Question

I have been struggling to hide another application from the taskbar from my application.
I have been using the SetWindowLong function in order to set/remove WS_EX_APPWINDOW on the extended style.

I have tried both setting and removing the property individually as well as taking the current WindowLong, and removing/adding it to that one, like so:

SetWindowLong(pMainWindow, GWL_EXSTYLE, GetWindowLong(pMainWindow) & WS_EX_APPWINDOW);

And tried removing it like so:

SetWindowLong(pMainWindow, GWL_EXSTYLE, GetWindowLong(pMainWindow) & ~WS_EX_APPWINDOW);

Also tried both those methods without first getting the window long. Here is my entire code:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    [DllImport("User32.dll")]
    public static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
    [DllImport("User32.dll")]
    public static extern int GetWindowLong(IntPtr hWnd, int nIndex);

    [DllImport("user32.dll")]
    static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

    private const int SW_HIDE = 0x00;
    private const int SW_SHOW = 0x05;

    private const int WS_EX_APPWINDOW = 0x40000;
    private const int GWL_EXSTYLE = -0x14;

    private void HideWindowFromTaskbar(IntPtr pMainWindow)
    {
        SetWindowLong(pMainWindow, GWL_EXSTYLE, ~WS_EX_APPWINDOW);

        ShowWindow(pMainWindow, SW_HIDE);
        ShowWindow(pMainWindow, SW_SHOW);
    }

    private void ButtonHide_Click(object sender, RoutedEventArgs e)
    {
        HideWindowFromTaskbar(Process.GetProcessesByName("notepad")[0].MainWindowHandle);
    }
}

I have noticed changes in Spy++ looking at the properties. I had a bunch of different results, like WS_EX_APPWINDOW being added, but also randomly have other attributes disappearing, etc.
When looking at the messages, I also saw that it DID get messages like STYLE_CHANGED.

Answer 1

The rules for determining which windows have buttons on the taskbar are documented on MSDN. Raymond Chen gives the following summary of these rules:

There are some basic rules on which windows go into the taskbar. In
short:

• If the WS_EX_APPWINDOW extended style is set, then it will show (when visible).
• If the window is a top-level unowned window, then it will show (when visible).
• Otherwise it doesn’t show.

The fact that you are trying to modify a window in another app severely hampers you. You are removing the WS_EX_APPWINDOW extended style. This is not enough because the window in question will be a top-level unowned window (see bullet point 2). You cannot change the owner of a window once it has been created and since the window is controlled by another process you are pretty much stuck.

The only option remaining is to remove the WS_EX_APPWINDOW extended style and replace it with WS_EX_TOOLWINDOW. This will indeed get the window off the taskbar but it will change the appearance of the window:

The window is intended to be used as a floating toolbar. A tool window
has a title bar that is shorter than a normal title bar, and the
window title is drawn using a smaller font. A tool window does not
appear in the taskbar or in the dialog that appears when the user
presses ALT+TAB. If a tool window has a system menu, its icon is not
displayed on the title bar. However, you can display the system menu
by right-clicking or by typing ALT+SPACE.