Solidity Bad Practices: Ether Balance Check

A function compares the contract balance to a specific Ether value.

Assuming the contract has a specific Ether balance can lead to erroneous or unexpected behavior because the balance of a contract can be forcibly altered, for example by sending Ether to the contract.

Example 1: The following code uses an assert to check that the balance of the Lock contract instance is a specific value (msg.value).

contract Lock {
    constructor (address owner, uint256 unlockTime) public payable {
        ...
    }
}

contract Lockdrop {
    ...
    function lock(...) {
        uint256 eth = msg.value;
        address owner = msg.sender;
        uint256 unlockTime = unlockTimeForTerm(term);

        Lock lockAddr = (new Lock).value(eth)(owner, unlockTime);

        assert(address(lockAddr).balance == msg.value);
    }
}