Do compiled bindings (x:Bind) require the ViewModel to derive from DependencyObject?

Question

I am trying to use compiled bindings on a project realised with the Catel MVVM Framework (should not matter). In the generated code however I get compiler errors because my ViewModel does not implement certain functions normally exposed by DependencyObjects:

CS1061 'MainViewModel' does not contain a definition for 'RegisterPropertyChangedCallback' and no extension method 'RegisterPropertyChangedCallback' accepting a first argument of type 'MainViewModel' could be found (are you missing a using directive or an assembly reference?)

Also:

CS0039 Cannot convert type 'Windows.UI.Xaml.DependencyObject' to 'CatelCompiledBindingTest.ViewModels.MainViewModel' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

The x:Bind compiler-generated code throwing the errors is the following:

public void DependencyPropertyChanged_VM_Visible(global::Windows.UI.Xaml.DependencyObject sender, global::Windows.UI.Xaml.DependencyProperty prop)
{
    MainView_obj1_Bindings bindings = TryGetBindingObject();
    if (bindings != null)
    {
        global::CatelCompiledBindingTest.ViewModels.MainViewModel obj = sender as global::CatelCompiledBindingTest.ViewModels.MainViewModel;
        if (obj != null)
        {
            bindings.Update_VM_Visible(obj.Visible, DATA_CHANGED);
        }
    }
}
private global::CatelCompiledBindingTest.ViewModels.MainViewModel cache_VM = null;
private long tokenDPC_VM_Visible = 0;
public void UpdateChildListeners_VM(global::CatelCompiledBindingTest.ViewModels.MainViewModel obj)
{
    if (obj != cache_VM)
    {
        if (cache_VM != null)
        {
            ((global::System.ComponentModel.INotifyPropertyChanged)cache_VM).PropertyChanged -= PropertyChanged_VM;
            cache_VM.UnregisterPropertyChangedCallback(global::CatelCompiledBindingTest.ViewModels.MainViewModel.VisibleProperty, tokenDPC_VM_Visible);
            cache_VM = null;
        }
        if (obj != null)
        {
            cache_VM = obj;
            ((global::System.ComponentModel.INotifyPropertyChanged)obj).PropertyChanged += PropertyChanged_VM;
            tokenDPC_VM_Visible = obj.RegisterPropertyChangedCallback(global::CatelCompiledBindingTest.ViewModels.MainViewModel.VisibleProperty, DependencyPropertyChanged_VM_Visible);
        }
    }
}

The MainViewModel is fairly simple (with Catel boilerplate code excluded):

public class MainViewModel : ViewModelBase
{
    public MainViewModel()
    {
        ToggleVisibility = new Command(OnToggleVisibilityExecute);
        AddText = new Command(OnAddTextExecute);
        SomeText = "";
    }

    //ViewModel properties

    public string SomeText
    {
        get { return GetValue<string>(SometextProperty); }
        set { SetValue(SometextProperty, value); }
    }

    public bool Visible
    {
        get { return GetValue<bool>(VisibleProperty); }
        set { SetValue(VisibleProperty, value); }
    }

    //ViewModel Commands

    public Command ToggleVisibility { get; private set; }

    public void OnToggleVisibilityExecute()
    {
        Visible = !Visible;
    }

    public Command AddText { get; private set; }

    public void OnAddTextExecute()
    {
        SomeText += "Random text! ";
    }
}

The View is the following:

<controls:Page
    x:Class="CatelCompiledBindingTest.Views.MainView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:CatelCompiledBindingTest.Views"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:controls="using:Catel.Windows.Controls"
    xmlns:converters="using:Catel.MVVM.Converters"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <Page.Resources>
        <converters:BooleanToCollapsingVisibilityConverter x:Key="BooleanToVisibilityConverter"/>
    </Page.Resources>

    <Grid>
        <Grid.KeyboardAccelerators>
            <KeyboardAccelerator Invoked="{x:Bind VM.OnToggleVisibilityExecute}" Key="T" Modifiers="Control"/>
            <KeyboardAccelerator Invoked="{x:Bind VM.OnAddTextExecute}" Key="Add" Modifiers="Control"/>
        </Grid.KeyboardAccelerators>

        <Border Width="500" Height="500" Visibility="{x:Bind VM.Visible, Mode=OneWay}" Background="SlateGray">
            <TextBlock Style="{ThemeResource HeaderTextBlockStyle}" Text="{x:Bind VM.SomeText, Mode=OneWay}"/>
        </Border>

    </Grid>
</controls:Page>

With the following code-behind to be able to use compiled bindings:

public sealed partial class MainView : Page
{
    public MainView()
    {
        this.InitializeComponent();
        VM = DataContext as MainViewModel;
    }

    public MainViewModel VM { get; set; }
}

The question now is: Why does the compiler assume MainViewModel to be of Type DependencyObjectin the compiler-generated DependencyPropertyChanged_VM_Visible? According to Data Binding in Depth the ViewModel simply neeeds to implement IPropertyChangedwhich it does.

How do I resolve this issue (apart from resorting to {Binding}again)? Which namespaces do I need to include in the ViewModel for compiled Bindings to work?

Answer 1

It assumes it's a dependency property because the VM property is declared on the view MainView. You should define the VM property as a dependency property so it can be bound to (so not the value / type of the property, but the property itself).