msg.value and msg.sender in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity event log?

Consider the following Solidity contract snippet. A user calls sendEther with 1 ether. What will be the values emitted in the event?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    event Log(address sender, uint amount);

    function sendEther() public payable {
        emit Log(msg.sender, msg.value);
    }
}

A The event logs the contract's address and zero ether.

B The event logs the caller's address and zero ether.

C The event logs the caller's address and the amount of ether sent (1 ether).

D The event logs the zero address and the amount of ether sent.

💡 Hint

Remember, msg.sender is the caller's address and msg.value is the amount of ether sent with the call.

When a function is called with ether, msg.sender is the caller's address and msg.value is the amount of ether sent. The event logs these values.

🧠 Conceptual

intermediate

What does msg.value represent in Solidity?

Choose the best description of msg.value in a Solidity function call.

A <code>msg.value</code> is the amount of ether (in wei) sent with the transaction.

B <code>msg.value</code> is the block timestamp when the transaction was mined.

C <code>msg.value</code> is the address of the sender.

D <code>msg.value</code> is the amount of gas used by the transaction.

💡 Hint

Think about what value is transferred along with a payable function call.

msg.value holds the amount of ether (in wei) sent with the current call or transaction.

❓ Predict Output

advanced

What happens if a non-payable function tries to access msg.value with ether sent?

Given this contract, what will happen if nonPayableFunction is called with 1 ether?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    function nonPayableFunction() public {
        uint amount = msg.value;
    }
}

A The transaction will revert because the function is not payable but ether was sent.

B The function will run and <code>amount</code> will be 1 ether.

C The function will run and <code>amount</code> will be zero.

D The function will ignore <code>msg.value</code> and continue without error.

💡 Hint

Consider the payable modifier and what happens when ether is sent to a non-payable function.

Sending ether to a non-payable function causes the transaction to revert. Accessing msg.value alone does not cause error, but the call with ether to a non-payable function is rejected.

🧠 Conceptual

advanced

Which statement about msg.sender is true?

Choose the correct statement about msg.sender in Solidity.

A <code>msg.sender</code> is the address of the contract currently executing the code.

B <code>msg.sender</code> can be a contract address if the call was made by another contract.

C <code>msg.sender</code> is always the original external account that started the transaction.

D <code>msg.sender</code> is always the zero address when called internally.

💡 Hint

Think about calls between contracts and external accounts.

msg.sender is the immediate caller of the function. It can be an external account or another contract.

🔧 Debug

expert

Why does this contract fail to receive ether?

Examine the contract below. Why does sending ether to receiveEther fail?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    function receiveEther() public {
        uint received = msg.value;
    }
}

A The function does not emit an event to confirm ether reception.

B The function does not have a fallback function to accept ether.

C The contract does not have a constructor to initialize ether reception.

D The function is not marked payable, so it cannot receive ether.

💡 Hint

Check the function modifiers required to accept ether.

In Solidity, functions must be marked payable to receive ether. Without it, sending ether causes the transaction to revert.