WPF Tips: Window.ShowDialog()方法：Cannot set Visibility or call Show, ShowDialog, or WindowInteropHelp

关于Window.ShowDialog()方法，有一个常见的容易犯的错误。下面给出这个错误的例子：

DemoA：错误的例子

1. 在WPF项目中，创建一个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();        }    }}

然后在MainWindow里面去调用它：

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();        }    }}

这个例子似乎非常寻常。看出它有什么问题了吗？如果您看出问题来了，那么本文对您来说纯属浪费时间，请移步其它文章，谢谢。

好，下面我们运行它。运行时，点击MainWindow窗口的按钮，第一次点击，弹出了DialogWindow对话框，没什么问题。点击“Cancel”按钮关闭DialogWindow对话框，再次点击MainWindow窗口的按钮，这时候问题出现了，抛出了以下异常：

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.

如何解决这个问题呢？解决方案如下：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();        }    }}

这样，每次调用Window.ShowDialog()方法的时候，都创建了一个新的DialogWindow实例，所以问题得以解决。

参考：

同样的疑问，在StackOverflow上的讨论如下：

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

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