msg.value and msg.sender in Blockchain / Solidity - Mini Project: Build & Apply

Understanding msg.value and msg.sender in Solidity

📖 Scenario: You are creating a simple smart contract for a crowdfunding campaign on the Ethereum blockchain. People can send money to support the project, and the contract keeps track of who sent money and how much.

🎯 Goal: Build a smart contract that records the sender's address and the amount of Ether sent using msg.sender and msg.value. Then display the total amount collected and the last sender's address.

📋 What You'll Learn

Create a public variable lastSender to store the address of the last person who sent Ether

Create a public variable totalCollected to store the total amount of Ether collected

Write a receive function to accept Ether and update lastSender and totalCollected

Write a function getSummary that returns lastSender and totalCollected

💡 Why This Matters

🌍 Real World

Smart contracts on Ethereum use <code>msg.sender</code> and <code>msg.value</code> to handle payments and identify users.

💼 Career

Understanding these variables is essential for blockchain developers building decentralized applications that handle money.

1

Set up the contract and variables

Create a contract called CrowdFund. Inside it, declare a public address variable called lastSender and a public unsigned integer variable called totalCollected.

Blockchain / Solidity

contract CrowdFund {
    // Your code here
}

Need a hint?

Use address public lastSender; and uint public totalCollected; inside the contract.

2

Add the receive function to accept Ether

Add a receive function that is external and payable. Inside it, set lastSender to msg.sender and add msg.value to totalCollected.

Blockchain / Solidity

contract CrowdFund {
    address public lastSender;
    uint public totalCollected;

    // Add receive function here
    // Your code here
}

Need a hint?

The receive function must be external payable. Use msg.sender and msg.value inside it.

3

Create a function to get the summary

Write a public view function called getSummary that returns the lastSender address and the totalCollected amount.

Blockchain / Solidity

contract CrowdFund {
    address public lastSender;
    uint public totalCollected;

    receive() external payable {
        lastSender = msg.sender;
        totalCollected += msg.value;
    }

    // Add getSummary function here
    // Your code here
}

Need a hint?

Use function getSummary() public view returns (address, uint) and return the two variables.

4

Test the contract output

Write a comment showing the expected output of calling getSummary() after someone sends 1 Ether from address 0x123.... The output should show lastSender as 0x123... and totalCollected as 1000000000000000000 (1 Ether in wei).

Blockchain / Solidity

contract CrowdFund {
    address public lastSender;
    uint public totalCollected;

    receive() external payable {
        lastSender = msg.sender;
        totalCollected += msg.value;
    }

    function getSummary() public view returns (address, uint) {
        return (lastSender, totalCollected);
    }
}

// Write expected output comment here
// Your code here

Need a hint?

Remember 1 Ether = 10^18 wei. Show the address and total in a comment.