msg.value and msg.sender in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the address of the caller.

Blockchain / Solidity

address caller = [1];

Drag options to blanks, or click blank then click option'

Amsg.value

Baddress(this)

Ctx.origin

Dmsg.sender

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.value instead of msg.sender

Using tx.origin which is less secure

Using address(this) which is the contract's address

msg.sender holds the address of the caller who invoked the function.

2fill in blank

medium

Complete the code to get the amount of Ether sent with the call.

Blockchain / Solidity

uint amount = [1];

Drag options to blanks, or click blank then click option'

Atx.gasprice

Bmsg.value

Cmsg.sender

Dmsg.gas

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.sender instead of msg.value

Using tx.gasprice which is gas price, not value

Using msg.gas which is deprecated

msg.value holds the amount of Ether (in wei) sent with the transaction.

3fill in blank

hard

Fix the error in the function to correctly check if Ether was sent.

Blockchain / Solidity

function deposit() public payable {
    require([1] > 0, "Send some Ether");
}

Drag options to blanks, or click blank then click option'

Amsg.value

Btx.origin

Caddress(this).balance

Dmsg.sender

Attempts:

3 left

💡 Hint

Common Mistakes

Checking msg.sender instead of msg.value

Using address(this).balance which is total contract balance

The function must check msg.value to see if Ether was sent.

4fill in blank

hard

Fill both blanks to create a function that returns the sender and the amount sent.

Blockchain / Solidity

function getInfo() public view returns (address, uint) {
    return ([1], [2]);
}

Drag options to blanks, or click blank then click option'

Amsg.sender

Bmsg.value

Ctx.origin

Daddress(this)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning tx.origin instead of msg.sender

Returning address(this) instead of msg.sender

Returning msg.sender and tx.origin mixed

The function returns the caller's address (msg.sender) and the Ether sent (msg.value).

5fill in blank

hard

Fill all three blanks to create a payable function that stores sender and amount in state variables.

Blockchain / Solidity

address public lastSender;
uint public lastAmount;

function recordPayment() public payable {
    lastSender = [1];
    lastAmount = [2];
    emit PaymentReceived([3]);
}

event PaymentReceived(address sender);

Drag options to blanks, or click blank then click option'

Amsg.sender

Bmsg.value

DlastSender

Attempts:

3 left

💡 Hint

Common Mistakes

Using lastSender instead of msg.sender in the event

Mixing up msg.sender and msg.value assignments

The function saves msg.sender and msg.value to state variables, then emits an event with the sender's address.

Practice

1. What does msg.sender represent in a smart contract?

easy

A. The amount of ether sent with the transaction

B. The address of the account that called the contract

C. The balance of the contract

D. The gas price of the transaction

Solution

1. Step 1: Understand the role of msg.sender

   msg.sender always holds the address of the account or contract that called the current function.

2. Step 2: Differentiate from msg.value

   msg.value holds the amount of ether sent, not the caller's address.

3. Final Answer:

   The address of the account that called the contract -> Option B

4. Quick Check:

   msg.sender = caller address [OK]

Hint: Remember: sender is who calls, value is what is sent [OK]

Common Mistakes:

• Confusing msg.sender with msg.value
• Thinking msg.sender is contract balance
• Assuming msg.sender is always a user (it can be a contract)

2. Which of the following is the correct way to check if the caller sent at least 1 ether in Solidity?

easy

A. if (msg.value >= 1 ether) {}

B. if (msg.value == msg.sender) {}

C. if (msg.sender >= 1 ether) {}

D. if (msg.sender == 1 ether) {}

Solution

1. Step 1: Identify the variable holding sent ether amount

   msg.value stores the amount of ether sent with the call.

2. Step 2: Use comparison to check minimum ether sent

   Checking if msg.value >= 1 ether correctly tests if at least 1 ether was sent.

3. Final Answer:

   if (msg.value >= 1 ether) {} -> Option A

4. Quick Check:

   Check ether amount with msg.value [OK]

Hint: Use msg.value to check ether sent, not msg.sender [OK]

Common Mistakes:

• Using msg.sender to compare ether amounts
• Using equality instead of >= for minimum check
• Mixing up msg.sender and msg.value in conditions

3. What will be the output of this Solidity function when called by address 0xABC with 2 ether sent?

function pay() public payable returns (address, uint) {
    return (msg.sender, msg.value);
}

medium

A. (0xABC, 0)

B. (msg.sender, msg.value)

C. (address(0), 2 ether)

D. (0xABC, 2 ether)

Solution

1. Step 1: Understand msg.sender and msg.value in the function

   msg.sender is the caller's address (0xABC), and msg.value is the ether sent (2 ether).

2. Step 2: Return values match caller and sent ether

   The function returns these two values exactly as (0xABC, 2 ether).

3. Final Answer:

   (0xABC, 2 ether) -> Option D

4. Quick Check:

   Caller and sent ether returned correctly [OK]

Hint: Returned values match caller address and sent ether [OK]

Common Mistakes:

• Assuming msg.value is zero if not explicitly set
• Confusing msg.sender with zero address
• Expecting variable names instead of actual values

4. Identify the error in this Solidity function that tries to check if the caller sent ether:

function deposit() public {
    require(msg.value > 0, "Send some ether");
}

medium

A. Missing payable keyword in function declaration

B. Incorrect use of require syntax

C. msg.value cannot be compared with > 0

D. Function should return a value

Solution

1. Step 1: Check function declaration for payable

   To receive ether and access msg.value, the function must be marked payable.

2. Step 2: Identify missing payable keyword

   The function lacks payable, so msg.value will always be zero and cause errors.

3. Final Answer:

   Missing payable keyword in function declaration -> Option A

4. Quick Check:

   Functions receiving ether must be payable [OK]

Hint: Add payable to receive ether and use msg.value [OK]

Common Mistakes:

• Forgetting payable keyword
• Thinking require syntax is wrong
• Expecting function to return value unnecessarily

5. You want to write a Solidity function that only accepts payments from a specific address and requires at least 0.5 ether sent. Which code snippet correctly enforces both conditions?

hard

A. function pay() public payable {
require(msg.sender != 0x123 && msg.value > 0.5 ether);
}

B. function pay() public {
require(msg.sender == 0x123);
require(msg.value >= 0.5 ether);
}

C. function pay() public payable {
require(msg.sender == 0x123 && msg.value >= 0.5 ether, "Unauthorized or insufficient funds");
}

D. function pay() public payable {
if (msg.sender == 0x123 || msg.value >= 0.5 ether) {}
}

Solution

1. Step 1: Ensure function is payable to accept ether

   Only options A, C, and D have payable, needed to receive ether.

2. Step 2: Check conditions for sender and ether amount

   function pay() public payable { require(msg.sender == 0x123 && msg.value >= 0.5 ether, "Unauthorized or insufficient funds"); } uses require with AND (&&) to enforce both sender address and minimum ether sent.

3. Step 3: Evaluate other options for correctness

   function pay() public { require(msg.sender == 0x123); require(msg.value >= 0.5 ether); } lacks payable, so msg.value is always zero. function pay() public payable { if (msg.sender == 0x123 || msg.value >= 0.5 ether) {} } uses OR (||), allowing either condition alone. function pay() public payable { require(msg.sender != 0x123 && msg.value > 0.5 ether); } uses != which rejects the correct sender.

4. Final Answer:

   function pay() public payable { require(msg.sender == 0x123 && msg.value >= 0.5 ether, "Unauthorized or insufficient funds"); } -> Option C

5. Quick Check:

   Use payable and require with AND for both checks [OK]

Hint: Use payable and require with && to check sender and value [OK]

Common Mistakes:

• Forgetting payable keyword
• Using OR instead of AND for conditions
• Checking msg.sender with != instead of ==
• Not combining conditions in one require