<controls:ChildWindow x:Class="ChessProject.MsgBox" ...省略一点...  Width="290" Height="141"  Title="系统消息">
    <Grid x:Name="LayoutRoot" Margin="2" Height="97" Width="270">
        <Button Visibility="Collapsed" x:Name="CancelButton" Content="取消" Click="CancelButton_Click" Width="75" Height="23" HorizontalAlignment="Right" Margin="0,62,93,12" />
        <Button x:Name="OKButton" Content="确定" Click="OKButton_Click" Width="75" Height="23" HorizontalAlignment="Right" Margin="0,62,10,12" />
        <TextBlock Height="41" TextWrapping="Wrap" HorizontalAlignment="Left" Margin="15,15,0,0" Name="tbMsg" Text="请按确定按钮确定" VerticalAlignment="Top" Width="224" />
    </Grid>
</controls:ChildWindow>

       DispatcherTimer timer;//定时器
        public MsgBox()
        {
            InitializeComponent();
            timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromSeconds(1);
            timer.Tick += new EventHandler(timer_Tick);
        }

       int defaultTime = 3;//默认N秒
        string userTitle;
       DispatcherTimer timer;//定时器
        public MsgBox()
        {
            //...省略...
        }
        void timer_Tick(object sender, EventArgs e)
        {
            Title = string.Format(userTitle + " [倒计时自动确定:{0}秒]", defaultTime);
            defaultTime--;
            if (defaultTime == 0)
            {
                ResetTimer();
            }
        }
        void ResetTimer()
        {
            timer.Stop();
            defaultTime = 3;
        }
        public void Show(string msg, string title)
        {
            tbMsg.Text = msg;
            userTitle = title;
            Show();
            timer.Start();
        }

首先：这个子窗口是异步的，所以，在点击完确定时，需要增加多一个回调函数；
接着：默认3秒，很明显情况不同，时间也要稍为增加变动一下；
然后：有时候按钮只是确定，有时候就是取消+确定；

最后：倒计时时间到了，默认执行确定，还是执行取消。

        public MsgBox()
        {
            InitializeComponent();
            timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromSeconds(1);
            timer.Tick += new EventHandler(timer_Tick);
            this.Closed += new EventHandler(MsgBox_Closed);
        }
        void MsgBox_Closed(object sender, EventArgs e)
        {
            //待实现
         }
        private void OKButton_Click(object sender, RoutedEventArgs e)
        {
            this.DialogResult = true;
            Close();//调用一下关闭
        }

private void CancelButton_Click(object sender, RoutedEventArgs e)
        {
            this.DialogResult = false;
            Close();//调用一下关闭
        }

    public partial class MsgBox : ChildWindow
    {
        int defaultTime = 3;//默认N秒
        string userTitle;
        DispatcherTimer timer;//定时器
        Action<bool> callBackEvent;
        bool autoOKConfirm = true;
        public MsgBox()
        {
            InitializeComponent();
            timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromSeconds(1);
            timer.Tick += new EventHandler(timer_Tick);
            this.Closed += new EventHandler(MsgBox_Closed);
        }
        void MsgBox_Closed(object sender, EventArgs e)
        {
            //待实现
        }
        void timer_Tick(object sender, EventArgs e)
        {
            Title = string.Format(userTitle + " [倒计时自动确定:{0}秒]", defaultTime);
            defaultTime--;
            if (defaultTime == 0)
            {
                ResetTimer();
                if (autoOKConfirm)
                {
                    OKButton_Click(null, null);
                }
                else
                {
                    CancelButton_Click(null, null);
                }
            }
        }
        void ResetTimer()
        {
            timer.Stop();
            defaultTime = 3;
        }
        public void Show(string msg, string title)
        {
            Show(msg, title, defaultTime, null, true, MessageBoxButton.OK);
        }
        public void Show(string msg, string title, int timeSecond, Action<bool> callBack)
        {
            Show(msg, title, timeSecond, callBack, false, MessageBoxButton.OKCancel);
        }
        public void Show(string msg, string title, int timeSecond, Action<bool> callBack, bool autoOK, MessageBoxButton button)
        {
            tbMsg.Text = msg;
            userTitle = title;
            if (button == MessageBoxButton.OK)
            {
                OKButton.Content = "确定";
                CancelButton.Visibility = System.Windows.Visibility.Collapsed;
            }
            else
            {
                CancelButton.Visibility = System.Windows.Visibility.Visible;
                OKButton.Content = "同意";
                CancelButton.Content = "拒绝";
            }
            defaultTime = timeSecond;
            autoOKConfirm = autoOK;
            callBackEvent = callBack;
            Show();
            timer.Start();
        }
        private void OKButton_Click(object sender, RoutedEventArgs e)
        {
            this.DialogResult = true;
        }

private void CancelButton_Click(object sender, RoutedEventArgs e)
        {
            this.DialogResult = false;
        }
    }