Problems with C# ArraySegment

Question

I have an array of all the bytes in a file.

{ 0xA0, 0x00, 0xFF, 0xFF, 0xA0, 0x10, 0x01, 0x00, 0x10, 0x32, 0x0D, 0x00, 0x48, 0x00, 0x65, 0x00, 0x6C, 0x00, 0x6C, 0x00, 0x6F, 0x00, 0x2C, 0x00, 0x20, 0x00, 0x77, 0x00, 0x6F, 0x00, 0x72, 0x00, 0x6C, 0x00, 0x64, 0x00, 0x21, 0x00, 0xF0, 0xF0 }

I also have a function that reads a certain section of the file, and moves the location farther forward. (Like BinaryReader.ReadX() except in an array.

    private static byte[] readBytes(int length)
    {
        cursorLocation += length;
        return new ArraySegment<byte>(bytes, cursorLocation-length, length).ToArray();
    }

However, when I try to test if a certain section of bytes are the same, it seems to not work. For example, this:

    byte[] startup = readBytes(4);

    if (startup == new byte[] { 0xA0, 0x00, 0xFF, 0xFF })
    {
        Console.WriteLine("Matches");
    } else
    {
        Console.WriteLine("Does not match.");
    }

returns "Does Not Match," instead of "Matches." I have also tried comparing each array piece in a row, but that also does not work. Printing them out in a line like so:

Console.WriteLine(startup[0]);
Console.WriteLine(startup[1]);
Console.WriteLine(startup[2]);
Console.WriteLine(startup[3]);

returns, as expected:

I am really not sure what is wrong, and any help would be greatly appreciated. Thanks so much!

== on arrays is a reference equality check, not a contents check. — Marc Gravell
– Marc Gravell, Commented May 1, 2020 at 18:07
+@MarcGravell Interesting, should I use === instead or something? Thanks — Daniel Reynolds
– Daniel Reynolds, Commented May 1, 2020 at 18:09
The === operator doesn't exist in C#. You have to write a custom method that compares the two arrays element by element. Of course you should also make sure first that they are of same length and not null. — Döharrrck
– Döharrrck, Commented May 1, 2020 at 18:57

Döharrrck · Accepted Answer · 2020-07-13 09:11:48Z

1

In addition to my comment above, here is an implementation of a method that you could use to compare the arrays.

    private static bool AreByteArraysEqual(byte[] expected, byte[] actual)
    {
        return expected != null
            && actual != null
            && expected.Length == actual.Length 
            && Enumerable.Range(0, actual.Length - 1)
                         .All(i => expected[i] == actual[i]);
    }

Using == to compare the two arrays returns false in your case because this operator checks whether both arrays are the very same instance by comparing their reference as Marc Gravell already pointed out in his comment.

Update:

There was a comment to my answer that pointed out there is already a Linq method 'SequenceEqual' to compare collections element by element.

edited Jul 13, 2020 at 9:11

answered May 1, 2020 at 19:19

Döharrrck

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