✅ WPF - how to approach a treeview displaying different types at different levels via MVVM

public sealed partial class MainViewModel
  : ObservableObject
{
  /// <summary>
  /// Determines whether the loading screen for loading gists is displayed
  /// </summary>
  [ObservableProperty] private bool _gistsLoading;

  /// <summary>
  /// Currently selected gist
  /// </summary>
  /// <remarks>
  /// <c>null</c> when nothing is selected
  /// </remarks>
  [ObservableProperty] private Gist? _selectedGist;
  
  public ObservableCollection<Gist> Gists { get; } = new();

  // Bind this to the view Loaded event via interaction behaviors
  [RelayCommand]
  private async Task LoadGists()
  {
    LoadingGists = true;
    try
    {
      // Load the fun stuff from Gist API
    }
    finally
    {
      LoadingGists = false;
    }
  }
}

public sealed partial class MainViewModel
  : ObservableObject
{
  /// <summary>
  /// Determines whether the loading screen for loading gists is displayed
  /// </summary>
  [ObservableProperty] private bool _gistsLoading;

  /// <summary>
  /// Currently selected gist
  /// </summary>
  /// <remarks>
  /// <c>null</c> when nothing is selected
  /// </remarks>
  [ObservableProperty] private Gist? _selectedGist;
  
  public ObservableCollection<Gist> Gists { get; } = new();

  // Bind this to the view Loaded event via interaction behaviors
  [RelayCommand]
  private async Task LoadGists()
  {
    LoadingGists = true;
    try
    {
      // Load the fun stuff from Gist API
    }
    finally
    {
      LoadingGists = false;
    }
  }
}

public sealed partial class MainViewModel
  : ObservableObject
{
  #region Fields

  /// <summary>
  /// Determines whether the loading screen for loading gists is displayed
  /// </summary>
  [ObservableProperty] private bool _gistsLoading;

  /// <summary>
  /// Currently selected gist
  /// </summary>
  /// <remarks>
  /// <c>null</c> when nothing is selected
  /// </remarks>
  [ObservableProperty] private Gist? _selectedGist;

  #endregion

  #region Properties
  
  public ObservableCollection<Gist> Gists { get; } = new();

  #endregion

  #region Commands

  // Bind this to the view Loaded event via interaction behaviors
  [RelayCommand]
  private async Task LoadGists()
  {
    LoadingGists = true;
    try
    {
      // Load the fun stuff from Gist API
    }
    finally
    {
      LoadingGists = false;
    }
  }

  #endregion

  public MainWindowViewModel()
  {
    // I put the constructor here, I do not region it, untless there is a need for multiple ctors
  }

  #region Methods

  // For non-command logic

  #endregion
}

public sealed partial class MainViewModel
  : ObservableObject
{
  #region Fields

  /// <summary>
  /// Determines whether the loading screen for loading gists is displayed
  /// </summary>
  [ObservableProperty] private bool _gistsLoading;

  /// <summary>
  /// Currently selected gist
  /// </summary>
  /// <remarks>
  /// <c>null</c> when nothing is selected
  /// </remarks>
  [ObservableProperty] private Gist? _selectedGist;

  #endregion

  #region Properties
  
  public ObservableCollection<Gist> Gists { get; } = new();

  #endregion

  #region Commands

  // Bind this to the view Loaded event via interaction behaviors
  [RelayCommand]
  private async Task LoadGists()
  {
    LoadingGists = true;
    try
    {
      // Load the fun stuff from Gist API
    }
    finally
    {
      LoadingGists = false;
    }
  }

  #endregion

  public MainWindowViewModel()
  {
    // I put the constructor here, I do not region it, untless there is a need for multiple ctors
  }

  #region Methods

  // For non-command logic

  #endregion
}