How to bind a collection of a subtype to a dependency property - wpf

Question

I'm creating a UserControl in WPF, that is able to work for any object of type IMyNode. Basically, it receives an ObservableCollection through a dependency property, register to it and do some stuff.
In one of my usecase, I use in a control that uses(and need), an ObservableCollection of SomeSpecificNode. SomeSpecificNode is an implementation of IMyNode.
Currently, I've a binding error:
System.Windows.Data Error: 1 : Cannot create default converter to perform 'one-way' conversions between types 'System.Collections.ObjectModel.ObservableCollection`1[SomeSpecificNode]' and 'System.Collections.ObjectModel.ObservableCollection`1[IMyNode]'.
I understand why it happens, it doesn't know how to convert automatically an ObservableCollection<SomeSpecificNode> to ObservableCollection<IMyNode>.
What would be the correct approach to do this?
Using a converter would break the NotifyPropertyChange. Using a ObservableCollection<IMyNode> in my parent ViewModel would not work for the other control in the same page.
Thank you!
Here some pseudo code:
public class SomeSpecificNode: IMyNode{
}
public interface IMyNode{
}
public class ParentViewModel {
public ObservableCollection<SomeSpecificNode> SelectedNodes {get;}=> new ObservableCollection<SomeSpecificNode>()
}
<UserControl x:Class="ParentView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:ch.VibroMeter.Xms.Configurators.Controls.ActionBar"
xmlns:dxb="http://schemas.devexpress.com/winfx/2008/xaml/bars"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800">
<UserControl.DataContext>
<ParentViewModel/>
</UserControl.DataContext>
<StackPanel Orientation="Horizontal" Name="RootContainer">
<SomeChildControl Nodes="{Binding SelectedNodes}" /><!-- This binding will fail !-->
</StackPanel
</UserControl>
public partial class ParentView : UserControl
{
public static readonly DependencyProperty NodesProperty = DependencyProperty.Register(
nameof(Nodes), typeof(ObservableCollection<IMyNode>), typeof(ParentView), new PropertyMetadata(default(ObservableCollection<IMyNode>), OnNodesChanged));
private static void OnNodesChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
//...
}
public ObservableCollection<IMyNode> Nodes
{
get { return (ObservableCollection<IMyNode>)GetValue(NodesProperty); }
set { SetValue(NodesProperty, value); }
}
}

Answer 1

You should change the type of the dependency property to a compatible type such as IEnumerable<IMyNode>.
You cannot set an ObservableCollection<IMyNode> property to anything else than an ObservableCollection<IMyNode> or null.
An ObservableCollection<SomeSpecificNode> is not an ObservableCollection<IMyNode> but it is an IEnumerable<IMyNode> assuming that SomeSpecificNode implements IMyNode.
So this compiles just fine;
IEnumerable<IMyNode> collection = new ObservableCollection<SomeSpecificNode>();
But this doesn't:
ObservableCollection<IMyNode> collection = new ObservableCollection<SomeSpecificNode>(); //Cannot implictly convert type...
The difference is that IEnumerable<T> is covariant. Please refer to the docs for more information.