WPF Tips: Window.ShowDialog()方法：Cannot set Visibility or cal

關於Window.ShowDialog()方法，有一個常見的容易犯的錯誤。下面給出這個錯誤的例子：

DemoA：錯誤的例子

1. 在WPF項目中，建立一個Windows：DialogWindow

DialogWindow.xaml

<Window x:Class="DemoA.DialogWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        Title="I am a dialog window" Height="200" Width="300">    <Grid>        <Grid.RowDefinitions>            <RowDefinition Height="140" />            <RowDefinition Height="60" />        </Grid.RowDefinitions>                <Label Grid.Row="0" Margin="12" Content="Hello!" />        <WrapPanel Grid.Row="1" Margin="0" HorizontalAlignment="Center">            <Button Content="OK" Margin="5,0,5,0" Width="60" />            <Button Content="Cancel" Margin="5,0,5,0" Width="60"                    Click="ButtonCancelOnClick"/>        </WrapPanel>    </Grid></Window>

DialogWindow.xaml.cs

using System.Windows;namespace DemoA{    /// <summary>    /// Interaction logic for DialogWindow.xaml    /// </summary>    public partial class DialogWindow : Window    {        public DialogWindow()        {            InitializeComponent();        }        private void ButtonCancelOnClick(object sender, RoutedEventArgs e)        {            this.Close();        }    }}

然後在MainWindow裡面去調用它：

MainWindow.xaml

<Window x:Class="DemoA.MainWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        Title="MainWindow" Height="350" Width="525">    <Grid>        <Button Width="Auto" Height="Auto" Content="Show dialog" Click="ButtonClick" />    </Grid></Window>

MainWindow.xaml.cs

using System.Windows;namespace DemoA{    /// <summary>    /// Interaction logic for MainWindow.xaml    /// </summary>    public partial class MainWindow : Window    {        private DialogWindow dialogWindow = new DialogWindow();        public MainWindow()        {            this.InitializeComponent();        }        private void ButtonClick(object sender, RoutedEventArgs e)        {            dialogWindow.ShowDialog();        }    }}

這個例子似乎非常尋常。看出它有什麼問題了嗎？如果您看出問題來了，那麼本文對您來說純屬浪費時間，請移步其它文章，謝謝。

好，下面我們運行它。運行時，點擊MainWindow視窗的按鈕，第一次點擊，彈出了DialogWindow對話方塊，沒什麼問題。點擊“Cancel”按鈕關閉DialogWindow對話方塊，再次點擊MainWindow視窗的按鈕，這時候問題出現了，拋出了以下異常：

An unhandled exception of type 'System.InvalidOperationException' occurred in PresentationFramework.dllAdditional information: Cannot set Visibility or call Show, ShowDialog, or WindowInteropHelper.EnsureHandle after a Window has closed.

如何解決這個問題呢？解決方案如下：DemoB

DialogWindow.xaml.cs

using System.Windows;namespace DemoB{    /// <summary>    /// Interaction logic for DialogWindow.xaml    /// </summary>    public partial class DialogWindow : Window    {        private static DialogWindow staticInstance = null;        public DialogWindow()        {            this.InitializeComponent();            this.Closed += WindowOnClosed;        }        public static DialogWindow GetInstance()        {            if (staticInstance == null)            {                staticInstance = new DialogWindow();            }            return staticInstance;        }        private void ButtonCancelOnClick(object sender, RoutedEventArgs e)        {            this.Close();        }        private void WindowOnClosed(object sender, System.EventArgs e)        {            staticInstance = null;        }    }}

MainWindow.xaml.cs

using System.Windows;namespace DemoB{    /// <summary>    /// Interaction logic for MainWindow.xaml    /// </summary>    public partial class MainWindow : Window    {        public MainWindow()        {            this.InitializeComponent();        }        private void ButtonClick(object sender, RoutedEventArgs e)        {            DialogWindow.GetInstance().ShowDialog();        }    }}

這樣，每次調用Window.ShowDialog()方法的時候，都建立了一個新的DialogWindow執行個體，所以問題得以解決。

參考：

同樣的疑問，在StackOverflow上的討論如下：

《WPF: Cannot reuse window after it has been closed》

http://stackoverflow.com/questions/3568233/wpf-cannot-reuse-window-after-it-has-been-closed