ObservableCollection is not updating Multibinding in C# WPF

Question

I have a TreeView that creates all its items from databound ObservableCollections. I have a hierarchy of GameNode objects, each object has two ObservableCollections. One collections has EntityAttrib objects and the other have GameNode objects. You could say that the GameNode object represents folders and EntityAttrib represents files. To display both attrib and GameNodes in the same TreeView I use Multibinding. This all works fine in startup, but when I add a new GameNode somewhere in the hierarchy the TreeView is not updated. I set a breakpoint in my converter method but it's not called when adding a new GameNode. It seems that the ObservableCollection is not notifying the MultiBinding of the change. If I comment out the MultiBinding and only bind the GameNode collection it works as expected.

XAML:

<HierarchicalDataTemplate DataType="{x:Type local:GameNode}">
        <HierarchicalDataTemplate.ItemsSource>
            <MultiBinding Converter="{StaticResource combineConverter}">
                <Binding Path="Attributes" />
                <Binding Path="ChildNodes" />
            </MultiBinding>
        </HierarchicalDataTemplate.ItemsSource>
        <TextBlock Text="{Binding Path=Name}" ContextMenu="{StaticResource EntityCtxMenu}"/>
    </HierarchicalDataTemplate>

C#:

public class GameNode
{
    string mName;
    public string Name { get { return mName; } set { mName = value; } }

    GameNodeList mChildNodes = new GameNodeList();
    public GameNodeList ChildNodes { get { return mChildNodes; } set { mChildNodes = value; } }

    ObservableCollection<EntityAttrib> mAttributes = new ObservableCollection<EntityAttrib>();
    public ObservableCollection<EntityAttrib> Attributes { get { return mAttributes; } set { mAttributes = value; } }
}

GameNodeList is a subclassed ObservableCollection

Answer 1

The best way to do (only if your EntityAttrib and GameNode are two different classes inherited from same base class) would be actually defining two data templates as below.

<HierarchicalDataTemplate DataType="{x:Type local:GameNode}">        

<HierarchicalDataTemplate DataType="{x:Type local:EntityAttrib}">

This is better because its easier to recollect later on. Consider file system objects, both FileInfo and DirectoryInfo classes are actually derived from FileSystemInfo class which shares common property.

You should have a baseclass, "BaseGameNode" , have something in it, "GameNode" and "GameEntityAttribNode" both derived from "BaseGameNode". And they should have only one Children property that is observable collection of type BaseGameNode but its item instance should be different as needed.

You can define multiple templates provided they have something to distinguish, the type will be automatically chosen for the children of the node.

Value you have bound will not refresh because they are not dependency property neither they notify on change. Since multibinding will not detect collection change event at all unless the reference/instance of collection changes. When you change an item in collection your actual instance of attributes/properties remains same.

When you bind ItemsSource = collection, its the ItemsSource that will listen for CollectionChange event and update the items accordingly.