Binding default value to visibility if default value is null in ViewModel

Question

The first time loading a window I want a textbox to be collapsed while keeping a binding to a property (SomeProp) in my Viewmodel.

Unfortunatly I am not able to do this.

what I have tried:

Attempt 1: I have tried to explicity set the value to null in the constructor of the viewmodel and also explicitely call the Onpropertychanged. The Convertor is not triggered.

Attempt 2: In the code behind I've set the default visibility to visibility.Collapsed. this seems to have a side-effect that the Visibility is no longer bound to the SomeProp property.

Attempt 3: After googling I found something about PriorityBinding, but this seems to only work on the TEXT property of the textbox Use a default value when binding cannot be evaluated because of a null value

Thank you in advance,

Extra Info:

I have a textbox and it should only be visible if a property (SomeProp) in my ViewModel has a value 'Other'. I am successfully using a Convertor for this. This means whenever I change the value of SomeProp the textbox becomes visible/invisible depending on the value of SomeProp. I have used the following code: WPF: Binding Visibility by string contents

Does anyone know how I can set te textbox Visibility to Collapsed while keeping the Binding to a property SomeProp.

XAML

        <TextBox Name="txtbox" Visibility="{Binding SomeProp.Description, Converter={StaticResource StringOtherToVisibilityConverter}}" TextWrapping="Wrap" Height="150" MaxLength="2000"
                 Text="{Binding SomeProp2.Text, Mode=TwoWay}"
             ScrollViewer.VerticalScrollBarVisibility="Auto" AcceptsReturn="True">
        </TextBox>

Convertor

class StringOtherToVisibilityConverter : System.Windows.Markup.MarkupExtension, IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{

    if (value != null)
    {
        if (value.ToString() == "Other")
        {
            return Visibility.Visible;
        }
        else
        {
            return Visibility.Collapsed;
        }


    }
    else
    {
        return Visibility.Collapsed;
    }

}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    return null; //DependencyProperty.UnsetValue;
}

public override object ProvideValue(IServiceProvider serviceProvider)
{
    return this;
}


}

Answer 1

Setting TargetNullValue={x:Static Visibility.Collapsed} should collapse the binding target when the source value is null.

You can also set FallbackValue, which is applied when the binding or conversion fails. Such failures may include the NullReferenceException that would occur when binding to X.Y if X is null.