Why does Image not appear when rendering it before? - silverlight

Question

Starting with a newly created project I add a button to my main page and do the following in the click handler:
I create an Image and assign a BitmapImage as its Source. Then I add the Image to my LayoutRoot. My expectation is that I see the image in the GUI after I click the button.
Now there is the twist: I also want to render this Image to a WriteableBitmap. Therefore I create such a bitmap and call its Render method to render the image.
Here is the problem: when I comment out the Render call then I immediately see the Image appear on my main page. When I include the Render call, then the Image appears not on the first button click, but on the second. Why?
Here is the code:
private void button1_Click(object sender, RoutedEventArgs e)
{
WriteableBitmap wbmp = new WriteableBitmap(62, 62);
BitmapImage bmp = new BitmapImage(new Uri("ApplicationIcon.png", UriKind.Relative));
Image img = new Image() { Width = 62, Height = 62, Source = bmp };
wbmp.Render(img, null); // <------ this line makes the difference
LayoutRoot.Children.Add(img);
}

Answer 1

There are some remarks of using Render (may be one of these makes your issue):
After calling this method, you must call Invalidate in order to render
the bitmap.
This method does support UIElement objects that are not part of the
visual tree. You are required to call Measure and Arrange for
UIElement objects that are not in the visual tree, before calling
Render.

Answer 2

try this..
private Image _img = new Image ();
public Image img
{
get
{
return _img;
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String PropertyName)
{
if (null != PropertyChanged)
{
PropertyChanged(this, new PropertyChangedEventArgs(PropertyName));
}
}
private void button1_Click(object sender, RoutedEventArgs e)
{
WriteableBitmap wbmp = new WriteableBitmap(62, 62);
BitmapImage bmp = new BitmapImage(new Uri("ApplicationIcon.png", UriKind.Relative));
Image img = new Image() { Width = 62, Height = 62, Source = bmp };
wbmp.Render(img, null); // <------ this line makes the difference
LayoutRoot.Children.Add(img);
NotifyPropertyChanged("Image");
}
this should work.. try once...