C#•2y ago

byte[] does not equal a matching byte[], but the hex values match.

I have two byte arrays, one is 4 bytes long containing a known sequence, the other is file data. I am attempting to locate the 4-byte sequence in the file data by checking every four bytes in the file data, and using indexing to get the four bytes. Here's the code:

byte[] data = File.ReadAllBytes(fileName);
byte[] header = new byte[] { 0x4B, 0x54, 0x53, 0x52 };
int pos = 0;
for (int i = 4; i < data.Length; i += 4) {
#if DEBUG
    Debug.WriteLine(Convert.ToHexString(data[pos..i]));
    bool isTrue = Convert.ToHexString(data[pos..i]) == Convert.ToHexString(header); // this is true
    bool isFalse = data[pos..i] == header; // this is false
#endif
    if (data[pos..i] == header) {
        break;
    }
    pos += 4;
}

byte[] data = File.ReadAllBytes(fileName);
byte[] header = new byte[] { 0x4B, 0x54, 0x53, 0x52 };
int pos = 0;
for (int i = 4; i < data.Length; i += 4) {
#if DEBUG
    Debug.WriteLine(Convert.ToHexString(data[pos..i]));
    bool isTrue = Convert.ToHexString(data[pos..i]) == Convert.ToHexString(header); // this is true
    bool isFalse = data[pos..i] == header; // this is false
#endif
    if (data[pos..i] == header) {
        break;
    }
    pos += 4;
}

Can anyone explain this behavior? And suggest how I can fix it? Thanks

13 Replies

Kouhai•2y ago

if (data[pos..i] == header) {
        break;
    }

if (data[pos..i] == header) {
        break;
    }

Is checking if the references are equal, not whether the contents are equal

Arch Leaders•2y ago

Ah, right I think I just found another answer on StackOverflow as well, I need to use data[pos..i].SequenceEqual(header). Which lined up with your answer. Thanks

Kouhai•2y ago

Jester•2y ago

@Arch Leaders i strongly recommend making a span for header and data. then use the IndexOf (does the loop and compare for you, but more optimised) method. it should be much faster than the code you wrote and allocate a million times less memory.

ero•2y ago

didn't .net 6 add list pattern matching or was that .net 7

Anar•2y ago

Arch Leaders•2y ago

Is it possible to read all bytes into a span?

Jester•2y ago

byte[] has a .AsSpan() method

Arch Leaders•2y ago

Also are you saying use IndexOf on the data instead of iterating the data? Ah okay, thanks.

Jester•2y ago

so var bytes = file.readallbytes var spanoverbytes = bytes.asspan() spanoverbytes.indexof(otherspan)

Arch Leaders•2y ago

What is otherspan here?

Jester•2y ago

the byte array youre looking for as span

Arch Leaders•2y ago

Wait that actually works? Not on my PC to test, but IndexOf will find a span of bytes at any place in another span? (The start pos of the span I’m looking for is indeterminate, all I know to is that it’s divisible by 4)

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byte[] does not equal a matching byte[], but the hex values match.