Why doesn't the window set cursor correctly?

Question

As MSDN describes:

When the mouse moves over a window, the window receives a WM_SETCURSOR message (unless another window has captured the mouse).

If the application passes WM_SETCURSOR to DefWindowProc, the DefWindowProc function uses the following algorithm to set the cursor image:

1. If the window has a parent, forward the WM_SETCURSOR message to the parent to handle.
2. Otherwise, if the window has a class cursor, set the cursor to the class cursor.
3. If there is no class cursor, set the cursor to the arrow cursor.

And here's my source code:

#include <tchar.h>
#include <Windows.h>

LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int APIENTRY _tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInsTance, LPTSTR lpCmdLine, int nCmdShow)
{
  WNDCLASSEX wcex = { 0 };
  HWND hWnd;
  BOOL ret;
  MSG msg;

  wcex.cbSize = sizeof(wcex);
  wcex.lpfnWndProc = WndProc;
  wcex.hInstance = hInstance;
  wcex.hIcon = LoadIcon(NULL, IDI_APPLICATION);
  // wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
  wcex.lpszClassName = TEXT("MainWindow");
  wcex.hIconSm = wcex.hIcon;

  RegisterClassEx(&wcex);
  hWnd = CreateWindow(wcex.lpszClassName, TEXT("Test"), WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, HWND_DESKTOP, NULL, hInstance, NULL);

  ShowWindow(hWnd, nCmdShow);
  UpdateWindow(hWnd);

  while (ret = GetMessage(&msg, NULL, 0, 0))
  {
    if (ret == -1)
    {
      return EXIT_FAILURE;
    }
    else
    {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
    }
  }
  return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
  switch (uMsg)
  {
  case WM_DESTROY:
    PostQuitMessage(EXIT_SUCCESS);
    break;
  default:
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
  }
  return 0;
}

When I move my cursor quickly over the window, the cursor doesn't change to an arrow, it become a resize arrow; if I set wcex.hCursor to LoadCursor(NULL, IDC_ARROW), everything will be all right. My question is: why doesn't my code work as MSDN says?

I mean, if I don't set wcex.hCursor, and I don't handle the WM_SETCURSOR message, the DefWindowProc should "set the cursor to the arrow cursor", but it seems it doesn't. Why is that?

Answer 1

See description of the hCursor member of the WNDCLASSEX:

A handle to the class cursor. This member must be a handle to a cursor resource. If this member is NULL, an application must explicitly set the cursor shape whenever the mouse moves into the application's window.

WNDCLASSEX structure