract.transfer(admin,  tokenContract.balanceOf(address(this))));


     admin.transfer(address(this).balance);
   }
}

Answer 1

In your code, tokenContract is a string, as defined here:

string public tokenContract;

There is no method balanceOf that can be called on a string.

Answer 2

There is a lot going wrong here.

The short answer is tokenContract is a string because of this:

string public tokenContract;

There is no reason to think a string should have a method balanceOf(). It's not possible.

Hope it helps.