WPARAM is 8 bytes on 64bit Windows

[softworkz]

Also it's unsigned, so it actually has to be UIntPtr, even though IntPtr is often used.
int is definitely wrong - it's kind of a mystery (or pure accidence) that it has even been working with int 😉

WinUI3_SplashScreen/SplashScreen.cs

Lines 259 to 260 in cfc18b8

	[DllImport("User32.dll", SetLastError = true, CharSet = CharSet.Auto)]
	public static extern int DefWindowProc(IntPtr hWnd, uint uMsg, int wParam, IntPtr lParam);

WinUI3_SplashScreen/SplashScreen.cs

Line 271 in cfc18b8

public delegate int WNDPROC(IntPtr hwnd, uint uMsg, int wParam, IntPtr lParam);

WinUI3_SplashScreen/SplashScreen.cs

Lines 595 to 604 in cfc18b8

	private int Win32WndProc(IntPtr hwnd, uint msg, int wParam, IntPtr lParam)
	{
	//int wmId, wmEvent;
	switch (msg)
	{
	default:
	break;
	}
	return DefWindowProc(hwnd, msg, wParam, lParam);
	}

[castorix]

Also it's unsigned, so it actually has to be UIntPtr, even though IntPtr is often used. int is definitely wrong - it's kind of a mystery (or pure accidence) that it has even been working with int 😉

Yes, right.
(I had copied old definitions I used for x86 only, I will fix it asap...)
It is defined like this in DotNet48ZDP2 source by MS :

        [DllImport(ExternDll.User32, CharSet=System.Runtime.InteropServices.CharSet.Auto)]
        [ResourceExposure(ResourceScope.None)]
        public static extern IntPtr DefWindowProc(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);

Thanks.