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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace hintergrundfenster
{
    public partial class Form1 : Form
    {
        [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);
        static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);
        static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);
        static readonly IntPtr HWND_TOP = new IntPtr(0);

        const UInt32 SWP_NOSIZE = 0x0001;
        const UInt32 SWP_NOMOVE = 0x0002;
        const UInt32 SWP_NOZORDER = 0x0004;
        const UInt32 SWP_NOREDRAW = 0x0008;
        const UInt32 SWP_NOACTIVATE = 0x0010;
        const UInt32 SWP_FRAMECHANGED = 0x0020;  /* The frame changed: send WM_NCCALCSIZE */
        const UInt32 SWP_SHOWWINDOW = 0x0040;
        const UInt32 SWP_HIDEWINDOW = 0x0080;
        const UInt32 SWP_NOCOPYBITS = 0x0100;
        const UInt32 SWP_NOOWNERZORDER = 0x0200;  /* Don't do owner Z ordering */
        const UInt32 SWP_NOSENDCHANGING = 0x0400;  /* Don't send WM_WINDOWPOSCHANGING */

        const UInt32 TOPMOST_FLAGS = SWP_NOMOVE | SWP_NOSIZE;


        public static void MakeBackMost(Form form)
        {
            SetWindowPos(form.Handle, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE);
        }
        public static void MakeTopMost(Form form)
        {
            SetWindowPos(form.Handle, HWND_TOPMOST, 0, 0, 0, 0, TOPMOST_FLAGS);
         }

        public static void MakeNormal(Form form)
        {
            SetWindowPos(form.Handle, HWND_NOTOPMOST, 0, 0, 0, 0, TOPMOST_FLAGS);
         }
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            test();
            //MakeTopMost(this);    //<-- funktioniert
            MakeBackMost(this);     //<-- funktioniert nicht
        }
        public void test()
        {
            
        }
    }
}					

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    public partial class Form1 : Form
    {
        [DllImport("user32.dll", EntryPoint = "GetDesktopWindow")]
        public static extern IntPtr GetDesktopWindow();
        
        protected override CreateParams CreateParams
        {
            get
            {
                new SecurityPermission(SecurityPermissionFlag.UnmanagedCode).Demand();

                CreateParams cp = base.CreateParams;
                cp.Parent = GetDesktopWindow();
                cp.Style |= 0x40000000; // WS_CHILD

                return cp;
            }
        }
    ...					

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