Win32: How to hide 3rd party windows in taskbar by hWnd

Before you use SetWindowLong, call ShowWindow(hWnd, SW_HIDE), then call SetWindowLong, then call ShowWindow again like ShowWindow(hWnd, SW_SHOW). So your code will look like this:

long style= GetWindowLong(hWnd, GWL_STYLE);
style &= ~(WS_VISIBLE);    // this works - window become invisible 

style |= WS_EX_TOOLWINDOW;   // flags don't work - windows remains in taskbar
style &= ~(WS_EX_APPWINDOW); 

ShowWindow(hWnd, SW_HIDE); // hide the window
SetWindowLong(hWnd, GWL_STYLE, style); // set the style
ShowWindow(hWnd, SW_SHOW); // show the window for the new style to come into effect
ShowWindow(hWnd, SW_HIDE); // hide the window so we can't see it

Here is a relevant quote from Microsoft's Website:

To prevent the window button from being placed on the taskbar, create the unowned window with the WS_EX_TOOLWINDOW extended style. As an alternative, you can create a hidden window and make this hidden window the owner of your visible window.

The Shell will remove a window's button from the taskbar only if the window's style supports visible taskbar buttons. If you want to dynamically change a window's style to one that doesn't support visible taskbar buttons, you must hide the window first (by calling ShowWindow with SW_HIDE), change the window style, and then show the window.

You must use GWL_EXSTYLE to get/set the EX flags, GWL_STYLE will not work for EX flags.