Memory offset of ref bool in extern method

[wegylexy]

Description

extern "C"
{
    void MyNativeFunction(bool* flags);
}

[DllImport("MyNativeLib")]
static extern void MyNativeFunction(ref bool flags);

Span<bool> flags = stackalloc bool[24576];
// more code / stackalloc in between increases the offset
MyNativeFunction(ref MemoryMarshal.GetReference(flags));
// now, values of flags seen here are offset by arbitrary number of bytes

Adding [MarshalAs(UnmanagedType.U1)] does not help.

Configuration

.NET 6 preview 6 Linux x64 docker image from mcr.microsoft.com/dotnet/aspnet:6.0

Regression?

It works fine when casted to byte first, or with pointers.

[DllImport("MyNativeLib")]
static extern void MyNativeFunction(ref byte flags);

Span<bool> flags = stackalloc bool[24576];
MyNativeFunction(ref MemoryMarshal.GetReference(MemoryMarshal.Cast<bool, byte>(flags)));

[DllImport("MyNativeLib")]
static extern void MyNativeFunction(nint flags);

Span<bool> flags;
unsafe
{
    bool* p = stackalloc bool[24576];
    MyNativeFunction((nint)p);
    flags = new(p, 24576);
}

Other information

Offsets that actually experienced were -96 bytes and -64 bytes.

[wegylexy]

I wonder if the marshaller uses a shadow copy elsewhere, passing the address of that memory location.
Is there a way to disable marshalling and pass ref bool by reference directly without casting to IntPtr/nint in unsafe and bool*, or to ref byte via Span<byte>?

[jkoritzinsky]

bool values are always marshalled when passed by value or by ref. The runtime does normalization on both managed->native and native->managed for true and false.

You'll either need to use bool* (where we then assume that you know what you are doing and leave the code be), or pass as a ref byte using unsafe code.

In general, if you want to have the marshaller not make any copies into locals in a marshalling stub, do not use any non-blittable types.

[wegylexy]

Thanks. I didn't expect bool to be non-blittable.
Should I use unsafe code or MemoryMarshal.Cast<bool, byte>()?

[jkoritzinsky]

MemoryMarshal works. It’s technically unsafe code as well, so be careful for other usages (this one is usually fine).