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我的 WP7 应用程序遇到问题：

仅当应用程序无法检测到任何存储的用户名或密码值时，我才尝试提示登录页面；但是，当我尝试导航到我的登录页面时，我一直遇到 NullReferenceException。

我的代码看起来像这样，它在构造函数中：

if (!checkLogin())
    {
        NavigationService.Navigate(new Uri("/LoginPage.xaml", UriKind.Relative));
    }

并且checkLogin只是一个返回 true 或 false 的函数，具体取决于隔离存储设置是否正确设置。

有人有什么建议吗？

谢谢。

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1 回答 1

3

Here is what i think you want to do (From Peter Torr's blog).

If you need more clarification here is a piece of code to illustrate that. Assume there are 2 pages A.XAML and B.xaml and you would want to detect in whether to load A.xaml or B.xaml based on checking some login credentials which is stored in the IsolatedStorage,

in App.xaml.cs of your project overwrite public App() with:

    public App()
    {
        // Global handler for uncaught exceptions. 
        UnhandledException += Application_UnhandledException;

        // Standard Silverlight initialization
        InitializeComponent();

        // Phone-specific initialization
        InitializePhoneApplication();

        // Show graphics profiling information while debugging.
        if (System.Diagnostics.Debugger.IsAttached)
        {
            // Display the current frame rate counters.
            Application.Current.Host.Settings.EnableFrameRateCounter = true;

            // Show the areas of the app that are being redrawn in each frame.
            //Application.Current.Host.Settings.EnableRedrawRegions = true;

            // Enable non-production analysis visualization mode, 
            // which shows areas of a page that are handed off to GPU with a colored overlay.
            //Application.Current.Host.Settings.EnableCacheVisualization = true;

            // Disable the application idle detection by setting the UserIdleDetectionMode property of the
            // application's PhoneApplicationService object to Disabled.
            // Caution:- Use this under debug mode only. Application that disables user idle detection will continue to run
            // and consume battery power when the user is not using the phone.
            PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Disabled;
        }

        RootFrame.Navigating += new NavigatingCancelEventHandler(RootFrame_Navigating);
    }

    void RootFrame_Navigating(object sender, NavigatingCancelEventArgs e)
    {
        if (e.Uri.ToString().Contains("/MainPage.xaml") != true)
        {
            return;
        }
        e.Cancel = true;
        RootFrame.Dispatcher.BeginInvoke(delegate
        {
            if (System.IO.IsolatedStorage.IsolatedStorageSettings.ApplicationSettings.Contains("Login_Credentials"))
            {
                RootFrame.Navigate(new Uri("/B.xaml", UriKind.Relative));
            }
            else
            {
                RootFrame.Navigate(new Uri("/A.xaml", UriKind.Relative));
            }
        });
    }

Then create the 2 dummy pages A.xaml and B.xaml so that for A.xaml you have some logic for saving the login credentials (in this case just a boolean flag):

A.XAML:

<Grid x:Name="LayoutRoot" Background="Transparent">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>

    <!--TitlePanel contains the name of the application and page title-->
    <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">
        <TextBlock x:Name="ApplicationTitle" Text="MY APPLICATION" Style="{StaticResource PhoneTextNormalStyle}"/>
        <TextBlock x:Name="PageTitle" Text="A Page" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
    </StackPanel>

    <!--ContentPanel - place additional content here-->
    <StackPanel x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
        <Button Content="Save Login Creds" Click="SaveLoginCreds"/>
    </StackPanel>
</Grid>

A.XAML.cs:

private void SaveLoginCreds(object sender, RoutedEventArgs e)
{
    System.IO.IsolatedStorage.IsolatedStorageSettings.ApplicationSettings.Add("Login_Credentials", true);
}

Now, when you run the application for the first time, it will load A.xaml because it could not find any login credentials. Then if you click on the button, it will save the login credentials data in IsolatedStorage. Next time, you start the app it will load B.xaml because it detected the login credentials.

I hope this helps.

于 2012-09-14T16:10:55.900 回答