Receive and fallback functions in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What happens when Ether is sent to this contract?

Consider the following Solidity contract. What will be the output when 1 Ether is sent to the contract address without calling any function explicitly?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    event Log(string func);

    receive() external payable {
        emit Log("receive called");
    }

    fallback() external payable {
        emit Log("fallback called");
    }
}

A Transaction reverts with an error.

B The event 'fallback called' is emitted.

C The event 'receive called' is emitted.

D No event is emitted, but Ether is accepted.

💡 Hint

Sending Ether without data triggers the receive function if it exists.

When Ether is sent with empty calldata, the receive() function is called if it is defined and payable. Otherwise, fallback() is called if payable. Here, receive() exists and is payable, so it emits 'receive called'.

❓ Predict Output

intermediate

Which function is called when sending data to this contract?

Given the contract below, what event is emitted when a transaction with non-empty calldata but no matching function signature is sent?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    event Log(string func);

    receive() external payable {
        emit Log("receive called");
    }

    fallback() external payable {
        emit Log("fallback called");
    }
}

A The event 'fallback called' is emitted.

B Transaction reverts with an error.

C No event is emitted, Ether is refunded.

D The event 'receive called' is emitted.

💡 Hint

Fallback is called when calldata is not empty and no matching function exists.

When calldata is non-empty and no matching function exists, the fallback() function is called if payable. Here, fallback() emits 'fallback called'.

🔧 Debug

advanced

Why does this contract reject Ether transfers?

Examine the contract below. Why does sending Ether to this contract cause the transaction to revert?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    fallback() external {
        // no payable modifier
    }
}

A Because fallback() is missing the receive() function, it reverts.

B Because fallback() is not payable, it rejects Ether transfers.

C Because fallback() has no code, it reverts automatically.

D Because the contract has no constructor, it rejects Ether.

💡 Hint

Only payable functions can receive Ether.

The fallback() function is not marked payable, so any Ether sent to the contract triggers a revert because no payable receive() or fallback() exists.

📝 Syntax

advanced

Identify the syntax error in this fallback function

Which option correctly fixes the syntax error in the fallback function below?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    fallback() external payable {
        // missing function body
    }
}

A Add empty braces: fallback() external payable {}

B Add a semicolon after fallback() external payable;

C Remove payable modifier: fallback() external {}

D Add returns keyword: fallback() external payable returns (bool) {}

💡 Hint

Functions must have a body or be declared abstract.

The fallback function must have a function body enclosed in braces. Adding empty braces fixes the syntax error.

🚀 Application

expert

How many times is fallback called in this scenario?

Given the contract below, if a user sends 0 Ether with non-empty calldata that does not match any function, how many times will the fallback function be called during the transaction?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint public count = 0;

    fallback() external payable {
        count++;
    }

    function getCount() public view returns (uint) {
        return count;
    }
}

A Fallback is called twice, count becomes 2.

B Fallback is called once, but count remains 0 due to no Ether.

C Fallback is not called, count remains 0.

D Fallback is called once, count becomes 1.

💡 Hint

Fallback is called once per transaction if calldata does not match any function.

The fallback function is called once per transaction when calldata does not match any function. The count increments by 1 regardless of Ether sent.