层叠两个组合框 - 儿童组合框不选择当前值

Question

问题总结

当使用2个级联组合框，它是从哪里来的孩子组合框不选择当前项目，而不是一个空项（我不知道从！）被选中。

问题详情

我在XAML中声明了两个组合框下面给出。在剧中的两个实体是StandardRack和RelayConfig

XAML：

<ComboBox ItemsSource="{Binding StandardRacks}" DisplayMemberPath="Name" 
      SelectedItem="{Binding StandardRack, Mode=TwoWay}" SelectedValuePath="Id" 

<ComboBox ItemsSource="{Binding RelayConfigs}" DisplayMemberPath="DisplayName" 
      SelectedValue="{Binding DefaultRelayConfig, Mode=TwoWay}" SelectedValuePath="Id" 

这里有后盾性能和代码加载组合框在视图模型

视图模型

private ObservableCollection<StandardRack> _standardRacks; 
public ObservableCollection<StandardRack> StandardRacks { 
    get { return _standardRacks; } 
    set { _standardRacks = value; RaisePropertyChanged(() => StandardRacks); } 
} 

private StandardRack _standardRack; 
public StandardRack StandardRack { 
    get { return _standardRack; } 
    set { 
     if (_standardRack != value) { 
      _standardRack = value; 
      LoadRelayConfigs(); 
      RaisePropertyChanged(() => StandardRack); 
     } 
    } 
} 

private ObservableCollection<RelayConfig> _relayConfigs; 
public ObservableCollection<RelayConfig> RelayConfigs { 
    get { return _relayConfigs; } 
    set { _relayConfigs = value; RaisePropertyChanged(() => RelayConfigs); } 
} 

private RelayConfig _defaultRelayConfig; 
public RelayConfig DefaultRelayConfig { 
    get { return _defaultRelayConfig; } 
    set { _defaultRelayConfig = value; RaisePropertyChanged(() => DefaultRelayConfig); } 
} 

private void LoadRack() { 
    StandardRacks = new ObservableCollection<StandardRack>(
        unitOfWork.StandardRackRepository.GetQueryable().Include(sr => sr.StandardRelay).ToList()); 
    if (StandardRacks.Count > 0) { 
     StandardRack = Rack.StandardRack; //Set the current value of StandardRacks combobox 
    } 
} 

//Loads RelayConfigs Combobox based on Current Value of StandardRacks Combobox 

private void LoadRelayConfigs() { 
    RelayConfigs = new ObservableCollection<RelayConfig>(
     unitOfWork.RelayConfigRepository.GetQueryable() 
     .Where(rc => rc.StandardRelays.Any(srl => srl.Id == StandardRack.StandardRelay.Id)).ToList()); 
    DefaultRelayConfig = Rack.DefaultRelayConfig; //Set Current Value of RelayConfigs Combobox. Does not work. 
} 

上面的代码正确加载了组合框（StandardRacks和RelayConfigs）项。但是，RelayConfigs选择的值未设置为从XAML指向的值。相反，我在RelayConfigs Combobox中获得一个空项目作为当前项目。

Answer 1

'RelayConfigs'选择的'DefaultRelayConfig'部分是？