C# and PInvoke Winapi program hangs

Question

So I'm trying to make a winapi application by using PInvoke. So far I've been able to open a window, but from there, I can't seem to make the WndProc work. Right now, the application starts, but it won't respond neither to resize, nor to movement or anything, except the hovering above the control buttons.

WindowLL.cs:

public delegate long WndProc(IntPtr hWnd, uint message, byte wParam, long lParam);
public static unsafe class WindowLL
{
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall, SetLastError = true)]
    public static extern ushort RegisterClassExA(ref WNDCLASSEX _class);
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall, SetLastError = true)]
    public static extern IntPtr CreateWindowExA(
        uint extraStyle,
        ushort className,
        //[MarshalAs(UnmanagedType.LPWStr)]
        //string className,
        [MarshalAs(UnmanagedType.LPStr)]
        string title,
        uint style,
        int x,
        int y,
        int width,
        int height,
        IntPtr parent,
        IntPtr menu,
        IntPtr instance,
        IntPtr parameter
    );
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern bool DestroyWindow(IntPtr hWnd);
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall, SetLastError = true)]
    public static extern bool ShowWindow(IntPtr hWnd, int cmdShow);
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool GetMessage(
        out MSG lpMsg,
        IntPtr hWnd,
        uint wMsgFilterMin,
        uint wMsgFilterMax
    );
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern uint DispatchMessage(ref MSG lpMsg);
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern uint TranslateMessage(ref MSG lpMsg);
    [DllImport("user32.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern long DefWindowProcA(
        IntPtr hWnd,
        uint Msg,
        byte wParam,
        long lParam
    );
}

Structures (split into different files):

[StructLayout(LayoutKind.Sequential)]
public struct MSG
{
    public IntPtr hwnd;
    public uint message;
    public byte wParam;
    public long lParam;
    public short time;
    public POINT pt;
    public short lPrivate;
}
[StructLayout(LayoutKind.Sequential)]
public struct POINT
{
    public long X;
    public long Y;
}
[StructLayout(LayoutKind.Sequential)]
public struct WNDCLASSEX
{
    public uint      cbSize;
    public uint      style;
    public WndProc   lpfnWndProc;
    public int       cbClsExtra;
    public int       cbWndExtra;
    public IntPtr    hInstance;
    public IntPtr    hIcon;
    public IntPtr    hCursor;
    public IntPtr    hbrBackground;
    [MarshalAs(UnmanagedType.LPStr)]
    public string    lpszMenuName;
    [MarshalAs(UnmanagedType.LPStr)]
    public string    lpszClassName;
    public IntPtr    hIconSm;
}

Program.cs:

class Program
{
    static long WndProc(IntPtr hWnd, uint message, byte wParam, long lParam)
    {
        return WindowLL.DefWindowProcA(hWnd, message, wParam, lParam);
    }
    static void Main(string[] args)
    {
        var winCalss = new WNDCLASSEX() {
            lpszClassName = "test",
            lpfnWndProc = WndProc,
            cbSize = (uint)Marshal.SizeOf(typeof(WNDCLASSEX)),
        };
        var classId = WindowLL.RegisterClassExA(ref winCalss);

        var overlapped =
            WindowStyle.Overlapped |
            WindowStyle.Caption |
            WindowStyle.SysMenu |
            WindowStyle.ThickFrame |
            WindowStyle.MinimizeBox |
            WindowStyle.MaximizeBox;

        IntPtr hWnd = WindowLL.CreateWindowExA(0,
            classId, "test",
            (uint)overlapped,
            0, 0,
            1280, 720,
            IntPtr.Zero, IntPtr.Zero,
            IntPtr.Zero, IntPtr.Zero
        );

        WindowLL.ShowWindow(hWnd, (int)NCmdShow.Show);

        while (WindowLL.GetMessage(out var msg, hWnd, 0, 0)) {
            WindowLL.TranslateMessage(ref msg);
            WindowLL.DispatchMessage(ref msg);
        }
    }
}

Answer 1

Your structures are mostly wrong.

wParam / lParam should both be IntPtr, time should be uint, point x/y should be int.

You don't even need to write them yourself, copy-paste from the framework, MIT license allow that:

https://source.dot.net/#WindowsBase/System/Windows/Interop/MSG.cs,8a0b462145bdb8d8

https://source.dot.net/#PresentationFramework/System/Windows/Standard/NativeMethods.cs,527982c752231945

One more thing. You should not be using A-functions and structures like CreateWindowExA. These are compatibility shims for Windows 95, 98 and Me, all 3 discontinued decades ago. Modern software should only use W-versions. As a nice side effect strings marshalling is way cheaper, C# strings are stored as UTF-16 which matches the encoding expected by W WinApi functions.