.net - WPF说，数据项不为空时为空

Question

我有一个与WPF和MVVM一起使用的ViewModel类:

public class ViewModel {

    /* Other members here... */

    public ReadOnlyObservableCollection<BackplaneViewModel> Backplanes {
        get { return _Backplanes; }
    }

    public BackplaneViewModel CurrentBackplane {
        get { 
            var cb = _CurrentBackplane ?? (_CurrentBackplane = Backplanes.First()); 
            return cb;
        }
        set { 
            if (_CurrentBackplane == value) return;
            _CurrentBackplane = value; 
            RaisePropertyChanged("CurrentBackplane");
        }
    }
}

_Backplanes集合是在构造函数中创建和填充的，并且永不更改。

我有一个控件使用此 ViewModel的实例作为其 DataContext。用户可以使用 CurrentBackplane选择 ComboBox:

<ComboBox ItemsSource="{Binding Backplanes}"
          SelectedItem="{Binding CurrentBackplane}"
          DisplayMemberPath="BackplaneIndex" />

CurrentBackplane也可以在代码中更改。

我在 get的 CurrentBackplane上设置了一个断点。 cb变量是 而不是null 。但是，在 WPF请求其值之后，我立即在 输出窗口中得到以下内容:

System.Windows.Data Information: 40 : BindingExpression path error: 'BackplaneIndex' property not found for 'object' because data item is null.  This could happen because the data provider has not produced any data yet. BindingExpression:Path=BackplaneIndex; DataItem=null; target element is 'ComboBox' (Name=''); target property is 'NoTarget' (type 'Object')
System.Windows.Data Information: 19 : BindingExpression cannot retrieve value due to missing information. BindingExpression:Path=BackplaneIndex; DataItem=null; target element is 'ComboBox' (Name=''); target property is 'NoTarget' (type 'Object')
System.Windows.Data Information: 20 : BindingExpression cannot retrieve value from null data item. This could happen when binding is detached or when binding to a Nullable type that has no value. BindingExpression:Path=BackplaneIndex; DataItem=null; target element is 'ComboBox' (Name=''); target property is 'NoTarget' (type 'Object')
System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=BackplaneIndex; DataItem=null; target element is 'ComboBox' (Name=''); target property is 'NoTarget' (type 'Object')

为什么 WPF告诉我数据项为空？

我不确定自己在做什么错。该程序实际上运行良好，但是我试图找出可能与该问题有关的内存泄漏。

Answer 1

在构造函数中将集合的值设置为第一个值并将 setter/getter 代码更改为仅返回_CurrentBackplane之后，是否尝试过设置_CurrentBackplane？

_CurrentBackplane = Backplanes.First();  

和

public BackplaneViewModel CurrentBackplane {  
    get {   
        return _CurrentBackplane;  
    }  
    set { _CurrentBackplane = value; }  
}