StackOverflow #1

MainWindow.cs

using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;

namespace TestApplication
{
    public interface INameable
    {
        string Name { get; set; }
    }
    
    public class BaseViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public class Quest : BaseViewModel, INameable
    {
        private string _name;

        public string Name
        {
            get { return _name; }
            set
            {
                _name = value; 
                
                OnPropertyChanged();
            }
        }

        public BindingList<Dialogue> Dialogues { get; set; }

        public Quest()
        {
            Dialogues = new BindingList<Dialogue>();
            Dialogues.ListChanged += (sender, args) =>
            {
                OnPropertyChanged(nameof(Dialogues));
            };
        }
    }

    public class Dialogue : BaseViewModel, INameable
    {
        private string _name;

        public string Name
        {
            get { return _name; }
            set
            {
                _name = value;
                
                OnPropertyChanged();
            }
        }
    }

    public partial class MainWindow : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        public INameable SelectedObject
        {
            get { return _selectedObject; }
            set
            {
                _selectedObject = value;

                OnPropertyChanged();
            }
        }

        public BindingList<Quest> Quests
        {
            get { return _quests; }
            set
            {
                _quests = value; 
                
                OnPropertyChanged();
            }
        }

        private BindingList<Quest> _quests;
        private INameable _selectedObject;

        public MainWindow()
        {
            Quests = new BindingList<Quest>();
            Quests.ListChanged += (sender, args) =>
            {
                OnPropertyChanged(nameof(Quests));
            };

            InitializeComponent();

            _treeView.SelectedItemChanged += (sender, args) =>
            {
                if (args.NewValue is INameable)
                {
                    SelectedObject = args.NewValue as INameable;
                }
            };
        }

        private void AddQuest(object sender, RoutedEventArgs e)
        {
            Quests.Add(new Quest
            {
                Name = "Quest"
            });
        }

        private void AddDialogue(object sender, RoutedEventArgs e)
        {
            if (_treeView.SelectedItem is Quest)
            {
                (_treeView.SelectedItem as Quest).Dialogues.Add(new Dialogue
                {
                    Name = "Dialogue"
                });
            }
        }

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

MainWindow.xaml

<Window x:Class="TestApplication.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:testApplication="clr-namespace:TestApplication"
        mc:Ignorable="d"
        Title="MainWindow" Width="256" DataContext="{Binding RelativeSource={RelativeSource Self}}" SizeToContent="WidthAndHeight">
    <DockPanel LastChildFill="True">
        <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
            <Button Content="Add Quest" Click="AddQuest" />
            <Button Content="Add Dialogue" Click="AddDialogue" />
            <TextBox Text="{Binding SelectedObject.Name, UpdateSourceTrigger=PropertyChanged}" MinWidth="100" />
        </StackPanel>
        <TreeView x:Name="_treeView" x:FieldModifier="private">
            <TreeViewItem IsExpanded="True">
                <TreeViewItem.Header>
                    <TextBlock Text="Project" Margin="5,0,0,0" />
                </TreeViewItem.Header>
                <TreeViewItem ItemsSource="{Binding Quests}" IsExpanded="True">
                    <TreeViewItem.Header>
                        <TextBlock Text="Quests" Margin="5,0,0,0" />
                    </TreeViewItem.Header>
                    <TreeViewItem.Resources>
                        <HierarchicalDataTemplate DataType="{x:Type testApplication:Quest}" ItemsSource="{Binding Dialogues}">
                            <TextBlock Text="{Binding Name}" Margin="5,0,0,0" />
                        </HierarchicalDataTemplate>
                        <DataTemplate DataType="{x:Type testApplication:Dialogue}">
                            <TextBlock Text="{Binding Name}" Margin="5,0,0,0" />
                        </DataTemplate>
                    </TreeViewItem.Resources>
                </TreeViewItem>
            </TreeViewItem>
        </TreeView>
    </DockPanel>
</Window>