Xaml error when using Interactivity wpf

Question

I'm getting a the following error when I use Interactivity :

Error Collection property 'System.Windows.Controls.Canvas'.'Triggers' is null.

Here's my code:

<ItemsControl x:Name="StreamCanvasItemSource" ItemsSource="{Binding StreamCanvasItems, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="{Binding ActualWidth, ElementName=StreamImage}" Height="{Binding ActualHeight, ElementName=StreamImage}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <Canvas x:Name="StreamCanvas" Background="Transparent" Width="{Binding ActualWidth, ElementName=StreamImage}" Height="{Binding ActualHeight, ElementName=StreamImage}" 
                AuxiliaryUtils:SizeObserver.Observe="True" AuxiliaryUtils:SizeObserver.ObservedWidth="{Binding CurrentCanvasWidth, Mode=OneWayToSource}" 
                AuxiliaryUtils:SizeObserver.ObservedHeight="{Binding CurrentCanvasHeight, Mode=OneWayToSource}" 
                AuxiliaryUtils:SizeObserver.ObservedLeft="{Binding CanvasLeft, Mode=OneWayToSource}" AuxiliaryUtils:SizeObserver.ObservedTop="{Binding CanvasTop, Mode=OneWayToSource}">
                <Interactivity:Interaction.Triggers>
                    <Interactivity:EventTrigger EventName="PreviewMouseWheel">
                        <Command:EventToCommand Command="{Binding OnMouseWheelCommand}" PassEventArgsToCommand="True" />
                    </Interactivity:EventTrigger>
                    <Interactivity:EventTrigger EventName="PreviewMouseDown">
                        <Interactivity:InvokeCommandAction Command="{Binding OnMouseDownCommand}" CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Canvas}}}"/>
                    </Interactivity:EventTrigger>
                    <Interactivity:EventTrigger EventName="PreviewMouseMove">
                        <Interactivity:InvokeCommandAction Command="{Binding OnMouseMoveCommand}" CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Canvas}}}"/>
                    </Interactivity:EventTrigger>
                    <Interactivity:EventTrigger EventName="PreviewMouseUp">
                        <Interactivity:InvokeCommandAction Command="{Binding OnMouseUpCommand}" CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Canvas}}}"/>
                    </Interactivity:EventTrigger>
                </Interactivity:Interaction.Triggers>
            </Canvas>
        </ItemsPanelTemplate>
  </ItemsControl.ItemsPanel>
</ItemsControl>

The thing is that everything works as expected when I run the program, I see the error when compiling, and I can't see the designer unless I comment the Interactivity block.

Any idea why?

Answer 1

How is the datacontext connected to the window/page? it would it advisable to add the following line to the window/page and setting IsDesignTimeCreatable to true:

d:DataContext="{d:DesignInstance Type=yourviewmodetype, IsDesignTimeCreatable=True}"

In this way the view will bind to the values of the viewmodel in designtime which is exactly what is going wrong atm in your application.