看来我问得太早了，因为我自己找到了答案。
问题1的答案
当您有一个从 DataTemplate 动态创建的 UserControl 时，它与另一个对象（属于 ViewModel 或资源）相关联，该对象被定义为 UserControl 的 DataContext。但是，您无法在 UserControl 的构造函数中访问它，您必须等到引发“Loaded”事件：
public CustomUserControl()
{
InitializeComponent();
Console.WriteLine(this.DataContext.ToString());
// This doesn't work : DataContext is null
}
private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
Console.WriteLine(this.DataContext.ToString());
// or
Console.WriteLine((sender as UserControl).DataContext.ToString());
// this is Ok.
}
问题2的答案
这就是你如何获得一个 ViewModel 在父 UserControl.Resources 中实例化的 UserControl：
你不这样做。
相反，您在其父 ViewModel 中实例化其 ViewModel。完整示例：
MainWindow.xaml：
<Window x:Class="MainWindow"
...
xmlns:local="clr-namespace:my_project_namespace"
xmlns:cust="clr-namespace:CustomUserControl;assembly=CustomUserControl"
...>
<Window.Resources>
<DataTemplate DataType="{x:Type cust:CustomControlViewModel}">
<cust:CustomControlView>
</DataTemplate>
<!-- Here are listed all the types inheriting from CustomControlViewModel and CustomControlView.-->
<!-- CustomControlViewModel and CustomControlView are used as "abstract" classes-->
</Window.Resources>
<Window.DataContext>
<local:MainWindowViewModel>
</Window.DataContext>
<Grid>
<ItemsControl ItemsSource="{Binding Path=CustomVMList}"/>
</Grid>
</Window>
MainWindowViewModel.cs：
namespace my_project_namespace
{
public class MainWindowViewModel
{
public ObservableCollection<CustomControlViewModel> CustomVMList { get; set; }
public MainWindowViewModel()
{
CustomVMList = new ObservableCollection<CustomControlViewModel>();
// Fill in the list...
}
}
}
CustomControlView.xaml
<UserControl x:class="CustomUserControl.CustomControlView"
...
xmlns:my="clr-namespace:IndexPicker;assembly=IndexPicker"
...>
<UserControl.Resources>
<DataTemplate DataType="{x:Type my:IndexPickerViewModel}">
<my:IndexPickerView/>
</DataTemplate>
</UserControl.Resources>
<StackPanel Orientation="Horizontal">
<Label Content="{Binding Name}/>
<ContentControl Content="{Binding Path=MyIndexPicker}"/>
</Grid>
</UserControl>
这就是有趣的地方：
CustomControlViewModel.cs：
namespace CustomUserControl
{
public class CustomControlViewModel : INotifyPropertyChanged
{
public IndexPickerViewModel MyIndexPicker{ get; set; }
public string Name { get ; set; }
public int Id
{
get
{
return MyIndexPicker.Index;
}
set
{
if (value != MyIndexPicker.Index)
{
MyIndexPicker.Index = value;
NotifyPropertyChanged("Id");
}
}
}
public CustomControlViewModel(string _name)
{
Name = _name;
MyIndexPicker = new IndexPickerViewModel();
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName);
}
}
}
IndexPickerView.xaml：
<UserControl x:Class="IndexPicker.IndexPickerView"
...
...>
<Grid>
<Combobox ItemsSource="{Binding Path=MyTable}"
DisplayMemberPath="ColumnXYZ"
SelectedItem={Binding Path=SelectedRow}/>
</Grid>
</UserControl>
终于
IndexPickerViewModel.cs：
namespace IndexPicker
{
public class IndexPickerViewModel : INotifyPropertyChanged
{
private DataAccess data;
public DataView MyTable { get; set; }
private DataRowView selectedRow;
public DataRowView SelectedRow
{
get { return selectedRow; }
set
{
selectedRow = value;
NotifyPropertyChanged("SelectedRow");
}
}
public int? Index
{
get
{
if (SelectedRow != null) return (int?)selectedRow.Row["Column_Id"];
else return null;
}
set
{
SelectedRow = MyTable[MyTable.Find((int)value)];
NotifyPropertyChanged("Index");
}
}
public IndexPickerViewModel()
{
data = new DataAccess();
MyTable = data.GetTableView("tableName");
MyTable.Sort = "Column_Id";
}
// And don't forget INotifyPropertyChanged implementation
}
}
此配置与几个不同的 UserControls 一起使用，这些 UserControls 继承自 CustomControlView，它们的 ViewModel 继承自 CustomControlViewModel。它们是动态创建的，并列在 CustomVMList 中。这里包含 IndexPicker 的 CustomControlViewModel 已经是一个特化了。
具体使用：CRUD数据库Tables的Generic Dialog，可以根据每个Table Columns动态创建UserControl。此处显示的特化用于包含外键的列的情况。
我希望它清楚。
上面列出的代码可能包含错误。欢迎批评和评论。