Question

Im trying to use SendInput to simulate keyboard presses in my app and want to support both 32-bit and 64-bit.

I’ve determined that for this to work, I need to have 2 different INPUT structs as such

        [StructLayout(LayoutKind.Sequential)]
    public struct KEYBDINPUT
    {
        public ushort wVk; // Virtual Key Code
        public ushort wScan; // Scan Code
        public uint dwFlags;
        public uint time;
        public IntPtr dwExtraInfo;
    }

    [StructLayout(LayoutKind.Explicit, Size = 28)]
    public struct INPUT32
    {
        [FieldOffset(0)]
        public uint type; // eg. INPUT_KEYBOARD
        [FieldOffset(4)]
        public KEYBDINPUT ki;
    }

    [StructLayout(LayoutKind.Explicit, Size = 40)]
    public struct INPUT64
    {
        [FieldOffset(0)]
        public uint type; // eg. INPUT_KEYBOARD
        [FieldOffset(8)]
        public KEYBDINPUT ki;
    }

I wanted to know if there was a way to set the StructLayout size and FieldOffsets at runtime so I could use just one INPUT struct and determine the size and fieldoffset depending on the machine.

I have tried the code below but I would like to know if the same is possible at runtime instead of compile time.

#if _M_IX86
    [StructLayout(LayoutKind.Explicit, Size = 28)]
#else
    [StructLayout(LayoutKind.Explicit, Size = 40)]
#endif
    public struct INPUT
    {
        [FieldOffset(0)]
        public uint type; // eg. INPUT_KEYBOARD
#if _M_IX86
        [FieldOffset(4)]
#else
        [FieldOffset(8)]
#endif
        public KEYBDINPUT ki;
    }

Answer 1

Unfortunately, no.

Attributes are “fused” to the type at compile-time, which is why all values passed to an attribute constructor must be constants.

And at runtime you can’t modify the attributes attached to the type. You can grab a copy and modify its values, but the actual attribute attached to the type will remain unchanged, so you can’t “trick” mscorlib code into seeing your changes instead of the original, either.