Setting a Windows form to be bottommost

Some background

One of my current clients runs a chain of Internet points where customers an access the net through PC:s set up as "kiosks" (a custom-built application "locks" the computer until a user has signed in, and the running account is heavily restricted through the Windows group policy). Currently, each computer is running Windows XP and uses Active Desktop to display advertisements in the background. However, since my client has got problems with Active Desktop crashing on a daily basis (in addition to generally slowing down the computer) I have been asked to develop an application that replaces it.

The problem

I am trying to investigate whether it is possible to build a Windows forms application (using C#) that always stays in the background. The application should lie above the desktop (so that it covers any icons, files etc) but always behind all other running applications. I guess I'm really looking for a BottomMost property of the Form class (which doesn't exist, of course).

Any tips or pointers on how to achieve this would be highly appreciated.

This isn't directly supported by the .NET Form class, so you have two options:

1) Use the Win32 API SetWindowPos function.

pinvoke.net shows how to declare this for use in C#:

[DllImport("user32.dll")]
static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

static readonly IntPtr HWND_BOTTOM = new IntPtr(1);
const UInt32 SWP_NOSIZE = 0x0001;
const UInt32 SWP_NOMOVE = 0x0002;
const UInt32 SWP_NOACTIVATE = 0x0010;

So in your code, call:

SetWindowPos(Handle, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_NOACTIVATE);

As you commented, this moves the form to the bottom of the z-order but doesn't keep it there. The only workaround I can see for this is to call SetWindowPos from the Form_Load and Form_Activate events. If your application is maximized and the user is unable to move or minimise the form then you might get away with this approach, but it's still something of a hack. Also the user might see a slight "flicker" if the form gets brought to the front of the z-order before the SetWindowPos call gets made.

2) subclass the form, override the WndProc function and intercept the WM_WINDOWPOSCHANGING Windows message, setting the SWP_NOZORDER flag (taken from this page).

I think the best way to do so is using the activated event handler and SendToBack method, like so:

private void Form1_Activated(object sender, EventArgs e)
{
    this.SendToBack();
}

Set your window to be a child window of the desktop (the "Program Manager" or "progman" process). I've succeeded with this method in Windows XP (x86) and Windows Vista (x64).

I stumbled on this method while searching for a way to make a screensaver display as if it were wallpaper. It turns out, this is sort of built in to the system's .scr handler. You use screensaver.scr /p PID, where PID is the process id of another program to attach to. So write a program to find progman's handle, then invoke the .scr with that as the /p argument, and you have screensaver wallpaper!

The project I'm playing with now is desktop status display (shows the time, some tasks, mounted disks, etc), and it's built on Strawberry Perl and plain Win32 APIS (mainly the Win32::GUI and Win32::API modules), so the code is easy to port to or understand any dynamic language with similar Win32 API bindings or access to Windows' Scripting Host (eg, ActivePerl, Python, JScript, VBScript). Here's a relevant portion of the class that produces the window:

do { Win32::API->Import(@$_) or die "Win32::API can't import @$_ ($^E)" } for
    [user32 => 'HWND FindWindow(LPCTSTR lpClassName, LPCTSTR lpWindowName)'],
    [user32 => 'HWND SetParent(HWND hWndChild, HWND hWndNewParent)'],

sub __screen_x {
    Win32::GUI::GetSystemMetrics(SM_CXSCREEN)
}
sub __screen_y {
    Win32::GUI::GetSystemMetrics(SM_CYSCREEN)
}
sub _create_window { # create window that covers desktop
    my $self = shift;
    my $wnd = $$self{_wnd} = Win32::GUI::Window->new(
        -width   => __screen_x(), -left => 0,
        -height  => __screen_y(), -top  => 0,
    ) or die "can't create window ($^E)";

    $wnd->SetWindowLong(GWL_STYLE,
        WS_VISIBLE
        | WS_POPUP # popup: no caption or border
    );
    $wnd->SetWindowLong(GWL_EXSTYLE, 
        WS_EX_NOACTIVATE # noactivate: doesn't activate when clicked
        | WS_EX_NOPARENTNOTIFY # noparentnotify: doesn't notify parent window when created or destroyed
        | WS_EX_TOOLWINDOW # toolwindow: hide from taskbar
    );
    SetParent($$wnd{-handle}, # pin window to desktop (bottommost)
        (FindWindow('Progman', 'Program Manager') or die "can't find desktop window ($^E)")
    ) or die "can't pin to desktop ($^E)";
    Win32::GUI::DoEvents; # allow sizing and styling to take effect (otherwise DC bitmaps are the wrong size)
}

This program buffers output to prevent flickering, which you'll probably want to do as well. I create a DC (device context) and PaintDesktop to it (you could use any bitmap with only a couple more lines -- CreateCompatibleBitmap, read in a file, and select the bitmap's handle as a brush), then create a holding buffer to keep a clean copy of that background and a working buffer to assemble the pieces -- on each loop, copy in background, then draw lines and brush bitmaps and use TextOut -- which is then copied to the original DC, at which time it appears on screen.