If you've worked with older graphic user interfaces, you are aware of a concept: in Windows Forms applications, only one control receives an event.  For example when you click a button, only this button receives the Click event.  But in Windows Presentation Foundation there is a key difference here: other elements can receive an event for an element.  For example you may click on a button but its preceding elements receive Click events as well.

Windows Presentation Foundation supports different types of routing strategies which are listed with a short description below.  Before stepping into these three types, it is worthwhile to have something in mind.  Imagine elements organized in a hierarchical manner.  That is, suppose that in a tree parent elements are in upper level and children are in lower level.

·         Bubble: A Bubble event starts from an element and follows for all its preceding elements.  When a Bubble event occurs, first the element itself receives the event then its parent, then the parent of parent and so on.

·         Direct: Direct events are similar to traditional Windows Forms events.  Only the element, itself, receives the event.

·         Tunnel: Tunnel events are in opposite direction of Bubble events.  It means when a Tunnel event occurs, first root element receives it, then its first child, then child of this child and so on until it arrives to element, itself.

In working with the above routing strategies you need to be aware of some points that I'll address in a moment:

·         As you see Bubble and Tunnel strategies are in opposite directions.  Here Microsoft has done a naming convention to help developers find their way easier.  Tunnel event names start with a "Preview" prefix.  So you can identify Tunnel events by their names.  For example MouseRightButtonDown is a Bubble event but PreviewMouseRightButtonDown is a Tunnel event.

·         It's recommended to mark events as Handled when you're sure there is no other element in the route.  This has a performance effect.  Handled is a property for event argument types.

All events in Windows Presentation Foundation belong to one of the above three types.  For example the Click event has a Direct routing strategy, MouseLeftButtonDown has a Bubble routing strategy and PreviewMouseLeftButtonDown has a Tunnel routing strategy.

Let's see an example of Bubble events.

Listing 1 and Listing 2 represent XAML and logic codes for a simple application which uses the MouseEnter event (with the Bubble strategy). 

Listing 1

<Window x:Class="EventsInWPF.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="EventsInWPF" Height="100" Width="250"
    >
    <Grid MouseEnter="GridHandler">
      <Grid.RowDefinitions>
        <RowDefinition />
        <RowDefinition />
      </Grid.RowDefinitions>
      <Grid.ColumnDefinitions>
        <ColumnDefinition />
        <ColumnDefinition />
      </Grid.ColumnDefinitions>
      <Button MouseEnter="ButtonHandler" Grid.Row="0" Grid.Column="0"
              Width="100" Height="20">Click</Button>
    </Grid>  
</Window>

Listing 2

private void ButtonHandler(object sender, MouseEventArgs e)
{
    Debug.WriteLine("Event Received By Button");
}
 
private void GridHandler(object sender, MouseEventArgs e)
{
    Debug.WriteLine("Event Received By Grid");
}

The output of Listing 2 shows the order of events by providing the output when you move mouse over Button as shown below

Event Received By Button
Event Received By Grid

Clearly the event was received by both the Button and its parent, as expected.