how to bind width of child element to width of parent element in silverlight

I have a grid whose width is "1*". So the actual width decided at runtime I think. Within that grid I have another grid whose width I want to set to the runtime width of parent grid. How Can I do that in xaml through binding.

This will actually help you I guess

Width="{Binding ActualWidth, ElementName=parentElementName}"

This binds the width to the parent element or the element name provided

This is generic solution which may work everywhere. You wouldn't need to write parent element name. This will identify its parent and will take parent's width.

Width="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=ActualWidth}"

I think that easiest way to do same thing is:

HorizontalAlignment="Stretch"

That is not using binding as you have been asked, but it is easier.

 Width="{Binding Width, RelativeSource={RelativeSource AncestorType={x:Type Parent}, Mode=FindAncestor}}"

if both controls DataContext is different.

If you are doing it in CodeBehind, this works for me. It has the added advantage that bindMe does not have to be a child of toMe:

public static void BindWidth(this FrameworkElement bindMe, FrameworkElement toMe)
{
  Binding b = new Binding();
  b.Mode = BindingMode.OneWay;
  b.Source = toMe.ActualWidth;
  bindMe.SetBinding(FrameworkElement.WidthProperty, b);
}

usage:

child.BindWidth(parent);

HorizontalAlignment doesn't work for buttons in Xamarin, you can use HorizontalOptions instead

HorizontalOptions="Fill"

Use HorizontalContentAlignment="Stretch" on ListBox:

<ListBox HorizontalContentAlignment="Stretch">
	<ListBox.ItemTemplate>
		<DataTemplate>
			<StackPanel Orientation="Vertical">
				<Image HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Source="TestImage.png"/>
				<TextBlock HorizontalAlignment="Center" Text="Test Text"/>
			</StackPanel>
		</DataTemplate>
	</ListBox.ItemTemplate>
</ListBox>

or if it is inside a Template use this:

<ListBox>
	<ListBox.ItemTemplate>
		<DataTemplate>
			<StackPanel Orientation="Vertical">
				<Image Width="{TemplateBinding Width}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Source="TestImage.png"/>
				<TextBlock HorizontalAlignment="Center" Text="Test Text"/>
			</StackPanel>
		</DataTemplate>
	</ListBox.ItemTemplate>
</ListBox>