Receive and fallback functions in Blockchain / Solidity - Mini Project: Build & Apply

Receive and fallback functions

📖 Scenario: You are creating a simple smart contract on Ethereum that can receive Ether payments. Sometimes, Ether is sent directly without calling any function, and sometimes it is sent with data that does not match any function. Your contract needs to handle both cases safely.

🎯 Goal: Build a Solidity contract that uses receive() and fallback() functions to accept Ether payments and log when each function is called.

📋 What You'll Learn

Create a contract named PaymentReceiver

Add a receive() function to accept plain Ether transfers

Add a fallback() function to accept Ether with unknown data

Emit events inside both functions to log which function was triggered

Test sending Ether with and without data to see which function runs

💡 Why This Matters

🌍 Real World

Smart contracts often need to accept payments safely. Using receive() and fallback() functions ensures the contract can handle Ether sent in different ways.

💼 Career

Understanding receive() and fallback() is essential for blockchain developers to write secure and reliable contracts that interact with Ether transfers.

1

Create the contract and event

Create a contract called PaymentReceiver and inside it, declare an event named ReceivedEther that logs an address sender and a uint amount.

Blockchain / Solidity

contract PaymentReceiver {
    // Your code here
}

Need a hint?

Use the event keyword to declare an event inside the contract.

2

Add the receive() function

Inside the PaymentReceiver contract, add a receive() external payable function that emits the ReceivedEther event with msg.sender and msg.value.

Blockchain / Solidity

contract PaymentReceiver {
    event ReceivedEther(address sender, uint amount);
    
    // Add receive() function below
    // Your code here
}

Need a hint?

The receive() function has no name and no arguments. It must be marked external payable.

3

Add the fallback() function

Inside the PaymentReceiver contract, add a fallback() external payable function that emits an event named FallbackCalled with msg.sender and msg.value. Declare the FallbackCalled event above.

Blockchain / Solidity

contract PaymentReceiver {
    event ReceivedEther(address sender, uint amount);
    
    receive() external payable {
        emit ReceivedEther(msg.sender, msg.value);
    }
    
    // Add fallback() function and event below
    // Your code here
}

Need a hint?

The fallback() function is called when no other function matches. It must be external payable to accept Ether.

4

Test sending Ether and print logs

Add a public view function named getBalance() that returns the contract's Ether balance as uint. Then add a print statement to display the balance after deployment and after sending Ether (simulate this in your test environment).

Blockchain / Solidity

contract PaymentReceiver {
    event ReceivedEther(address sender, uint amount);
    event FallbackCalled(address sender, uint amount);
    
    receive() external payable {
        emit ReceivedEther(msg.sender, msg.value);
    }
    
    fallback() external payable {
        emit FallbackCalled(msg.sender, msg.value);
    }
    
    // Add getBalance() function below
    // Your code here
}

Need a hint?

Use address(this).balance to get the contract's Ether balance.