Q:

why not to send WM_PAINT manually

I have read that I should never send WM_PAINT manually and should call InvalidateRect instead but didn't found anything about why not, however. So why not?
update works with InvalidateRect but not with SendMessage(WM_PAINT)
LRESULT CALLBACK window_proc(HWND wnd, UINT msg, WPARAM w_param, LPARAM l_param)
{
  switch (msg)
  {
    case WM_PAINT:
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(wnd, &ps);

        Polyline(..);

        EndPaint(wnd, &ps);
        return 0;

    case WM_USER:           
        // SendMessage(wnd, WM_PAINT, NULL, NULL);
        // InvalidateRect(wnd, NULL, FALSE);

        return 0;
  }
}

A:

Official docs for WM_PAINT state that you shouldn't in the very first sentence of the remarks section. Seriously, that should be enough of a reason not to.
As for technical reasons why, I guess this is one of them, taken from BeginPaint remarks section:

The update region is set by the InvalidateRect or InvalidateRgn function and by the system after sizing, moving, creating, scrolling, or any other operation that affects the client area.

Thus BeginPaint might not work correctly if you send WM_PAINT manually.
There might be more reasons/surprises.

A:

If you want to trigger an immediate repaint, the correct approach is to either:

Use InvalidateRect() followed by UpdateWindow().
Use RedrawWindow().

Those will trigger a new WM_PAINT message to be generated.

