Can anyone explain this strange behavior with signed floats in C#?

Question

Here is the example with comments:

class Program
{
    // first version of structure
    public struct D1
    {
        public double d;
        public int f         


        

Answer 1

Vilx's conjecture is correct. What "CanCompareBits" does is checks to see whether the value type in question is "tightly packed" in memory. A tightly packed struct is compared by simply comparing the binary bits that make up the structure; a loosely packed structure is compared by calling Equals on all the members.

This explains SLaks' observation that it repros with structs that are all doubles; such structs are always tightly packed.

Unfortunately as we've seen here, that introduces a semantic difference because bitwise comparison of doubles and Equals comparison of doubles gives different results.