TreeViewItemViewModel.cs

public class TreeViewItemViewModel : INotifyPropertyChanged
    {
        #region Data

        static readonly TreeViewItemViewModel DummyChild = new TreeViewItemViewModel();

        readonly ObservableCollection<TreeViewItemViewModel> _children;
        readonly TreeViewItemViewModel _parent;

        bool _isExpanded;
        bool _isSelected;

        #endregion // Data

        #region Constructors

        protected TreeViewItemViewModel(TreeViewItemViewModel parent, bool lazyLoadChildren)
        {
            _parent = parent;

            _children = new ObservableCollection<TreeViewItemViewModel>();

            if (lazyLoadChildren)
                _children.Add(DummyChild);
        }

        // This is used to create the DummyChild instance.
        private TreeViewItemViewModel()
        {
        }

        #endregion // Constructors

        #region Presentation Members

        #region Children

        /// <summary>
        /// Returns the logical child items of this object.
        /// </summary>
        public ObservableCollection<TreeViewItemViewModel> Children
        {
            get { return _children; }
        }

        #endregion // Children

        #region HasLoadedChildren

        public bool HasLoadedChildren
        {
            get { return !HasDummyChild; }
        }

        /// <summary>
        /// Returns true if this object's Children have not yet been populated.
        /// </summary>
        public bool HasDummyChild
        {
            get { return Children.Count == 1 && Children[0] == DummyChild; }
        }

        #endregion // HasLoadedChildren

        #region IsExpanded

        /// <summary>
        /// Gets/sets whether the TreeViewItem 
        /// associated with this object is expanded.
        /// </summary>
        public bool IsExpanded
        {
            get { return _isExpanded; }
            set
            {
                if (value != _isExpanded)
                {
                    _isExpanded = value;
                    OnPropertyChanged("IsExpanded");
                }

                // Expand all the way up to the root.
                if (_isExpanded && _parent != null)
                    _parent.IsExpanded = true;

                // Lazy load the child items, if necessary.
                if (HasDummyChild)
                {
                    Children.Remove(DummyChild);
                    LoadChildren();
                }
            }
        }

        #endregion // IsExpanded

        #region IsSelected

        /// <summary>
        /// Gets/sets whether the TreeViewItem 
        /// associated with this object is selected.
        /// </summary>
        public bool IsSelected
        {
            get { return _isSelected; }
            set
            {
                if (value != _isSelected)
                {
                    _isSelected = value;
                    OnPropertyChanged("IsSelected");
                }
                
                SelectOU();
            }
        }

        #endregion // IsSelected

        #region LoadChildren

        /// <summary>
        /// Invoked when the child items need to be loaded on demand.
        /// Subclasses can override this to populate the Children collection.
        /// </summary>
        protected virtual void LoadChildren()
        {
        }

        #endregion // LoadChildren

        #region SelectOU

        /// <summary>
        /// Invoked when the item is selected.
        /// Subclasses can override this to react to the selection.
        /// </summary>
        protected virtual void SelectOU()
        {
        }

        #endregion // SelectOU

        #region Parent

        public TreeViewItemViewModel Parent
        {
            get { return _parent; }
        }

        #endregion // Parent

        #endregion // Presentation Members

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        #endregion // INotifyPropertyChanged Members
    }