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WTF Solidity 33. Sending Airdrops

Recently, I have been revisiting Solidity, consolidating the finer details, and writing "WTF Solidity" tutorials for newbies.

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In the world of cryptocurrency, one of the most exciting things is receiving an airdrop of free tokens. In this tutorial, we will learn how to use a smart contract to send ERC20 tokens as an airdrop.


An airdrop is a marketing strategy used in the cryptocurrency industry where a project distributes tokens for free to a specific group of users. To qualify for an airdrop, users are usually required to complete simple tasks such as testing a product, sharing news, or referring friends. The project gains seed users through the airdrop, while users receive a sum of wealth, making it a win-win situation.

Since there are usually a large number of users receiving an airdrop, it is not practical for the project to send each transfer one by one. By using a smart contract to batch-send ERC20 tokens, the efficiency of the airdrop can be significantly increased.

Airdrop Token Contract

The logic of the airdrop contract is simple: by using a loop, a single transaction sends ERC20 tokens to multiple addresses. The contract contains 2 functions:

  • The getSum() function: returns the sum of a uint array.

    // sum function for arrays
    function getSum(uint256[] calldata _arr) public pure returns(uint sum)
    for(uint i = 0; i < _arr.length; i++)
    sum = sum + _arr[i];
  • Function multiTransferToken(): sends airdrop of ERC20 tokens, including 3 parameters:

    • _token: Address of the token contract (address type)

    • _addresses: Array of user addresses receiving the airdrop (address[] type)

    • _amounts: Array of airdrop amounts that correspond to the quantity of each address in _addresses (uint[] type)

      This function contains 2 checks: The first require checks if the length of _addresses array is equal to the length of _amounts array . The second require checks if the authorization limit of the airdrop contract is greater than the total amount of tokens to be airdropped.

/// @notice Transfer ERC20 tokens to multiple addresses, authorization is required before use
/// @param _token The address of ERC20 token for transfer
/// @param _addresses The array of airdrop addresses
/// @param _amounts The array of amount of tokens (airdrop amount for each address)
function multiTransferToken(
address _token,
address[] calldata _addresses,
uint256[] calldata _amounts
) external {
// Check: The length of _addresses array should be equal to the length of _amounts array
require(_addresses.length == _amounts.length, "Lengths of Addresses and Amounts NOT EQUAL");
IERC20 token = IERC20(_token); // Declare IERC contract variable
uint _amountSum = getSum(_amounts); // Calculate the total amount of airdropped tokens
// Check: The authorized amount of tokens should be greater than or equal to the total amount of airdropped tokens
require(token.allowance(msg.sender, address(this)) >= _amountSum, "Need Approve ERC20 token");

// for loop, use transferFrom function to send airdrops
for (uint8 i; i < _addresses.length; i++) {
token.transferFrom(msg.sender, _addresses[i], _amounts[i]);
  • multiTransferETH() function: Send ETH airdrop, with 2 parameters:
    • _addresses: An array of user addresses that receive the airdrop (address[] type)
    • _amounts: An array of airdrop amounts, corresponding to the quantity for each address in _addresses (uint[] type)
/// Transfer ETH to multiple addresses
function multiTransferETH(
address payable[] calldata _addresses,
uint256[] calldata _amounts
) public payable {
// Check: _addresses and _amounts arrays should have the same length
require(_addresses.length == _amounts.length, "Lengths of Addresses and Amounts NOT EQUAL");
// Calculate total amount of ETH to be airdropped
uint _amountSum = getSum(_amounts);
// Check: transferred ETH should equal total amount
require(msg.value == _amountSum, "Transfer amount error");
// Use a for loop to transfer ETH using transfer function
for (uint256 i = 0; i < _addresses.length; i++) {

Airdrop Practice

  1. Deploy the ERC20 token contract and mint 10,000 units of the token for yourself.

Deploy `ERC20`


  1. Deploy the Airdrop contract.

Deploy `Airdrop` contract

  1. Use the approve() function in the ERC20 contract to grant authorization of 10,000 units of the token to the Airdrop contract.

Authorize `Airdrop` contract

  1. Execute the multiTransferToken() function in the Airdrop contract to perform the airdrop. Set _token as the ERC20 token address, and fill in _addresses and _amounts as follows:
// input _addresses like below
["0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2", "0x5B38Da6a701c568545dCfcB03FcB875f56beddC4"]

// input _amounts like below
[100, 200]


  1. Use the balanceOf() function of the ERC20 contract to check the token balance of the user address above. The airdrop is successful if the balance becomes 100 and 200 respectively!

Check the token balance of the airdrop users


In this lesson, we introduced how to use solidity to write an ERC20 token airdrop contract, greatly increasing the efficiency of airdrops. The biggest airdrop I ever received was from ENS, how about you?