Creating a BOTTOMMOST Window

Introduction

To create a topmost window is simple, thanks to window style WS_EX_TOPMOST or Form.TopMost propertie in .NET. But unfortunately the counter part style BOTTOMMOST is not out of the box.

Background

An application window Z order, along with other parameters, can be enforced at runtime by modifying WINDOWPOS structure received with WM_WINOWPOSCHANGING message.

Using the code

To create a bottommost window, on the window message WM_WINOWPOSCHANGING add this code:
((LPWINDOWPOS)lParam)->hwndInsertAfter = HWND_BOTTOM;
This works in C or C++. For .NET, the code can be included in a dll and used inside the window procedure.
The resulted window behave like it is embedded in the wallpaper.

 dll code (C):

__declspec(dllexport) void Bottommost(LPARAM lParam);

void Bottommost(LPARAM lParam)
{
((LPWINDOWPOS)lParam)->hwndInsertAfter = HWND_BOTTOM;
}

C# code:

using System.Runtime.InteropServices;

[DllImport("mydll.dll")]
static extern void Bottommost(IntPtr lParam);

[System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
protected override void WndProc(ref Message Message)
{
if (Message.Msg == 0x46)//WM_WINDOWPOSCHANGING
{
Bottommost(Message.LParam);
}
base.WndProc(ref Message);
}

VB code:

Imports System.Runtime.InteropServices

<DllImport("mydll.dll")>
Shared Function Bottommost(lParam As IntPtr) As Boolean
End Function

<System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name:="FullTrust")>
Protected Overrides Sub WndProc(ByRef Message As Message)
If (Message.Msg = &H46) Then 'WM_WINDOWPOSCHANGING
Bottommost(Message.LParam)
End If
MyBase.WndProc(Message)
End Sub

 Win32 code (C, C++):

LRESULT CALLBACK WndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
switch(Message)
{
case WM_WINDOWPOSCHANGING:
((LPWINDOWPOS)lParam)->hwndInsertAfter = HWND_BOTTOM;
return 0L;    
break;
//...
}
return DefWindowProc(hwnd, Message, wParam, lParam);
}