error comparing two bytes memory

Question

I am testing a smart contract in Remix and I get the following error when trying to compare two bytes memory using "==":

TypeError: Operator == not compatible with types bytes memory and bytes memory.

This is my code :

function removeCertificate(bytes memory CertificateHash, address holder) external 
    isAProvider 
    isTheProviderOrHimself(holder, CertificateHash) 
{
    address provider = _CertificatesPerHolder[holder]._CertificateFromProvider[CertificateHash];
    bytes[] memory listOfCert = _CertificatesPerHolder[holder]._ListOfCertificates;
    
    for(uint i=0; i < listOfCert.length; i++){
        if(CertificateHash == listOfCert[i]){
            ArrayRemoveResize(i, _CertificatesPerHolder[holder]._ListOfCertificates);
            break;
        }
    }
    
    delete _CertificatesPerHolder[holder]._CertificateFromProvider[CertificateHash];
    
    emit _RemoveCertificateIdEvent(provider, holder);

}

I get the error at line : " if(CertificateHash == listOfCert[i]){"

Answer 1

listOfCert is of type bytes, listOfCert[i] is one element of those bytes. CertificateHash is of type bytes. So you cannot compare bytes to a single byte. Remix must have drawn your attention to that.

If you want to compare two bytes values, you can either check that they are of equal lengths, and if they are then proceed to loop through the elements one by one. But since two equal byte sequences will have equal keccak256 hashes, a more gas-efficient way to compare two byte sequences in Solidity is to compare their hashes:

bytes memory a = hex"00112233";
bytes memory b = hex"00112233";
bool areEqual = keccak256(a) == keccak256(b);

You could even try:

bool areEqual = a.length == b.length && keccak256(a) == keccak256(b);

This might bring some gains if you will often be comparing long bytes sequences of unequal lengths, at the cost of extra code.

Best would be to implement all three methods and compare how gas usage varies for different scenarios, and choose the best one to suit your needs.