空值异常的专属非空

Question

获取有关非空

下面是一个ViewModel的开端，创建视图模型对象，并打开窗户，我的代码，该方法的属性一NullValueException我正在使用ViewModelt。在SwitchName属性上抛出异常。 _ciscoswitch.SwitchName由于在SwitchVewModel中的_ciscoswitch为空而变为空值。异常是在SwitchBrowser构造函数中的InitializeComponent（）处抛出。在调试器中查看SwitchVewModel实例，_ciscoswitch不为空。我尝试将SwitchName访问器更改为使用CiscoSwitch.switchName而不是_ciscoswitch.SwitchName，但仍然失败。

class SwitchViewModel : INotifyPropertyChanged 
{ 
     #region Construction 
    /// Constructs the default instance of a SwitchViewModel 
    public SwitchViewModel() 
    { 

    } 


    public SwitchViewModel(CiscoSwitch cs) 
    { 
     _ciscoswitch = cs; 
    } 
     #endregion 
    #region Members 

    CiscoSwitch _ciscoswitch; 
    #endregion 

    #region Properties 
    public CiscoSwitch CiscoSwitch 
    { 
     get 
     { 
      return _ciscoswitch; 
     } 
     set 
     { 
      _ciscoswitch = value; 
     } 
    } 

    public string SwitchName 
    { 
     get { return _ciscoswitch.switchName; } 
     set 
     { 
      if (_ciscoswitch.switchName != value) 
      { 
       _ciscoswitch.switchName = value; 
       RaisePropertyChanged("switchName"); 
      } 
     } 
    } 
    #endregion 

    #region INotifyPropertyChanged Members 

    public event PropertyChangedEventHandler PropertyChanged; 

    #endregion 

    #region Methods 

    private void RaisePropertyChanged(string propertyName) 
    { 
     // take a copy to prevent thread issues 
     PropertyChangedEventHandler handler = PropertyChanged; 
     if (handler != null) 
     { 
      handler(this, new PropertyChangedEventArgs(propertyName)); 
     } 
    } 
    #endregion 

} 
} 

XAML的SwitchBrowserWindow我使用的是现在唯一的财产就是试图得到这方面的工作

<Window x:Class="CiscoDecodeWPF.SwitchBrowser" 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:CiscoDecodeWPF="clr-namespace:CiscoDecodeWPF" IsEnabled="{Binding Enabled}" 
    Title="SwitchBrowser" Height="500" Width="500" Background="GhostWhite"> 

<Window.Resources> 
    <Style TargetType="{x:Type TreeViewItem}" x:Key="ModuleStyle"> 
     <Setter Property="Foreground" Value="Blue"/> 
     <Setter Property="FontSize" Value="12"/> 
    </Style> 

    <Style TargetType="{x:Type TreeViewItem}" x:Key="RedModuleStyle" BasedOn="{StaticResource ModuleStyle}"> 
     <Setter Property="Foreground" Value="Red"/> 
    </Style> 
</Window.Resources> 

<Window.DataContext> 
    <CiscoDecodeWPF:SwitchViewModel/> 
</Window.DataContext> 
<Grid Margin="0,0,-211.4,-168"> 
    <StackPanel HorizontalAlignment="Stretch" Name="StackPanel1" VerticalAlignment="Stretch" Width="Auto" Margin="0,0,188.6,114"> 

     <StackPanel.Resources> 
      <Style TargetType="{x:Type Label}" x:Key="LabelStyle"> 
       <Setter Property="Foreground" Value="Blue"/> 
       <Setter Property="FontSize" Value="12"/> 
       <Setter Property="FontWeight" Value="Bold"/> 
      </Style> 
     </StackPanel.Resources> 
     <Label Content="Switch Name:" Name="Label1" Height="25" HorizontalAlignment="Left"/> 
     <Label Content="Software Version:" Name="Label2" HorizontalAlignment="Left" /> 
     <Label Content="Model Number:" Name="Label3" HorizontalAlignment="left"/> 
     <Label Content="IP Address:" Name="Label4" HorizontalAlignment="left"></Label> 
     <Label Content="Serial Number:" Name="Label5" HorizontalAlignment="Left"></Label> 
     <Label Content="Show Tech Taken:" Name="Label6" HorizontalAlignment="left"/> 
    </StackPanel> 
    <StackPanel Margin="105,0,218,489"> 
     <StackPanel.Resources> 
      <Style TargetType="{x:Type Label}" x:Key="LabelStyle"> 
       <Setter Property="FontSize" Value="12"/> 
       <Setter Property="FontWeight" Value="Bold"/> 
      </Style> 
     </StackPanel.Resources> 
     <Label Content="{Binding Path=SwitchName}" Name="SwitchNameLabel" HorizontalAlignment="left"/> 
     <Label Content="Version" Name="VersionLabel" HorizontalAlignment="left"/> 
     <Label Content="Model" Name="ModelNumberLabel" HorizontalAlignment="Left"/> 
     <Label Content="IP" Name="IPAddressLabel" HorizontalAlignment="Left"/> 
     <Label Content="Serial" Name="SerialLabel" HorizontalAlignment="Left"/> 
      <Label Content="ST" Name="ShowTechLabel" HorizontalAlignment="Left"/> 

    </StackPanel> 

异常，堆栈跟踪和调用堆栈

System.NullReferenceException was unhandled by user code 
    HResult=-2147467261 
Message=Object reference not set to an instance of an object. 
    Source=CiscoDecodeWPF 
StackTrace: 
    at CiscoDecodeWPF.SwitchViewModel.get_SwitchName() in 

d:\Projects\CiscoDecodeWPF\CiscoDecodeWPF\SwitchViewModel.cs:line 50 

的InnerException的交换机名称：

思科DecodeWPF.exe！CiscoDecodeWPF.SwitchViewModel.SwitchName.get（）线50 + 0xf字节C＃ [外部代码] CiscoDecodeWPF.exe！CiscoDecodeWPF.SwitchBrowser.SwitchBrowser（CiscoDecodeWPF.CiscoSwitch CS）线35 + 0x8中字节C＃ CiscoDecodeWPF.exe ！CiscoDecodeWPF.MainWindow.BrowseSwitchMenuItem_Click（对象发件人，System.Windows.RoutedEventArgs E）线1050个+ 0x34字节C＃ [外部代码]

Answer 1

尝试以下代码SwitchName：

public string SwitchName 
{ 
    get { 
     if (_ciscoswitch != null) 
     { 
      return _ciscoswitch.switchName; 
     } 
     else 
     { 
      return string.empty; 
     } 
     } 
    set 
    { 
     if (_ciscoswitch != null) 
     { 
     if (_ciscoswitch.switchName != value) 
     { 
      _ciscoswitch.switchName = value; 
      RaisePropertyChanged("switchName"); 
     } 
     } 
    } 
} 

如果你不我不想检查_ciscoswitch != null在SwitchName然后把DataContext = svm InitizlizeComponent（）

代码之前：

public SwitchBrowser(CiscoSwitch cs) 
{ 
    SwitchViewModel svm = new SwitchViewModel(cs); 
    DataContext = svm; 
    InitializeComponent(); 
} 

而且从XAML删除下面的代码。

<Window.DataContext> 
<CiscoDecodeWPF:SwitchViewModel/> 
</Window.DataContext> 

希望它应该工作。