Receiving Ether in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when sending Ether to this contract?

Consider the following Solidity contract that can receive Ether. What will be the value of received after sending 1 Ether?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Receiver {
    uint public received = 0;

    receive() external payable {
        received = msg.value;
    }
}

A received will be 1 ether (1000000000000000000 wei)

B received will be 0

C received will be the sender's address

D The contract will reject the Ether and revert

💡 Hint

The receive() function is called when Ether is sent without data. msg.value holds the amount of Ether sent in wei.

The receive() function updates received with msg.value, which is the amount of Ether sent in wei. Sending 1 Ether means msg.value is 1000000000000000000 wei.

❓ Predict Output

intermediate

What happens if a contract has no receive or fallback function and receives Ether?

Given this contract, what happens when it receives Ether?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract NoReceive {
    uint public balance = 0;
}

A The contract accepts Ether but balance stays zero

B The contract accepts Ether and balance updates automatically

C The contract rejects the Ether and the transaction reverts

D The contract throws a warning but accepts Ether

💡 Hint

Contracts without receive() or payable fallback() cannot accept plain Ether transfers.

Without a receive() or payable fallback(), the contract cannot accept Ether sent without data, so the transaction reverts.

🔧 Debug

advanced

Why does this contract fail to receive Ether?

Examine the contract below. It is supposed to receive Ether and update receivedAmount. Why does sending Ether fail?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ReceiverFail {
    uint public receivedAmount;

    fallback() external {
        receivedAmount = msg.value;
    }
}

A The contract does not have a constructor to initialize receivedAmount

B The fallback function is not marked payable, so it rejects Ether

C msg.value is always zero in fallback functions

D The fallback function should be named receive to accept Ether

💡 Hint

To accept Ether, fallback or receive functions must be marked payable.

The fallback function is missing the payable keyword, so it cannot accept Ether and the transaction reverts.

📝 Syntax

advanced

Identify the syntax error in this receive function

What is wrong with this receive function code?

Blockchain / Solidity

receive() external {
    // receive Ether
}

A Missing the payable keyword in the receive function declaration

B The receive function cannot be external

C The receive function must have a return type

D The receive function must have a function name

💡 Hint

Receive functions must be able to accept Ether.

Receive functions must be declared payable to accept Ether transfers. Missing payable causes a syntax error.

🚀 Application

expert

How many times can a contract receive Ether in this scenario?

Consider this contract deployed on Ethereum. A user sends 0.5 Ether twice in separate transactions. How many times is the receive() function called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract MultiReceive {
    uint public count = 0;

    receive() external payable {
        count++;
    }
}

A It depends on the gas limit of the transactions

B 1 time, because the contract only accepts Ether once

C 0 times, receive is only called on contract creation

D 2 times, once for each Ether transfer

💡 Hint

The receive() function is triggered every time Ether is sent without data.

Each Ether transfer triggers the receive() function, so sending Ether twice calls it twice, incrementing count twice.

Practice

1.

What is the main purpose of the receive function in a Solidity contract?

easy

A. To reject incoming Ether transfers

B. To send Ether from the contract to an external address

C. To initialize contract state variables

D. To accept Ether sent to the contract without any data

Solution

1. Step 1: Understand the role of the receive function

   The receive function is a special function in Solidity that is triggered when the contract receives Ether without any calldata.

2. Step 2: Identify its purpose

   Its purpose is to accept Ether sent directly to the contract without any function call data.

3. Final Answer:

   To accept Ether sent to the contract without any data -> Option D

4. Quick Check:

   receive function = accept Ether without data [OK]

Hint: Remember: receive handles plain Ether transfers only [OK]

Common Mistakes:

• Confusing receive with fallback function
• Thinking receive sends Ether out
• Assuming receive initializes variables

2.

Which of the following is the correct syntax for a receive function in Solidity?

?

easy

A. receive() internal payable {}

B. receive() external payable {}

C. receive() external {}

D. receive() public payable {}

Solution

1. Step 1: Recall receive function requirements

   The receive function must be declared as external and payable to accept Ether without data.

2. Step 2: Match syntax

   receive() external payable {} matches the required syntax: receive() external payable {}.

3. Final Answer:

   receive() external payable {} -> Option B

4. Quick Check:

   receive must be external and payable [OK]

Hint: receive must be external and payable, no public or internal [OK]

Common Mistakes:

• Using public instead of external
• Omitting payable keyword
• Making receive internal or private

3.

Consider this Solidity contract snippet:

contract Test {
    event Received(address sender, uint amount);
    receive() external payable {
        emit Received(msg.sender, msg.value);
    }
}

What happens when someone sends 1 Ether to this contract without calling any function?

medium

A. The contract rejects the Ether and transaction fails

B. The contract accepts Ether but does not emit any event

C. The contract accepts the Ether and emits the Received event

D. The contract calls fallback function instead

Solution

1. Step 1: Analyze the receive function

   The receive function is payable and emits an event with sender and amount when Ether is received without data.

2. Step 2: Understand the behavior on Ether transfer

   Sending 1 Ether without data triggers receive, so Ether is accepted and event emitted.

3. Final Answer:

   The contract accepts the Ether and emits the Received event -> Option C

4. Quick Check:

   receive payable + emit event = accept and notify [OK]

Hint: Sending Ether without data triggers receive, not fallback [OK]

Common Mistakes:

• Thinking fallback is called instead
• Assuming no event is emitted
• Believing Ether is rejected

4.

Identify the error in this Solidity contract snippet that prevents it from receiving Ether:

contract Example {
    receive() public payable {
        // code
    }
}

medium

A. receive function must be external, not public

B. receive function must not be payable

C. receive function must have a return type

D. receive function must be internal

Solution

1. Step 1: Check receive function visibility

   The receive function must be declared as external, not public.

2. Step 2: Confirm payable requirement

   It is correctly marked payable, so no error there.

3. Final Answer:

   receive function must be external, not public -> Option A

4. Quick Check:

   receive must be external, not public [OK]

Hint: receive must be external, not public or internal [OK]

Common Mistakes:

• Using public instead of external
• Removing payable keyword
• Adding return type to receive

5.

You want to create a contract that tracks the total Ether received and rejects any Ether sent with data. Which combination of functions should you implement?

hard

A. Implement receive() external payable to accept Ether and fallback() external to revert on data

B. Implement only fallback() external payable to accept all Ether

C. Implement receive() external without payable and fallback() external payable

D. Implement receive() external payable and fallback() external payable both accepting Ether

Solution

1. Step 1: Accept Ether without data using receive

   The receive function accepts Ether sent without data and can update total received.

2. Step 2: Reject Ether sent with data using fallback

   The fallback function should revert to reject any Ether sent with data.

3. Final Answer:

   Implement receive() external payable to accept Ether and fallback() external to revert on data -> Option A

4. Quick Check:

   receive accepts plain Ether, fallback rejects data [OK]

Hint: Use receive for plain Ether, fallback to reject data [OK]

Common Mistakes:

• Making fallback payable and accepting Ether with data
• Omitting fallback to reject data
• Not marking receive payable