How to find out if an Ethereum address is a contract?

Question

An address in Solidity can be an account or a contract (or other things, such as a transaction). When I have a variable x, holding an address, how can I test if it is a cont

Answer 1

Short answer:

require(tx.origin == msg.sender);

tx.origin is a reference of the original address who initiates this serial function call, while msg.sender is the address who directly calls the target function. Which means, tx.origin must be a human, msg.sender can be a contract or human. Thus, if someone calls you from a contract, then the msg.sender is a contract address which is different from tx.origin.

I know most contracts may use @Manuel Aráoz's code, which works in most cases. But if you call a function within the constructor of a contract, extcodesize will return 0 which fails the isContract check.

NOTE: DON'T use tx.origin under other circumstances if you are not clear about what it represents because .