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
{
    public static int SW_SHOW = 5;
    public static int SW_NORMAL = 1;
    public static int SW_MAX = 3;
    public static int SW_HIDE = 0;
    public static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);    //窗体置顶
    public static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);    //取消窗体置顶
    public const uint SWP_NOMOVE = 0x0002;    //不调整窗体位置
    public const uint SWP_NOSIZE = 0x0001;    //不调整窗体大小
    public bool isFirst = true;

    [DllImport("user32.dll")]
    public static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int x, int y, int cx, int cy, uint uFlags);

    [DllImport("user32.dll", EntryPoint = "ShowWindow")]
    public static extern bool ShowWindow(System.IntPtr hWnd, int nCmdShow);

    [DllImport("user32.dll")]
    public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
    /// <summary>
    /// 查找子窗口
    /// </summary>
    /// <param name="hwndParent"></param>
    /// <param name="hwndChildAfter"></param>
    /// <param name="lpClassName"></param>
    /// <param name="lpWindowName"></param>
    /// <returns></returns>
    [DllImport("User32.dll", EntryPoint = "FindWindowEx")]
    public static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpClassName, string lpWindowName);

    /// <summary>
    /// 窗体置顶，可以根据需要传入不同的值(需要置顶的窗体的名字Title)
    /// </summary>
    public static void SetTopWindow()
    {
        IntPtr frm = FindWindow(null, "窗体的名字Title");    // 程序中需要置顶的窗体的名字
        if (frm != IntPtr.Zero)
        {
            SetWindowPos(frm, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);

            var child = FindWindowEx(frm, IntPtr.Zero, null, "子窗体的名字Title");
        }
    }

    public static void SetTopmost(IntPtr handle)
    {
        SetWindowPos(handle, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | 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();