Question

我有一个MainWindow，其中的Stackpanel充满了按钮。我单击一个按钮，然后在Frame（Frame1）中填充Page（page1）。

Page1有自己的堆栈面板，其中包含按钮列表。

我在Frame1的右边也有Frame2。 Frame1和Frame 2都位于MainWindow的xaml中。

我可以单击o Mainwindow按钮，然后将页面加载到框架中。效果很好-无论我加载Frame1还是Frame2。

我需要做的是用PAge1加载Frame1（这很好），然后当我在Page1中选择一个按钮时，我希望Page2加载到Frame2中。这就是我的问题所在。

我遇到的问题是，当我尝试在frame2中加载page2时，它表示存在空异常。

我是否正确设计了这个-为什么我无法从page1代码中看到对frame2的引用-我必须添加一个Public Property CentralFrame As Frame以便可以看到它，但仍然会收到null异常。

我已经搜索过但找不到任何人进行设置-我确信我缺少一些简单的东西，因为这对我来说似乎是一个标准的UI设置。

MainWindow XAML在这里：

<Window x:Class="MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:MPS_Documentation"
        mc:Ignorable="d"
        Title="MainWindow" Height="600" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="auto"/>
            <RowDefinition Height="auto"/>
        </Grid.RowDefinitions>
        <DockPanel>
            <TextBlock DockPanel.Dock="Top" Grid.Row="0" Text="Managed Print Services Documentation" FontWeight="Bold" FontFamily="HP Simplified" FontSize="20">
            </TextBlock>
            <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                <Button Content="General" Margin="5,0,0,0" Click="btnGeneralClick"/>
                <Button Content="Copy/Print" Margin="5,0,0,0" Click="btnCopyPrintClick"/>
                <Button Content="Scan/Digital Send" Margin="5,0,0,0"/>
                <Button Content="Fax" Margin="5,0,0,0"/>
                <Button Content="Supplies" Margin="5,0,0,0"/>
                <Button Content="Toubleshooting" Margin="5,0,0,0"/>
                <Button Content="Security" Margin="5,0,0,0"/>
                <Button Content="HP Web Services" Margin="5,0,0,0"/>
                <Button Content="Networking" Margin="5,0,0,0"/>
            </StackPanel>
            <Frame x:Name="LeftFrame" DockPanel.Dock="Left" NavigationUIVisibility="Hidden" Width="160"/>
            <Frame x:Name="CentralFrame" NavigationUIVisibility="Hidden" />

        </DockPanel>

    </Grid>
</Window>

在单击按钮时打开左框架的代码

Private Sub btnGeneralClick(sender As Object, e As RoutedEventArgs)
        LeftFrame.Content = New LGeneralPage()
        'CentralFrame.Content = New CGeneralPage()

    End Sub

请注意，如果我使用第二行代码，它将在第二帧中打开一个页面

在第1帧中加载的PAge 1的XAML

<Page x:Class="LGeneralPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:MPS_Documentation"
      mc:Ignorable="d" 
      d:DesignHeight="450" d:DesignWidth="160"
      Title="LGeneralPage">
    <Grid>
        <StackPanel Orientation="Vertical">
            <Button Content="Quick Sets" Margin="5" Click="btnQuickSetsPageClick" />
            <Button Content="Alerts" Margin="5"/>
            <Button Content="Control Panel Settings App" Margin="5"/>
            <Button Content="General Settings" Margin="5"/>
            <Button Content="AutoSend" Margin="5"/>
            <Button Content="Edit Other Links" Margin="5"/>
        </StackPanel>
    </Grid>
</Page>

在第1页上单击按钮的代码，应在第2帧中打开第2页

Class LGeneralPage
    Public Property CentralFrame As Frame

    Private Sub btnQuickSetsPageClick(sender As Object, e As RoutedEventArgs)
        CentralFrame.Content = New CGeneralPage()
    End Sub


End Class

我必须添加公共属性才能引用frame2（CentralFrame）

当我单击此按钮时，出现以下错误

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=MPS Documentation
  StackTrace:
   at MPS_Documentation.LGeneralPage.btnQuickSetsPageClick(Object sender, RoutedEventArgs e) in C:\Users\u492748\source\repos\MPS Documentation\MPS Documentation\LGeneralPage.xaml.vb:line 5
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
   at System.Windows.Controls.Primitives.ButtonBase.OnClick()
   at System.Windows.Controls.Button.OnClick()
   at System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
   at System.Windows.UIElement.OnMouseLeftButtonUpThunk(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
   at System.Windows.UIElement.OnMouseUpThunk(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
   at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
   at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
   at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Application.RunDispatcher(Object ignore)
   at System.Windows.Application.RunInternal(Window window)
   at System.Windows.Application.Run(Window window)
   at System.Windows.Application.Run()
   at MPS_Documentation.Application.Main()

Answer 1

我必须承认我什至不了解视觉基础，我是C＃程序员，但我想我已经解决了。

我给CentralFrame引用了Page1的“标记”属性，并在需要时从Page1的“ Tag”属性中引用了CentralFrame。

MainWindow

Class MainWindow
    Private Sub btnGeneralClick(sender As Object, e As RoutedEventArgs)
        Dim Page1 As Page1 = New Page1()
        Page1.Tag = CentralFrame
        LeftFrame.Content = Page1
    End Sub
End Class

第1页

XAML

<Page x:Class="Page1"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:WpfApp21"
      mc:Ignorable="d" 
      d:DesignHeight="450" d:DesignWidth="800"
      Title="Page1">
    <Grid Background="Pink">
        <TextBlock Text="1" VerticalAlignment="Top" HorizontalAlignment="Left" FontSize="50"/>
        <Button Name="btnPage1" Width="100" Height="25" Content="Click" />
    </Grid>
</Page>

VB

Class Page1
    Private Sub BtnPage1_Click(sender As Object, e As RoutedEventArgs) Handles btnPage1.Click
        Dim CentralFrame As Frame = Me.Tag
        CentralFrame.Content = New Page2()
    End Sub
End Class

第2页

XAML

<Page x:Class="Page2"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:WpfApp21"
      mc:Ignorable="d" 
      d:DesignHeight="450" d:DesignWidth="800"
      Title="Page2">
    <Grid Background="Aqua">
        <TextBlock Text="2" VerticalAlignment="Center" HorizontalAlignment="Center" FontSize="50"/>
    </Grid>
</Page>

我的项目上的GIF（MainWindow是您的副本）

您怎么了？ 您将变量声明为：

Public Property CentralFrame As Frame

但是它为空，它没有保存CentralFrame的引用。我想这就是为什么您会得到NullReferenceException的原因。

注意：标记属性是一个“对象”。您可以根据需要设置此属性。

从单独的框架/页面中打开框架中的页面

1 个答案: