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

It must be related to a bit by bit comparison, since 0.0 should differ from -0.0 only by the signal bit.

Answer 2

The bug is in the following two lines of System.ValueType: (I stepped into the reference source)

if (CanCompareBits(this)) 
    return FastEqualsCheck(thisObj, obj);

(Both methods are [MethodImpl(MethodImplOptions.InternalCall)])

When all of the fields are 8 bytes wide, CanCompareBits mistakenly returns true, resulting in a bitwise comparison of two different, but semantically identical, values.

When at least one field is not 8 bytes wide, CanCompareBits returns false, and the code proceeds to use reflection to loop over the fields and call Equals for each value, which correctly treats -0.0 as equal to 0.0.

Here is the source for CanCompareBits from SSCLI:

FCIMPL1(FC_BOOL_RET, ValueTypeHelper::CanCompareBits, Object* obj)
{
    WRAPPER_CONTRACT;
    STATIC_CONTRACT_SO_TOLERANT;

    _ASSERTE(obj != NULL);
    MethodTable* mt = obj->GetMethodTable();
    FC_RETURN_BOOL(!mt->ContainsPointers() && !mt->IsNotTightlyPacked());
}
FCIMPLEND

Answer 3

Simpler test case:

Console.WriteLine("Good: " + new Good().Equals(new Good { d = -.0 }));
Console.WriteLine("Bad: " + new Bad().Equals(new Bad { d = -.0 }));

public struct Good {
    public double d;
    public int f;
}

public struct Bad {
    public double d;
}

EDIT: The bug also happens with floats, but only happens if the fields in the struct add up to a multiple of 8 bytes.

Answer 4

If you make D2 like this

public struct D2
{
    public double d;
    public double f;
    public string s;
}

it's true.

if you make it like this

public struct D2
{
    public double d;
    public double f;
    public double u;
}

It's still false.

it seems like it's false if the struct only holds doubles.

Answer 5

It must be zero related, since changing the line

d.d = -0.0

to:

d.d = 0.0

results in the comparison being true...

Answer 6

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.