Timelock pattern in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity timelock contract snippet?

Consider this simplified Solidity contract snippet implementing a timelock for a withdrawal function. What will be the output when withdraw() is called before the timelock expires?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleTimelock {
    uint256 public unlockTime;
    address public owner;

    constructor(uint256 _unlockTime) {
        owner = msg.sender;
        unlockTime = _unlockTime;
    }

    function withdraw() public {
        require(block.timestamp >= unlockTime, "Timelock not expired");
        payable(owner).transfer(address(this).balance);
    }

    receive() external payable {}
}

// Assume unlockTime is set to block.timestamp + 1 hour
// withdraw() is called 30 minutes after deployment

A Transaction reverts with error 'Timelock not expired'

B Funds are transferred to the owner

C Transaction succeeds but no funds are transferred

D Compilation error due to missing payable keyword

💡 Hint

Think about the require condition and the current block timestamp compared to unlockTime.

The withdraw function requires the current block timestamp to be greater than or equal to unlockTime. Since the call happens before the timelock expires, the require fails and the transaction reverts with the error message.

🧠 Conceptual

intermediate

Which statement best describes the purpose of a timelock pattern in smart contracts?

Choose the best description of why developers use the timelock pattern in blockchain smart contracts.

A To delay execution of critical functions to allow users to react or audit changes

B To permanently lock funds so they cannot be withdrawn

C To speed up transaction processing by batching calls

D To encrypt contract data for privacy

💡 Hint

Think about security and transparency in decentralized systems.

The timelock pattern delays execution of sensitive functions, giving users time to review or react before changes take effect. This improves security and trust.

🔧 Debug

advanced

What error does this Solidity timelock contract produce when calling execute() too early?

Analyze the following Solidity contract snippet. What error message will the execute() function produce if called before the timelock expires?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Timelock {
    uint256 public releaseTime;
    address public admin;

    constructor(uint256 _releaseTime) {
        admin = msg.sender;
        releaseTime = _releaseTime;
    }

    function execute() public {
        require(block.timestamp > releaseTime, "Too early to execute");
        // perform critical action
    }
}

A Transaction succeeds and performs critical action

B Transaction reverts with 'Too early to execute'

C Compilation error due to missing visibility on constructor

D Transaction reverts with 'Unauthorized caller'

💡 Hint

Check the require condition and error message.

The require statement checks if the current time is greater than releaseTime. If called too early, it reverts with 'Too early to execute'.

📝 Syntax

advanced

Which option correctly declares a timelock variable in Solidity to store a future timestamp?

Choose the correct Solidity code snippet that declares a public variable named unlockTime to store a UNIX timestamp for a timelock.

A uint256 unlockTime = public;

B uint public unlockTime() public;

C public uint256 unlockTime;

D uint256 public unlockTime;

💡 Hint

Remember Solidity variable declaration syntax and visibility keywords.

Option D correctly declares a public unsigned integer variable. Other options have syntax errors or incorrect order.

🚀 Application

expert

How many items are in the queue after scheduling three timelock transactions and executing one?

A timelock contract maintains a queue of scheduled transactions. Initially empty, it schedules three transactions with unique IDs. Then, it executes one transaction and removes it from the queue. How many transactions remain in the queue?

A 1

B 3

C 2

D 0

💡 Hint

Think about adding and removing items from a list.

Scheduling three transactions adds three items. Executing one removes it, leaving two remaining in the queue.