I have a contract (Implementation.sol) which delegatecalls the other contract (Attacker.sol) to selfdestruct. When I try to compile the Implementation.sol on Remix, I receive Expected primary expression error for the line require(success && dat != byte(0x0), "call failed");. Not sure what to do.

contract Implementation {

  function delegateTheCall (address target, string calldata action, address to) public returns (bytes memory) {

    (bool success,bytes memory dat) = target.delegatecall(abi.encodeWithSignature(action, to));
    require(success && dat != byte(0x0), "call failed"); 
    return dat;
  • You probably want dat.length != 0 instead of dat != byte(0x0). But why having this condition? It's common for a function to return nothing and still succeed.
    – 0xSanson
    Commented Aug 27, 2022 at 14:36
  • 1
    @0xSanson This is a better way to check the value. You're right. Thank you a lot. I'm just trying some experimental contracts to understand the concepts better. I definitely will not use these in prod.
    – bic
    Commented Aug 27, 2022 at 16:30

1 Answer 1


It's tricky to compare strings and bytes in Solidity. These two types are treated exactly the same in Solidity, only that string gets decoded in UTF-8 when returning it outside of the smart contract.

To compare these types, use keccak256:

require(success && keccak256(dat) != keccak256(""), "call failed"); 

The empty string ("") is treated as an empty bytes array, which is what you need.

Try it doing something like this to check the result and you'll see what I mean. Comparing the keccak256 hash of an an empty bytes array with the keccak256 hash of an empty string (which is the same as an empty bytes array):

bytes memory dat;
require(keccak256(dat) != keccak256(""));
  • 1
    Thank you @Jeremy Then. I definitely missed the point that you reminded me. It worked.
    – bic
    Commented Aug 27, 2022 at 16:27

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