Odd Loaded behavior in markup versus code - wpf

Question

I'm currently very confused by the differing behavior regarding the FrameworkElement.Loaded event. I've put together a small example application that demonstrates this.
Xaml:
<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300"
Loaded="Window_Loaded">
<Grid>
<TabControl>
<TabItem Header="Tab 1" />
<TabItem Header="Tab 2" >
<WindowsFormsHost Name="formHost" Loaded="formHost_Loaded" />
</TabItem>
</TabControl>
</Grid>
</Window>
Code:
using System.Windows;
namespace WpfApplication1
{
public partial class Window1 : Window
{
public Window1()
{
InitializeComponent();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
formHost.Loaded += delegate
{
MessageBox.Show("Delegate");
};
}
private void formHost_Loaded(object sender, RoutedEventArgs e)
{
MessageBox.Show("Markup");
}
}
}
As it is, when I run the application I get two immediate MessageBoxes - "Markup" and "Delegate". If, however, I remove Loaded="formHost_Loaded" from the WindowsFormsHost, I get neither on startup. It obviously makes sense why I don't get the "Markup" dialog, but why does this also remove the "Delegate"? I would imagine it has to do with the order in which the events are called (Window versus its children), but I'm having a tough time figuring it out.
Note: You can replace the WindowsFormsHost with other controls, it really shouldn't matter - I was just using it for another few tests.

Answer 1

The Loaded event handlers are determined before any of them are called, as the Loaded event is effectively broadcast all at once starting at the top of the tree. Since you added a handler after WPF has decided what handlers need to be called, it is ignored.
This can be verified using Reflector. Specifically, the BroadcastEventHelper.BroadcastLoadedSynchronously method will call the BroadcastEventHelper.BroadcastEvent method with the LoadedEvent routed event.
BroadcastEvent method gathers all the objects in the visual tree that have a Loaded event handler first, then loops through and raises the events on those objects.