0001_di-with-xaml-using-dependency-property.md

Notes

• I want to do dependency injection where the MainWindowViewModel provides the QueryBox control with it's QueryBoxViewModel.

  My approach was to define a dependency property and then access it in the constructor of QueryBox:

  public QueryBox()
  {
      InitializeComponent();
      DataContext = ViewModel;
  }
  
  public static readonly DependencyProperty ViewModelProperty = DependencyProperty.Register(
      name: nameof(ViewModel),
      propertyType: typeof(QueryBoxViewModel),
      ownerType: typeof(QueryBox)
  );
  public QueryBoxViewModel ViewModel
  {
      get => (QueryBoxViewModel)GetValue(ViewModelProperty);
      set => SetValue(ViewModelProperty, value);
  }

  This doesn't work, because ViewModel is always null.

• My understanding is that I should do dependency injection in the constructor, but I do not know how to provide constructor parameters.

• There is something like <x:Arguments /> but it seems that this is no longer supported.

• It's possible to assign to DataContext directly without any fancy work.

Tasks

• Try to get an event handler when the dependeny property changes.

• Research how to inject a view model into another component.

Theories

• I suspect, that the constructor runs before it starts looking at the properties.

• I suspect, that this is only possible to do from the view model.

  • I suspect, that overriding DataContext messes things up.

Result

• It is possible to assign to the DataContext property directly from the parent:

  <QueryBox DataContext="{Binding QueryBoxVM}" />