Update ImageBrush from ResourceDictionary at runtime - silverlight

Question

In my WP7 application, all the pages use an ImageBrush as the background that I have defined in a ResourceDictionay. This ResourceDictionary is merged globally through the App.xaml. The ImageBrush in the ResourceDictionary is defined like this:
<ImageBrush x:Key="PhonePageBackground" ImageSource="/Background1.jpg"/>
Im trying to update the ImageSource of the ImageBrush at runtime, but its not working.
Making some tests, where I have a page with a button on it to change the background, I realized that the code below works fine:
ImageBrush image;
public MainPage()
{
InitializeComponent();
image = new ImageBrush { ImageSource = new BitmapImage(new Uri("/Background1.jpg", UriKind.Relative)) };
LayoutRoot.Background = image;
}
private void button1_Click(object sender, RoutedEventArgs e)
{
image.ImageSource = new BitmapImage(new Uri("/Background2.jpg", UriKind.Relative));
}
But the code below, where I use the ImageBrush from the dictionary, does not work. The background of the page becomes transparent, as if the image could not be found:
ImageBrush image;
public MainPage()
{
InitializeComponent();
image = (ImageBrush)Application.Current.Resources["PhonePageBackground"];
LayoutRoot.Background = image;
}
private void button1_Click(object sender, RoutedEventArgs e)
{
image.ImageSource = new BitmapImage(new Uri("/Background2.jpg", UriKind.Relative));
}
Both images (Background1.jpg and Background2.jpg) build action are set as Content. I've tested with Resource set, but without success.
Any knowledge of why this behavior?

Answer 1

This works for me.
ImageBrush image;
public MainPage()
{
InitializeComponent();
image = (ImageBrush)Application.Current.Resources["PhonePageBackground"];
LayoutRoot.Background = image;
}
private void button1_Click(object sender, RoutedEventArgs e)
{
image = new ImageBrush { ImageSource = new BitmapImage(new Uri("/Background2.jpg", UriKind.Relative)) };
LayoutRoot.Background = image;
}