Checks-Effects-Interactions pattern in Blockchain / Solidity - Mini Project: Build & Apply

Checks-Effects-Interactions Pattern in Solidity

📖 Scenario: You are building a simple Ethereum smart contract for a wallet that allows users to withdraw their funds safely.In blockchain programming, it is important to follow the Checks-Effects-Interactions pattern to avoid security issues like reentrancy attacks.

🎯 Goal: Learn how to implement the Checks-Effects-Interactions pattern in Solidity by creating a withdraw function that first checks conditions, then updates state, and finally interacts with external contracts.

📋 What You'll Learn

Create a mapping called balances to store user balances

Add a function deposit to allow users to add funds

Add a function withdraw that uses the Checks-Effects-Interactions pattern

Print the remaining balance after withdrawal

💡 Why This Matters

🌍 Real World

Smart contracts on Ethereum must be secure to protect users' funds. The Checks-Effects-Interactions pattern is a key security practice.

💼 Career

Blockchain developers use this pattern daily to write safe contracts that handle money and interact with other contracts.

1

Create the balances mapping and deposit function

Create a public mapping called balances that maps address to uint. Then write a public deposit function that increases the sender's balance by msg.value.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    // Create balances mapping
    // Write deposit function
    
    // Your code here
}

Hint

Use mapping(address => uint) public balances; to store balances.

The deposit function should be payable and add msg.value to balances[msg.sender].

2

Add a withdraw function with a check for sufficient balance

Add a public function called withdraw that takes a uint amount parameter. Inside, first check that balances[msg.sender] is at least amount using require.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    mapping(address => uint) public balances;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }

    // Add withdraw function with balance check
    
    // Your code here
}

Hint

Use require to check if the sender has enough balance before proceeding.

3

Apply the Effects step by updating the balance before interaction

Inside the withdraw function, after the require check, subtract amount from balances[msg.sender]. This is the Effects step.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    mapping(address => uint) public balances;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw(uint amount) public {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        // Subtract amount from balance
        
        // Your code here
    }
}

Hint

Update the user's balance by subtracting the withdrawal amount before sending funds.

4

Complete the withdraw function with the interaction and print remaining balance

In the withdraw function, after updating the balance, send amount to msg.sender using payable(msg.sender).transfer(amount). Then add a print statement to output the remaining balance of msg.sender.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    mapping(address => uint) public balances;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw(uint amount) public {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        balances[msg.sender] -= amount;
        // Send amount to sender
        // Print remaining balance
        
        // Your code here
    }
}

Hint

Use payable(msg.sender).transfer(amount) to send Ether.

Solidity cannot print to console, so use an event to show the remaining balance.

Practice

1.

What is the main purpose of the Checks-Effects-Interactions pattern in smart contracts?

easy

A. To reduce the gas cost of transactions

B. To prevent reentrancy attacks by ordering operations safely

C. To increase the speed of contract execution

D. To allow contracts to call external APIs

Solution

1. Step 1: Understand the pattern's purpose

   The Checks-Effects-Interactions pattern is designed to prevent security issues by ordering operations carefully.

2. Step 2: Identify the main security benefit

   By checking conditions first, updating state second, and interacting last, it prevents reentrancy attacks.

3. Final Answer:

   To prevent reentrancy attacks by ordering operations safely -> Option B

4. Quick Check:

   Pattern purpose = Prevent reentrancy [OK]

Hint: Remember: check first, update second, interact last [OK]

Common Mistakes:

• Thinking it speeds up contract execution
• Believing it reduces gas costs
• Assuming it enables external API calls

2.

Which of the following Solidity code snippets correctly follows the Checks-Effects-Interactions pattern?

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
  balances[msg.sender] -= amount;
}

easy

A. Yes, it checks, then interacts, then effects

B. Yes, it updates state before checks

C. No, it interacts before updating state

D. No, it updates state before interaction

Solution

1. Step 1: Analyze the order of operations

   The code first checks the balance, then calls the external address, then updates the balance.

2. Step 2: Compare with pattern order

   The pattern requires checks, then effects (state update), then interactions. Here, interaction happens before effects.

3. Final Answer:

   No, it interacts before updating state -> Option C

4. Quick Check:

   Interaction before effect = Incorrect order [OK]

Hint: State update must come before external calls [OK]

Common Mistakes:

• Ignoring the order of state update and interaction
• Thinking checks alone are enough
• Confusing effects with interactions

3.

What will be the output of the following Solidity function if balances[msg.sender] is 100 and amount is 50?

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount;
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
}

Assume the call succeeds.

medium

A. The balance becomes 50 and 50 ether is sent

B. The balance remains 100 and 50 ether is sent

C. The balance becomes 50 but no ether is sent

D. Transaction reverts due to incorrect order

Solution

1. Step 1: Check balance and update state

   The require passes since 100 >= 50, then balance is reduced to 50.

2. Step 2: Send ether and confirm success

   The call sends 50 ether and returns success, so require(success) passes.

3. Final Answer:

   The balance becomes 50 and 50 ether is sent -> Option A

4. Quick Check:

   Balance updated then ether sent = Correct [OK]

Hint: State updates before sending ether means balance changes [OK]

Common Mistakes:

• Thinking balance stays the same after withdrawal
• Assuming call fails without reason
• Confusing order causing revert

4.

Identify the bug in this Solidity function violating the Checks-Effects-Interactions pattern:

function withdraw(uint amount) public {
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount;
}

medium

A. It correctly follows the pattern

B. It updates balance before checking conditions

C. It does not check for call success

D. It calls external address before checking balance

Solution

1. Step 1: Review order of operations

   The function calls the external address first, then checks balance, then updates state.

2. Step 2: Compare with pattern rules

   Pattern requires checks first, then effects, then interactions. Here, interaction is first, violating the pattern.

3. Final Answer:

   It calls external address before checking balance -> Option D

4. Quick Check:

   Interaction before checks = Bug [OK]

Hint: Always check before calling external contracts [OK]

Common Mistakes:

• Ignoring the order of require and call
• Assuming call success check is missing
• Thinking balance update order is wrong

5.

You want to write a Solidity function that sends a reward to a user only if they have enough points. Which code snippet correctly applies the Checks-Effects-Interactions pattern?

function rewardUser(address user, uint points) public {
  // Option A
  require(userPoints[user] >= points);
  userPoints[user] -= points;
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option B
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  require(userPoints[user] >= points);
  userPoints[user] -= points;

  // Option C
  userPoints[user] -= points;
  require(userPoints[user] >= 0);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option D
  require(userPoints[user] >= points);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  userPoints[user] -= points;
}

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Analyze the order of operations

   The correct snippet first requires userPoints[user] >= points (checks), then subtracts points (effects), then sends ether via call (interactions).

2. Step 2: Identify violations in others

   The second snippet interacts before checks; the third updates state before checks; the fourth interacts before effects.

3. Final Answer:

   Option A -> Option A

4. Quick Check:

   Checks -> Effects -> Interactions = Correct snippet [OK]

Hint: Check first, update second, call last for safety [OK]

Common Mistakes:

• Calling external before checking conditions
• Updating state before checks
• Mixing order of effects and interactions