Question

Consider this attached property:

public static readonly DependencyProperty TestImageProperty =
    DependencyProperty.RegisterAttached("TestImage",
    typeof(BitmapSource), typeof(TestView), new FrameworkPropertyMetadata(null,
    FrameworkPropertyMetadataOptions.AffectsRender));

public static void SetTestImage(UIElement element, BitmapSource value)
{
    element.SetValue(TestImageProperty, value);
}

public static BitmapSource GetTestImage(UIElement element)
{
    return (BitmapSource)element.GetValue(TestImageProperty);
}

public BitmapSource TestImage
{
    get
    {
        return GetTestImage(this);
    }
    set
    {
        SetTestImage(this, value);
    }
}

And the XAML:

<common:UserControlBase x:Class="MyViews.TestView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:views="clr-namespace:MyViews"
    xmlns:common="clr-namespace:MyCommon.Common;assembly=MyCommon.Common"
    xmlns:mefed="clr-namespace:MEFedMVVM.ViewModelLocator;assembly=MEFedMVVM.WPF"
    mefed:ViewModelLocator.SharedViewModel="MyViewModel"
    views:TestView.TestImage="{Binding TestImage}"

Hence, I’m binding the view’s TestImage property to the viewmodel’s TestImage property! Then, from code-behind I update the property:

this.TestImage = image;

This does NOT trigger the setter in my viewmodel.

What is causing this?

Answer 1

Put the Binding on TwoWay Mode will resolve the problem.