Windows窗體數據綁定和屬性路徑：如何處理可空性？

Question

我試圖在Windows窗體項目中使用MVVM模式實現master/detail場景（我瘋了，我知道）。請看下面的視圖模型：

public class MasterViewModel 
{ 
    public BindingList<DetailViewModel> Details { get; set; } 

    public DetailViewModel SelectedDetail 
    { 
     get 
     { 
      // 
     } 
     set 
     { 
      // raises SelectedDetailChanged 
     } 
    } 
} 

public class DetailViewModel 
{ 
    public string SubProperty 
    { 
     get 
     { 
      // ... 
     } 
     set 
     { 
      // ... raises SubPropertyChanged 
     } 
    } 
} 

我試圖DetailViewModel的子屬性綁定到一個文本框，使用下面的代碼（和屬性路徑，Windows支持的窗體數據綁定）：

  MasterViewModel masterViewModel; 
      TextBox textBox; 
      // ... 

      Binding binding = new Binding("Text", masterViewModel, "SelectedDetail.SubProperty"); 
      binding.FormattingEnabled = true; 
      binding.DataSourceUpdateMode = DataSourceUpdateMode.OnPropertyChanged; 
      binding.ControlUpdateMode = ControlUpdateMode.OnPropertyChanged; 

      textBox.DataBindings.Add(binding); 

它（如WPF !!!）...直到SelectedDetail爲null（對於我的邏輯，SelectedDetail null表示在主視圖中沒有選擇任何東西）。如果SelectedDetail爲null，那麼我得到一個ArgumentNullException（參數名稱：component）。

有沒有辦法處理「父」屬性（在導航路徑內）的可空性？

這裏是異常堆棧跟蹤：

in System.ComponentModel.ReflectPropertyDescriptor.AddValueChanged(Object component, EventHandler handler) 
    in System.Windows.Forms.BindToObject.CheckBinding() 
    in System.Windows.Forms.BindToObject.SetBindingManagerBase(BindingManagerBase lManager) 
    in System.Windows.Forms.Binding.SetListManager(BindingManagerBase bindingManagerBase) 
    in System.Windows.Forms.ListManagerBindingsCollection.AddCore(Binding dataBinding) 
    in System.Windows.Forms.BindingsCollection.Add(Binding binding) 
    in System.Windows.Forms.BindingContext.UpdateBinding(BindingContext newBindingContext, Binding binding) 
    in System.Windows.Forms.Control.UpdateBindings() 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnParentBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnParentBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnParentBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnParentBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnParentBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.set_BindingContextInternal(BindingContext value) 
    in System.Windows.Forms.ContainerControl.set_BindingContext(BindingContext value) 
    in System.Windows.Forms.ContainerControl.get_BindingContext() 
    in System.Windows.Forms.Control.get_BindingContextInternal() 
    in System.Windows.Forms.ContainerControl.get_BindingContext() 
    in System.Windows.Forms.Control.get_BindingContextInternal() 
    in System.Windows.Forms.ContainerControl.get_BindingContext() 
    in System.Windows.Forms.Control.get_BindingContextInternal() 
    in System.Windows.Forms.ContainerControl.get_BindingContext() 
    in System.Windows.Forms.Control.get_BindingContextInternal() 
    in System.Windows.Forms.Control.get_BindingContext() 
    in System.Windows.Forms.Control.get_BindingContextInternal() 
    in System.Windows.Forms.Control.get_BindingContext() 
    in System.Windows.Forms.Control.UpdateBindings() 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.DataGridView.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnParentBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnParentBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.Control.OnBindingContextChanged(EventArgs e) 
    in System.Windows.Forms.ContainerControl.OnCreateControl() 
    in System.Windows.Forms.UserControl.OnCreateControl() 
    in System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible) 
    in System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible) 
    in System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible) 
    in System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible) 
    in System.Windows.Forms.Control.CreateControl() 
    in System.Windows.Forms.Control.WmShowWindow(Message& m) 
    in System.Windows.Forms.Control.WndProc(Message& m) 
    in System.Windows.Forms.ScrollableControl.WndProc(Message& m) 
    in System.Windows.Forms.ContainerControl.WndProc(Message& m) 
    in System.Windows.Forms.Form.WmShowWindow(Message& m) 
    in System.Windows.Forms.Form.WndProc(Message& m) 
... 
    in System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) 
    in System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) 
    in System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) 

Answer 1

您需要BindingSource提供一個間接層。

bindingSource1 = new BindingSource(components); 
bindingSource1.DataMember = "Details"; 
bindingSource1.DataSource = typeof(MasterViewModel); 

Binding binding = new Binding("Text", bindingSource1, "SubProperty"); 
binding.FormattingEnabled = true; 
binding.DataSourceUpdateMode = DataSourceUpdateMode.OnPropertyChanged; 
binding.ControlUpdateMode = ControlUpdateMode.OnPropertyChanged; 

textBox.DataBindings.Add(binding); 

更詳細的例子：BindingSource and BindingNavigator in C# 2.0

Answer 2

你可能想嘗試像這樣代替：

Binding binding = 
    new Binding("Text", masterViewModel.SelectedDetail, "SubProperty");