How do I get a binding to an item in a list to update?

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XAML

WPF

data-binding

I have a Windows WPF application, following a fairly standard MVVM architecture. I am having trouble getting a list of items to respond to changes in the properties they are bound to. Here are some key components that are involved in my problem:

public class Contact
{
	public string Name
	{
		get
		{
			return _name;
		}
		set
		{
			SetProperty( ref _name, value );
			OnPropertyChanged( nameof(IsNotPlaceholder) );
		}
	}

	public string Telephone
	{
		get
		{
			return _telephone;
		}
		set
		{
			SetProperty( ref _telephone, value );
		}
	}

	public bool IsNotPlaceholder { get { return _name.Length > 0; } }

	private string _name = String.Empty;
	private string _telephone = String.Empty;
}


public class CustomerViewModel : INotifyPropertyChanged
{
	public ObservableCollection<Contact> Contacts { get { return SelectedCustomer.Contacts } }
}

So far, so good. Now, in the view, I want to display a list of contacts, and I want the Name field to be editable always but I want the Telephone field to become editable only when the contact record is not a placeholder. (The placeholders are used when the user creates a new contact, sort-of a dummy record that only gets saved to the database once it has been given a valid name.)

My XAML looks something like this:

<Control.Resources>
	<ResourceDictionary>

		<!--- One of two templates for displaying a contact.  This version blocks editing of the record. -->
		<DataTemplate x:Key="DataTemplateContact">
			<Grid Grid.Row="0" Grid.Column="0">
				<Grid.RowDefinitions>
					<RowDefinition/>
					<RowDefinition/>
				</Grid.RowDefinitions>
				<Grid.ColumnDefinitions>
					<ColumnDefinition Width="75"/>
						<ColumnDefinition Width="*"/>
				</Grid.ColumnDefinitions>
				<TextBlock Grid.Row="0" Grid.Column="0" Text="Name:"/>
				<TextBox Grid.Row="0" Grid.Column="1" Focusable="False" Text="{Binding Name}"/>
				<TextBlock Grid.Row="1" Grid.Column="0" Text="Telephone:"/>
				<TextBox Grid.Row="1" Grid.Column="1" Focusable="False" Text="{Binding Telephone}"/>
			</Grid>
		</DataTemplate>

		<!--- One of two templates for displaying a contact.  This one is switched in to allow record editing. -->
		<DataTemplate x:Key="DataTemplateContactSelected">
			<Grid Grid.Row="0" Grid.Column="0">
				<Grid.RowDefinitions>
					<RowDefinition/>
					<RowDefinition/>
				</Grid.RowDefinitions>
				<Grid.ColumnDefinitions>
					<ColumnDefinition Width="75"/>
					<ColumnDefinition Width="*"/>
				</Grid.ColumnDefinitions>
				<TextBlock Grid.Row="0" Grid.Column="0" Text="Name:"/>
				<TextBox Grid.Row="0" Grid.Column="1" Focusable="True" Text="{Binding Name,UpdateSourceTrigger=PropertyChanged}"/>
				<TextBlock Grid.Row="1" Grid.Column="0" Text="Telephone:"/>
				<TextBox Grid.Row="1" Grid.Column="1" Focusable="True" IsEnabled="{Binding IsNotPlaceholder}"
					Text="{Binding Telephone,UpdateSourceTrigger=PropertyChanged}"/>
			</Grid>
		</DataTemplate>

		<!--- This list item container is used to swap between the editable and non-editable templates above. -->
		<Style TargetType="{x:Type ListBoxItem}" x:Key="ContactListContainerStyle">
			<Setter Property="ContentTemplate" Value="{StaticResource DataTemplateContact}"/>
			<Style.Triggers>
				<Trigger Property="IsSelected" Value="True">
					<Setter Property="ContentTemplate" Value="{StaticResource DataTemplateContactSelected}" />
				</Trigger>
			</Style.Triggers>
		</Style>

	</ResourceDictionary>
</Control.Resources>


<ListBox
	ItemsSource="{Binding Contacts}"
	ItemContainerStyle="{StaticResource ContactListContainerStyle}"
	SelectedItem="{Binding SelectedContact}"/>

Everything works as expected, with the exception that the IsEnabled="{Binding IsNotPlaceholder}" does not 'realise' when it needs to update itself. The Contact is in my model-layer. I have set this up so that any data changes fire an event, which in turn fires an event in the view-model layer. The event in the view-model layer calls OnPropertyChanged( "" ), which should cause all UI elements to re-evaluate their bindings. But still the Telephone field does not become editable.

How can I get this working?

What I have tried:

I have tried setting the IsEnabled="true" in x:Key="DataTemplateContactSelected", and the Telephone field is editable when I select the contact in the list, so this part is working fine.

It is my binding to IsNotPlaceholder that is failing.