What is the difference between structures containing bool vs uint when using PInvoke?

Question

Ok, I'm now very confused. After my last question had several people comment about changing bool to uint I verified they are the same size by:

    Console.WriteLine("sizeof bool = {0}", Marshal.SizeOf(typeof(bool)));
    Console.WriteLine("sizeof uint = {0}", Marshal.SizeOf(typeof(uint)));

Which of course prints:

sizeof bool = 4
sizeof uint = 4

That said, I then broke down and gave their suggestions a try anyway... Changing a single bool inside a structure to a uint. What I can't figure out for the life of me is why this made it work...

So this works:

[StructLayout(LayoutKind.Sequential)]
public struct KEY_EVENT_RECORD
{
    public bool bKeyDown;
    public short wRepeatCount;
    public short wVirtualKeyCode;
    public short wVirtualScanCode;
    public char UnicodeChar;
    public int dwControlKeyState;
}

When used in this structure:

[StructLayout(LayoutKind.Explicit)]
public struct INPUT_RECORD
{
    [FieldOffset(0)] public short EventType;
    [FieldOffset(4)] public KEY_EVENT_RECORD KeyEvent;
}

But in this structure it breaks:

[StructLayout(LayoutKind.Explicit)]
public struct INPUT_RECORD
{
    [FieldOffset(0)] public short EventType;
    [FieldOffset(4)] public KEY_EVENT_RECORD KeyEvent;
    [FieldOffset(4)] public MOUSE_EVENT_RECORD MouseEvent;
    [FieldOffset(4)] public WINDOW_BUFFER_SIZE_RECORD WindowBufferSizeEvent;
    [FieldOffset(4)] public MENU_EVENT_RECORD MenuEvent;
    [FieldOffset(4)] public FOCUS_EVENT_RECORD FocusEvent;
}

Yet when I change bool bKeyDown to uint in the KEY_EVENT_RECORD structure it starts working again...

Can someone please explain this behavior?

I really would like to know the why of it so that I can avoid this undocumented feature (aka bug) in the future.

Answer 1

Try setting the field type to bool and add the attribute [MarshalAs(UnmanagedType.Bool)].

[StructLayout(LayoutKind.Sequential)]
public struct KEY_EVENT_RECORD
{
    [MarshalAs(UnmanagedType.Bool)]
    public bool bKeyDown;
    public short wRepeatCount;
    public short wVirtualKeyCode;
    public short wVirtualScanCode;
    public char UnicodeChar;
    public int dwControlKeyState;
}

Docs for MarshalAsAttribute Docs for UnmanagedType

Answer 2

bools are 1byte --> sizeof (C# Reference)

Also, see Default Marshaling for Boolean Types

Answer 3

Question reworded with a new sample:

Boolean Marshalling with LayoutKind.Explicit, Is this broken or failing as designed?