using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using System.Runtime.InteropServices;
using System.Windows.Interop;

namespace MMDesktop
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {

        const UInt32 SWP_NOSIZE = 0x0001;
        const UInt32 SWP_NOMOVE = 0x0002;
        const UInt32 SWP_NOACTIVATE = 0x0010;
        const UInt32 SWP_NOZORDER = 0x0004;
        const int WM_ACTIVATEAPP = 0x001C;
        const int WM_ACTIVATE = 0x0006;
        const int WM_SETFOCUS = 0x0007;
        static readonly IntPtr HWND_BOTTOM = new IntPtr(1);
        const int WM_WINDOWPOSCHANGING = 0x0046;

        [DllImport("user32.dll")]
        static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X,
           int Y, int cx, int cy, uint uFlags);
        [DllImport("user32.dll")]
        static extern IntPtr DeferWindowPos(IntPtr hWinPosInfo, IntPtr hWnd,
           IntPtr hWndInsertAfter, int x, int y, int cx, int cy, uint uFlags);
        [DllImport("user32.dll")]
        static extern IntPtr BeginDeferWindowPos(int nNumWindows);
        [DllImport("user32.dll")]
        static extern bool EndDeferWindowPos(IntPtr hWinPosInfo);



        public Window1()
        {
            InitializeComponent();
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            IntPtr hWnd = new WindowInteropHelper(this).Handle;
            SetWindowPos(hWnd, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_NOACTIVATE);

            IntPtr windowHandle = (new WindowInteropHelper(this)).Handle;
            HwndSource src = HwndSource.FromHwnd(windowHandle);
            src.AddHook(new HwndSourceHook(WndProc));
        }

        private IntPtr WndProc(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
        {
            if (msg == WM_SETFOCUS)
            {
                IntPtr hWnd2 = new WindowInteropHelper(this).Handle;
                SetWindowPos(hWnd2, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_NOACTIVATE);
                handled = true;
            }
            return IntPtr.Zero;
        }

        private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
        {
            IntPtr windowHandle = (new WindowInteropHelper(this)).Handle;
            HwndSource src = HwndSource.FromHwnd(windowHandle);
            src.RemoveHook(new HwndSourceHook(this.WndProc));
        }

    }
}

UI：

<Window x:Class="MMDesktop.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window1" Height="300" Width="300"
        Closing="Window_Closing" Loaded="Window_Loaded"
        >
    <Grid></Grid>
</Window>

c# WPF 設置窗口一直在其中窗口後面/底層窗口