的NullReferenceException而組合框結合

Question

我有一個下拉框：

<ComboBox Height="23" Name="DriveSelection" Width="120" 
           ItemsSource="{Binding Path=FixedDrives}" 
           DisplayMemberPath="Name" 
           SelectedItem="{Binding Path=DriveSelection_SelectionChanged}" 
           IsSynchronizedWithCurrentItem="True" 
           IsEnabled="{Binding DriveIsEnabled}" 
           SelectedValue="{Binding DriveSelected}" 
           /> 

與此綁定：

private ObservableCollection<DriveInfo> fixedDrives; 
public ObservableCollection<DriveInfo> FixedDrives 
{ 
    get 
    { 
     if (this.fixedDrives != null) 
      return this.fixedDrives; 
     this.fixedDrives = new ObservableCollection<DriveInfo>(Enumerable.Where<DriveInfo>((IEnumerable<DriveInfo>)DriveInfo.GetDrives(), (Func<DriveInfo, bool>)(driveInfo => driveInfo.DriveType == DriveType.Fixed))); 
     return this.fixedDrives; 
    } 
} 


public DriveInfo DriveSelection_SelectionChanged 
{ 
    get 
    { 
     return this.driveSelection; 
    } 
    set 
    { 
     if (value == this.driveSelection) 
      return; 
     this.driveSelection = value; 
     UpdatePathManager(); 
     this.OnPropertyChanged("DriveSelection_SelectionChanged"); 
    } 
} 
public object DriveSelected 
{ 
    get 
    { 
     return _driveSelected; 
    } 
    set 
    { 
     _driveSelected = value; 
     RaisePropertyChanged("DriveSelected"); 
    } 
} 

，並在做頁面初始化：最後一行

public PathSelectionPageViewModel(PathSelectionPage _page) 
{ 
    this.page = _page; 
    this.root = Path.GetPathRoot(App.Instance.PathManager.InstallRoot).ToUpperInvariant(); 
    this.DriveSelected = (object)this.root; 
    //this.page.DriveSelection.SelectedValue = (object)this.root; 
    this.DriveIsEnabled = true 
    //this.page.DriveSelection.IsEnabled = true 
    this.driveSelection = new DriveInfo(this.root); 
} 

： this.driveSelection = new DriveInfo(this.root);我在這一行創建空引用異常：

private void UpdatePathManager() 
{ 
    string newRoot = this.driveSelection.ToString(); <--- this line 
    //string newRoot = this.page.DriveSelection.SelectedValue.ToString(); 
} 

正如你所看到的，我只是試圖改變直接從視圖讀取數據到綁定，但我有這個問題。 爲了解決這個問題需要改變什麼？

@Update 正如我剛剛發現的： 問題在處理綁定期間。 WPF正在處理的綁定的順序 - >

1. FixedDrives
2. 選擇改變
3. DriveIsEnabled
4. DriveSelected

和處理DriveSelected與值= NULL燒製`DriveSelection_SelectionChanged」和這是造成問題的原因。

Answer 1

真正的問題是，new DriveInfo(this.root)您使用此代碼

this.driveSelection = new DriveInfo(this.root); 

分配是不是你FixedDevices的一部分採集。這導致null被WPF綁定傳遞給你的屬性。

後，辦理入住手續

if (value == this.driveSelection) 

在屬性DriveSelection_SelectionChanged結果false，因爲你已經分配new DriveInfo(this.root)給變量driveSelection。

發生故障的檢查導致該driveSelection被設置爲空，然後拋出NullReferenceException在UpdatePathManager()

Answer 2

看起來像可能DriveIsEnabled setter（未包含在您的代碼中）正在調用UpdatePathManger（）。 您應該確保this.driveSelection永遠不能爲null，通過改變構造函數：這裏

public PathSelectionPageViewModel(PathSelectionPage _page) 
    { 
     this.page = _page; 
     this.root = Path.GetPathRoot(App.Instance.PathManager.InstallRoot).ToUpperInvariant(); 
     this.driveSelection = new DriveInfo(this.root); 
     this.DriveSelected = (object)this.root; 
     //this.page.DriveSelection.SelectedValue = (object)this.root; 
     this.DriveIsEnabled = true 
     //this.page.DriveSelection.IsEnabled = true 
    }