Receiving Ether in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Ether in the context of blockchain?

Ether is the native cryptocurrency of the Ethereum blockchain. It is used to pay for transactions and computational services on the network.

beginner

How can a smart contract receive Ether?

A smart contract can receive Ether by having a payable function or a payable fallback/receive function that allows it to accept Ether transfers.

intermediate

What is the purpose of the 'receive() external payable' function in Solidity?

The 'receive() external payable' function is a special function in Solidity that is executed when the contract receives Ether without any data. It must be marked payable to accept Ether.

intermediate

What happens if a contract does not have a payable fallback or receive function and someone sends Ether to it?

The transaction will fail and the Ether will be rejected because the contract cannot accept Ether without a payable function to handle it.

beginner

How do you check the amount of Ether sent to a contract in a payable function?

You can check the amount of Ether sent using 'msg.value', which holds the amount of Wei (smallest Ether unit) sent with the transaction.

Which keyword must a Solidity function have to accept Ether?

A pure

B external

C view

D payable

Only functions marked with 'payable' can receive Ether in Solidity.

What is the special function called that handles plain Ether transfers without data?

A fallback()

B receive()

C constructor()

D transfer()

The 'receive()' function is called when Ether is sent without data.

What does 'msg.value' represent in a payable function?

A The amount of Ether sent in Wei

B The sender's address

C The gas price

D The block timestamp

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

If a contract has no payable fallback or receive function, what happens when Ether is sent to it?

A Ether is burned

B Ether is accepted and stored

C Transaction fails and Ether is rejected

D Ether is sent back automatically

Without a payable fallback or receive function, the contract rejects Ether and the transaction fails.

Which Solidity function is called when Ether is sent with data but no matching function exists?

A fallback()

B receive()

C constructor()

D transfer()

The 'fallback()' function is called when Ether is sent with data and no other function matches.

Explain how a smart contract can receive Ether and what functions are involved.

Think about special functions that accept Ether and how to check the amount sent.

Describe what happens if Ether is sent to a contract without a payable receive or fallback function.

Consider what Solidity requires to accept Ether.

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