RoutedEventArgs.Source vs Sender

Question

What is the difference between sender and source in wpf event handling?

For example, say I had an ellipse in a canvas, and clicked on the ellipse: the ellipse would be both the sender and the source.

However if the ellipse doesn't handle the event but the main window does, the event will pass through the canvas... so the canvas would be the sender of the event to the main window but the source would be the ellipse.

Do I have that right?

Answer 1

The difference between the two is not often seen, as usually the sender and Source are the same. Most code written like Windows Forms will basically ignore the difference and send them along as the same reference. However, given how WPF's event routing works they represent two different concepts.

sender is the object at which the event handler was attached. This is the owner that raised the handler to begin routing the event. From MSDN:

A difference between sender and Source is the result of the event being routed to different elements, during the traversal of the routed event through an element tree.

Source is the object where the event originates. In the case of tunneling and bubbling, the Source will be one of their child elements. You can use the OriginalSource property to peel back any event tree encapsulation.

Answer 2

Bubbles!

The sender is the object that the event is being raised from, whereas source was the original element that is causing the event to be raised.

Like in this case:

<TabControl Name="tc1" SelectionChanged="tc1_SelectionChanged">
    <TabItem Header="1">
        <TabControl Name="tc2">
            <TabItem Header="1" />
            <TabItem Header="2" />
        </TabControl>
    </TabItem>
    <TabItem Header="2" />
</TabControl>

private void tc1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
}

In this case, if you change the SelectedItem on the Sub-TabControl, sender will equal tc1, Source will equal tc2.