廢話不多說，直接上圖：

上程式碼：

先搞一個基類，方便子窗體複用

    public partial class BaseWindow : Window
    {
        public BaseWindow() : base()
        {
            this.Closed += Window_Closed;
            this.WindowStartupLocation = WindowStartupLocation.CenterOwner;
            this.ResizeMode = ResizeMode.NoResize;
        }

        public
 
 void SyncComplete()
        {
            this.Dispatcher.Invoke(() =>
            {
                MessageBox.Show(this, "操作完成！", "提示");
            });
        }

        private void Window_Closed(object sender, EventArgs e)
        {
            //容器Grid
            Grid grid = this.Owner.Content as
 
 Grid;
            //父級窗體原來的內容
            UIElement original = VisualTreeHelper.GetChild(grid, 0) as UIElement;
            //將父級窗體原來的內容在容器Grid中移除
            grid.Children.Remove(original);
            //賦給父級窗體
            this.Owner.Content = original;
        }

        public bool? ShowDialog(Window owner)
        {
            //蒙板
 

            Grid layer = new Grid() { Background = new SolidColorBrush(Color.FromArgb(128, 0, 0, 0)) };
            //父級窗體原來的內容
            UIElement original = owner.Content as UIElement;
            owner.Content = null;
            //容器Grid
            Grid container = new Grid();
            container.Children.Add(original);//放入原來的內容
            container.Children.Add(layer);//在上面放一層蒙板
                                          //將裝有原來內容和蒙板的容器賦給父級窗體
            owner.Content = container;
            this.Owner = owner;
            return this.ShowDialog();
        }
    }

子窗體實現：
XAML

<local:BaseWindow x:Class="Modules.wnd_TimeSync"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Modules"
        mc:Ignorable="d" WindowStyle="None" AllowsTransparency="True" Background="Transparent"
        Title="wnd_TimeSync" Height="20" Width="220" Closed="Window_Closed" >
    <StackPanel>
        <ProgressBar Name="pb" Height="20" Width="200" Maximum="100"></ProgressBar>
    </StackPanel>
</local:BaseWindow>

cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
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;

namespace Modules
{
    /// <summary>
    /// wnd_TimeSync.xaml 的互動邏輯
    /// </summary>
    public partial class wnd_TimeSync : BaseWindow
    {
        public wnd_TimeSync()
        {
            InitializeComponent();
        }

        public static void ShowDialog(Window owner)
        {
            //彈出訊息框
            wnd_TimeSync box = new wnd_TimeSync();
            box.ShowDialog(owner);
        }
    }
}