WPF：窗口居中置顶

主窗口居中置顶

public MainWindow()
{
    InitializeComponent();
    WindowStartupLocation = WindowStartupLocation.CenterScreen;
    this.Topmost = true;
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
    IntPtr hwnd = new WindowInteropHelper(this).Handle;
    TopMostTool.SetTopmost(hwnd);
}

using System;
using System.Runtime.InteropServices;
/// <summary>
/// 置顶帮助类
/// </summary>
public class TopMostTool
{
    /// <summary>
    /// 窗体置顶
    /// </summary>
    /// <param name="windowName">需要置顶的窗体的名字 Title</param>
    public static void SetTopWindow(string windowName)
    {
        IntPtr frm = Win32ApiHelper.FindWindow(null, windowName);    // 程序中需要置顶的窗体的名字
        if (frm != IntPtr.Zero)
        {
            Win32ApiHelper.SetWindowPos(frm, (IntPtr)Win32ApiHelper.HWND_TOPMOST, 0, 0, 0, 0, (int)Win32ApiHelper.WindowFlags.SWP_NOMOVE | (int)Win32ApiHelper.WindowFlags.SWP_NOSIZE);

            var child = Win32ApiHelper.FindWindowEx(frm, IntPtr.Zero, null, windowName);
        }
    }
    /// <summary>
    /// 窗体置顶
    /// </summary>
    /// <param name="handle">窗体句柄</param>
    public static void SetTopmost(IntPtr handle)
    {
        Win32ApiHelper.SetWindowPos(handle, (IntPtr)Win32ApiHelper.HWND_TOPMOST, 0, 0, 0, 0, (int)Win32ApiHelper.WindowFlags.SWP_NOMOVE | (int)Win32ApiHelper.WindowFlags.SWP_NOSIZE);
    }
}

子窗口居中显示

var dlg = new ChildDlg()
{
    Background = Brushes.Black,
    Opacity = 0.8,
    AllowsTransparency = true,
    WindowStyle = WindowStyle.None,
    WindowState = WindowState.Normal,
    Topmost = true,
    WindowStartupLocation = WindowStartupLocation.CenterOwner,
    Owner = mainWindow,
    Width = mainWindow.Width,
    Height  = mainWindow.Height,
};
dlg.ShowDialog();