Best way to hide a window from the Alt-Tab program

Question

I've been a .NET developer for several years now and this is still one of those things I don't know how to do properly. It's easy to hide a window from the taskbar via a property in both Windows Forms and WPF, but as far as I can tell, this doesn't guarantee (or necessarily even affect) it being hidden from the Alt+↹Tab dialog. I've seen invisible windows show up in Alt+↹Tab, and I'm just wondering what is the best way to guarantee a window will never appear (visible or not) in the Alt+↹Tab dialog.

Update: Please see my posted solution below. I'm not allowed to mark my own answers as the solution, but so far it's the only one that works.

Update 2: There's now a proper solution by Franci Penov that looks pretty good, but haven't tried it out myself. Involves some Win32, but avoids the lame creation of off-screen windows.

Answer 1

I tried setting the main form's visibility to false whenever it is automatically changed to true:

private void Form1_VisibleChanged(object sender, EventArgs e)
{
    if (this.Visible)
    {
        this.Visible = false;
    }
}

It works perfectly :)

Answer 2

In XAML set ShowInTaskbar="False":

<Window x:Class="WpfApplication5.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    ShowInTaskbar="False"    
    Title="Window1" Height="300" Width="300">
    <Grid>

    </Grid>
</Window>

Edit: That still shows it in Alt+Tab I guess, just not in the taskbar.

Answer 3

Why trying so much codes? Just set the FormBorderStyle propety to FixedToolWindow. Hope it helps.

Answer 4

see it:(from http://bytes.com/topic/c-sharp/answers/442047-hide-alt-tab-list#post1683880)

[DllImport("user32.dll")]
public static extern int SetWindowLong( IntPtr window, int index, int
value);
[DllImport("user32.dll")]
public static extern int GetWindowLong( IntPtr window, int index);


const int GWL_EXSTYLE = -20;
const int WS_EX_TOOLWINDOW = 0x00000080;
const int WS_EX_APPWINDOW = 0x00040000;

private System.Windows.Forms.NotifyIcon notifyIcon1;


// I use two icons depending of the status of the app
normalIcon = new Icon(this.GetType(),"Normal.ico");
alertIcon = new Icon(this.GetType(),"Alert.ico");
notifyIcon1.Icon = normalIcon;

this.WindowState = System.Windows.Forms.FormWindowState.Minimized;
this.Visible = false;
this.ShowInTaskbar = false;
iconTimer.Start();

//Make it gone frmo the ALT+TAB
int windowStyle = GetWindowLong(Handle, GWL_EXSTYLE);
SetWindowLong(Handle, GWL_EXSTYLE, windowStyle | WS_EX_TOOLWINDOW);

Answer 5

Don't show a form. Use invisibility.

More here: http://code.msdn.microsoft.com/TheNotifyIconExample

Answer 6

Update:

According to @donovan, modern days WPF supports this natively, through setting ShowInTaskbar="False" and Visibility="Hidden" in the XAML. (I haven't tested this yet, but nevertheless decided to bump the comment visibility)

Original answer:

There are two ways of hiding a window from the task switcher in Win32 API:

1. to add the WS_EX_TOOLWINDOW extended window style - that's the right approach.
2. to make it a child window of another window.

Unfortunately, WPF does not support as flexible control over the window style as Win32, thus a window with WindowStyle=ToolWindow ends up with the default WS_CAPTION and WS_SYSMENU styles, which causes it to have a caption and a close button. On the other hand, you can remove these two styles by setting WindowStyle=None, however that will not set the WS_EX_TOOLWINDOW extended style and the window will not be hidden from the task switcher.

To have a WPF window with WindowStyle=None that is also hidden from the task switcher, one can either of two ways:

• go with the sample code above and make the window a child window of a small hidden tool window
• modify the window style to also include the WS_EX_TOOLWINDOW extended style.

I personally prefer the second approach. Then again, I do some advanced stuff like extending the glass in the client area and enabling WPF drawing in the caption anyway, so a little bit of interop is not a big problem.

Here's the sample code for the Win32 interop solution approach. First, the XAML part:

<Window x:Class="WpfApplication1.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Height="300" Width="300"
        ShowInTaskbar="False" WindowStyle="None"
        Loaded="Window_Loaded" >

Nothing too fancy here, we just declare a window with WindowStyle=None and ShowInTaskbar=False. We also add a handler to the Loaded event where we will modify the extended window style. We can't do that work in the constructor, as there's no window handle at that point yet. The event handler itself is very simple:

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        WindowInteropHelper wndHelper = new WindowInteropHelper(this);

        int exStyle = (int)GetWindowLong(wndHelper.Handle, (int)GetWindowLongFields.GWL_EXSTYLE);

        exStyle |= (int)ExtendedWindowStyles.WS_EX_TOOLWINDOW;
        SetWindowLong(wndHelper.Handle, (int)GetWindowLongFields.GWL_EXSTYLE, (IntPtr)exStyle);
    }

And the Win32 interop declarations. I've removed all unnecessary styles from the enums, just to keep the sample code here small. Also, unfortunately the SetWindowLongPtr entry point is not found in user32.dll on Windows XP, hence the trick with routing the call through the SetWindowLong instead.

    #region Window styles
    [Flags]
    public enum ExtendedWindowStyles
    {
        // ...
        WS_EX_TOOLWINDOW = 0x00000080,
        // ...
    }

    public enum GetWindowLongFields
    {
        // ...
        GWL_EXSTYLE = (-20),
        // ...
    }

    [DllImport("user32.dll")]
    public static extern IntPtr GetWindowLong(IntPtr hWnd, int nIndex);

    public static IntPtr SetWindowLong(IntPtr hWnd, int nIndex, IntPtr dwNewLong)
    {
        int error = 0;
        IntPtr result = IntPtr.Zero;
        // Win32 SetWindowLong doesn't clear error on success
        SetLastError(0);

        if (IntPtr.Size == 4)
        {
            // use SetWindowLong
            Int32 tempResult = IntSetWindowLong(hWnd, nIndex, IntPtrToInt32(dwNewLong));
            error = Marshal.GetLastWin32Error();
            result = new IntPtr(tempResult);
        }
        else
        {
            // use SetWindowLongPtr
            result = IntSetWindowLongPtr(hWnd, nIndex, dwNewLong);
            error = Marshal.GetLastWin32Error();
        }

        if ((result == IntPtr.Zero) && (error != 0))
        {
            throw new System.ComponentModel.Win32Exception(error);
        }

        return result;
    }

    [DllImport("user32.dll", EntryPoint = "SetWindowLongPtr", SetLastError = true)]
    private static extern IntPtr IntSetWindowLongPtr(IntPtr hWnd, int nIndex, IntPtr dwNewLong);

    [DllImport("user32.dll", EntryPoint = "SetWindowLong", SetLastError = true)]
    private static extern Int32 IntSetWindowLong(IntPtr hWnd, int nIndex, Int32 dwNewLong);

    private static int IntPtrToInt32(IntPtr intPtr)
    {
        return unchecked((int)intPtr.ToInt64());
    }

    [DllImport("kernel32.dll", EntryPoint = "SetLastError")]
    public static extern void SetLastError(int dwErrorCode);
    #endregion