How does my boolean HashSet from a span have 3 values?

Question

I vaguely understand this, but I would like a concrete explanation as to what's happening. If I construct a HashSet<bool> with data originally from a byte array, why exactly does it keep duplicate values? I've tried to debug this but once I have a bool array, all the elements appear like standard bools.

.Net Fiddle: https://dotnetfiddle.net/QOll01

byte[] bytes = new byte[] { 0, 1, 2 };
ReadOnlySpan<byte> span = new(bytes);
ReadOnlySpan<bool> boolSpan = MemoryMarshal.Cast<byte, bool>(span);
bool[] bools = boolSpan.ToArray();

Console.WriteLine(string.Join(", ", bools)); // False, True, True

Console.WriteLine(new HashSet<bool>(bools).Count); // 3??
Console.WriteLine(string.Join(", ", new HashSet<bool>(bools))); // False, True, True

Answer 1

My guess would be that the memory for a bool can actually be populated with any data. If you set it explicitly then it is probably populated with all 0s or all 1s but it can actually be populated with anything and any value that is not zero is interpreted as true. Because you are creating your values from numbers, the two true values actually do contain different numbers in memory so, while they are the same when interpreted as bool values, they are actually being compared as numbers under the hood and are thus not equal.

To test, I changed this:

byte[] bytes = new byte[] { 0, 1, 2 };

to this:

byte[] bytes = new byte[] { 0, 1, 2, 1, 2 };

and the output was this:

False, True, True, True, True
3
False, True, True

That appears to support the theory.