Pass view to viewmodel with datatemplate

Question

I have a window named ParameterEditorView with a ParameterEditorViewModel as DataContext. In the ParameterEditorViewModel I have a list of ParameterViewModel. In the ParameterEditorView I have an ItemsControl whose ItemsSource is binded to the list of ParameterViewModel in the ParameterEditorViewModel. I need the ParameterViewModel to have a reference to the ParameterView (more on that later). In the Resources section of the ParameterEditorView I add the DataTemplate:

<DataTemplate DataType="{x:Type my:ParameterViewModel}" > <my:ParameterView HorizontalAlignment="Left"/> </DataTemplate>

So, how can I pass a reference of the ParameterView that is created to show the ParameterViewModel to it?

The reason I need the ParameterView in the ParameterViewModel is the following: I have a TextBox whose Text property is binded to the PropertyModelView.Name property. But I want to display a default string when the Name is empty or Null. I've tried to set the property value to the default string I want when that happens but the TextBox.Text is not set in this scenario. I do something like this:

private string _name; public string Name { get { return _name; } set { if (value == null || value.Length == 0) Name = _defaultName; else _name = value; } }

I've also tried to specifically set the TextBox.Text binding mode to TwoWay without success. I think this is a defense mechanism to prevent an infinite loop from happening but I don't know for sure. Any help on this front would also be highly appreciated.

Thanks, José Tavares

score 1 · Accepted Answer · 2010-05-17 17:36:04Z

{Binding } has a FallbackValue, btw.

Your question, it confuses me. I'd assume your PVM has a collection of PV's as a public property, which is bound within the UI. Also, I think you're mixing terms. Its Model-View-ViewModel where the ViewModel is the DataContext of the View, and the Model is exposed by the ViewModel via a public property. Sounds like if you're binding the window to a collection of ViewModels they are actually Models. It may seem pedantic, but getting your terms correct will help you research and ask questions.

Another solution would be to add a Converter to your Binding in combination with FallbackValue (I've had to do this, IIRC). That converter would be an IValueConverter that returns "DependencyProperty.UnsetValue" if the string is null or empty. I think this works sometimes because the TextBox will set the bound property to the empty string rather than null if the TB is empty. Here's a little sample to whet your whistle (not guaranteed to work; you need to debug this and tweak it):

public class ThisMightWorkConverter : IValueConverter { public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) { var temp = value as string; if(string.IsNullOrWhiteSpace(temp)) return System.Windows.DependencyProperty.UnsetValue; return temp; } public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) { return value; // you might need to change this } }

Well I may have not express myself clearly. I have a window named ParameterEditorView and a ParameterEditorViewModel. In the ParameterEditorView I have a list of ParameterViewModel. In the ParameterEditorView I have a ItemsControl whose ItemsSource is binded to the list of ParameterViewModel in the ParameterEditorViewModel. I also define the DataTemplate shown above in the ParameterEditorView. The binding is working fine, but I want to know if I can pass the reference of the ParameterView, that is automatically created to display a ParameterViewModel, to the ParameterViewModel.
@jps assuming ParameterView is a UserControl. If you're trying to pass a chunk of UI to the ViewModel or a Model you're doing something wrong. I think it would be to your benefit to state your problem and ask for solutions rather than go down the road you're on. FallbackValue is one option. I edited to add a second option.
I guess this second option is good option to overcome the problem. Thanks for the help.

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