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

Answer 1

{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
    }
}