Question

当我尝试更新LongListSelector控件的ItemsSource属性时，会发生ArgumentNullReference异常。

XAML代码：

<phone:LongListSelector x:Name="LongListSelectorMain" Margin="-12,0,0,0" ItemsSource="{Binding Items}" LayoutMode="List">
            <phone:LongListSelector.DataContext>
                <local:MainViewModel/>
            </phone:LongListSelector.DataContext>
            <phone:LongListSelector.ItemTemplate>
                <DataTemplate>
                    <StackPanel Margin="0,0,0,30">
                        <TextBlock FontSize="25" Text="{Binding LineOne}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
                        <TextBlock FontSize="25" Text="{Binding LineTwo}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
                        <TextBlock FontSize="25" Text="{Binding LineThree}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
                    </StackPanel>
                </DataTemplate>
            </phone:LongListSelector.ItemTemplate>
        </phone:LongListSelector>

项目类

Public Class ItemViewModel
Implements INotifyPropertyChanged
Private _lineOne As String
''' <summary>
''' Proprietà ViewModel di esempio: questa proprietà viene utilizzata per visualizzare il relativo valore mediante un'associazione.
''' </summary>
''' <returns></returns>
Public Property LineOne() As String
    Get
        Return _lineOne
    End Get
    Set(ByVal value As String)
        If Not value.Equals(_lineOne) Then
            _lineOne = value
            NotifyPropertyChanged("LineOne")
        End If
    End Set
End Property

Private _lineTwo As String
''' <summary>
''' Proprietà ViewModel di esempio: questa proprietà viene utilizzata per visualizzare il relativo valore mediante un'associazione.
''' </summary>
''' <returns></returns>
Public Property LineTwo() As String
    Get
        Return _lineTwo
    End Get
    Set(ByVal value As String)
        If Not value.Equals(_lineTwo) Then
            _lineTwo = value
            NotifyPropertyChanged("LineTwo")
        End If
    End Set
End Property

Private _Skipass As Skipass
''' <summary>
''' Proprietà ViewModel di esempio: questa proprietà viene utilizzata per visualizzare il relativo valore mediante un'associazione.
''' </summary>
''' <returns></returns>
Public Property Skipass() As Skipass
    Get
        Return _Skipass
    End Get
    Set(ByVal value As Skipass)
        If Not value.Equals(_Skipass) Then
            _Skipass = value
            NotifyPropertyChanged("Skipass")
        End If
    End Set
End Property

Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
Private Sub NotifyPropertyChanged(ByVal propertyName As String)
    Dim handler As PropertyChangedEventHandler = PropertyChangedEvent
    If Nothing IsNot handler Then
        handler(Me, New PropertyChangedEventArgs(propertyName))
    End If
End Sub

结束班

的DataContext

Public Class MainViewModel
Implements INotifyPropertyChanged

Public Sub New()
    Me.Items = New ObservableCollection(Of ItemViewModel)()
End Sub

''' <summary>
''' Raccolta per oggetti ItemViewModel.
''' </summary>
Private _items As ObservableCollection(Of ItemViewModel)
Public Property Items() As ObservableCollection(Of ItemViewModel)
    Get
        Return _items
    End Get
    Private Set(ByVal value As ObservableCollection(Of ItemViewModel))
        _items = value
    End Set
End Property

Private _sampleProperty As String = "Sample Runtime Property Value"
''' <summary>
''' Proprietà ViewModel di esempio: questa proprietà viene utilizzata per visualizzare il relativo valore mediante un'associazione
''' </summary>
''' <returns></returns>
Public Property SampleProperty() As String
    Get
        Return _sampleProperty
    End Get
    Set(ByVal value As String)
        If Not value.Equals(_sampleProperty) Then
            _sampleProperty = value
            NotifyPropertyChanged("SampleProperty")
        End If
    End Set
End Property

Private _isDataLoaded As Boolean
Public Property IsDataLoaded() As Boolean
    Get
        Return _isDataLoaded
    End Get
    Private Set(ByVal value As Boolean)
        _isDataLoaded = value
    End Set
End Property

''' <summary>
''' Crea e aggiunge alcuni oggetti ItemViewModel nella raccolta di elementi.
''' </summary>
Public Sub LoadData()
    'Dati di esempio; sostituirli con dati reali
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime one", .LineTwo = "Maecenas praesent accumsan bibendum", .LineThree = "Facilisi faucibus habitant inceptos interdum lobortis nascetur pharetra placerat pulvinar sagittis senectus sociosqu"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime two", .LineTwo = "Dictumst eleifend facilisi faucibus", .LineThree = "Suscipit torquent ultrices vehicula volutpat maecenas praesent accumsan bibendum dictumst eleifend facilisi faucibus"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime three", .LineTwo = "Habitant inceptos interdum lobortis", .LineThree = "Habitant inceptos interdum lobortis nascetur pharetra placerat pulvinar sagittis senectus sociosqu suscipit torquent"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime four", .LineTwo = "Nascetur pharetra placerat pulvinar", .LineThree = "Ultrices vehicula volutpat maecenas praesent accumsan bibendum dictumst eleifend facilisi faucibus habitant inceptos"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime five", .LineTwo = "Maecenas praesent accumsan bibendum", .LineThree = "Maecenas praesent accumsan bibendum dictumst eleifend facilisi faucibus habitant inceptos interdum lobortis nascetur"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime six", .LineTwo = "Dictumst eleifend facilisi faucibus", .LineThree = "Pharetra placerat pulvinar sagittis senectus sociosqu suscipit torquent ultrices vehicula volutpat maecenas praesent"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime seven", .LineTwo = "Habitant inceptos interdum lobortis", .LineThree = "Accumsan bibendum dictumst eleifend facilisi faucibus habitant inceptos interdum lobortis nascetur pharetra placerat"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime eight", .LineTwo = "Nascetur pharetra placerat pulvinar", .LineThree = "Pulvinar sagittis senectus sociosqu suscipit torquent ultrices vehicula volutpat maecenas praesent accumsan bibendum"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime nine", .LineTwo = "Maecenas praesent accumsan bibendum", .LineThree = "Facilisi faucibus habitant inceptos interdum lobortis nascetur pharetra placerat pulvinar sagittis senectus sociosqu"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime ten", .LineTwo = "Dictumst eleifend facilisi faucibus", .LineThree = "Suscipit torquent ultrices vehicula volutpat maecenas praesent accumsan bibendum dictumst eleifend facilisi faucibus"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime eleven", .LineTwo = "Habitant inceptos interdum lobortis", .LineThree = "Habitant inceptos interdum lobortis nascetur pharetra placerat pulvinar sagittis senectus sociosqu suscipit torquent"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime twelve", .LineTwo = "Nascetur pharetra placerat pulvinar", .LineThree = "Ultrices vehicula volutpat maecenas praesent accumsan bibendum dictumst eleifend facilisi faucibus habitant inceptos"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime thirteen", .LineTwo = "Maecenas praesent accumsan bibendum", .LineThree = "Maecenas praesent accumsan bibendum dictumst eleifend facilisi faucibus habitant inceptos interdum lobortis nascetur"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime fourteen", .LineTwo = "Dictumst eleifend facilisi faucibus", .LineThree = "Pharetra placerat pulvinar sagittis senectus sociosqu suscipit torquent ultrices vehicula volutpat maecenas praesent"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime fifteen", .LineTwo = "Habitant inceptos interdum lobortis", .LineThree = "Accumsan bibendum dictumst eleifend facilisi faucibus habitant inceptos interdum lobortis nascetur pharetra placerat"})
    'Me.Items.Add(New ItemViewModel() With {.LineOne = "runtime sixteen", .LineTwo = "Nascetur pharetra placerat pulvinar", .LineThree = "Pulvinar sagittis senectus sociosqu suscipit torquent ultrices vehicula volutpat maecenas praesent accumsan bibendum"})

    Me.IsDataLoaded = True
End Sub

Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
Private Sub NotifyPropertyChanged(ByVal propertyName As String)
    Dim handler As PropertyChangedEventHandler = PropertyChangedEvent
    If handler IsNot Nothing Then
        handler(Me, New PropertyChangedEventArgs(propertyName))
    End If
End Sub

结束班

正如您所看到的，我刚刚使用了Microsoft提供的默认示例......但说实话，我无法弄清楚为什么我会得到NullReference。

Answer 1

您可以将XAML发布到您声明ItemsSource绑定的资源的位置吗？

这是一个猜测，因为你没有发布这个，但是如果你提供了一个Name和一个Key，那么资源没有初始化，这会导致你的null引用异常...

<UserControl.Resources>
    <viewModel:FeedsViewModel x:Key="FeedsViewModel" x:Name="FeedsViewModel"/>
</UserControl.Resources>

如果您只声明一个密钥，那么它会自动初始化...

<UserControl.Resources>
    <viewModel:FeedsViewModel x:Key="FeedsViewModel"/>
</UserControl.Resources>

但请记住，您必须通过用户控件的Resources字典引用它，而不是作为用户控件的属性。

在LongListSelector中绑定数据

1 个答案: