Information Technology Reference
In-Depth Information
public
Window1()
{
InitializeComponent();
}
private void
OnMouseDown(
object
sender,
MouseButtonEventArgs
e)
{
DoMouseThings(e);
}
private void
DoMouseThings(
MouseButtonEventArgs
e)
{
throw new
NotImplementedException
();
}
}
The first solution is preferred. This may seem surprising given the empha-
sis on declarative code in WPF applications. Even so, if the logic must be
implemented in code, you should use the virtual method. If an event han-
dler throws an exception, no other handlers in the chain for that event are
called (see Items 24 and 25). Some other ill-formed code prevents the sys-
tem from calling your event handler. By overriding the protected
virtual
function, your handler gets called first. The
base
class version of the
virtual
function is responsible for calling any event handlers attached to
the particular event. That means that if you want the event handlers called
(and you almost always do), you must call the
base
class. In some rare
cases, you will want to replace the default behavior instead of calling the
base
class version so that none of the event handlers gets called. You can't
guarantee that all the event handlers will be called because some ill-formed
event handler might throw an exception, but you can guarantee that your
derived class's behavior is correct.
Using the override is more efficient than attaching the event handler. You
will remember from Item 25 that events are built on top of multicast del-
egates. That enables any event source to have multiple observers. The
event-handling mechanism takes more work for the processor because it
must examine the event to see if any event handlers have been attached. If
so, it must iterate the entire invocation list, which may contain any num-
ber of target methods. Each method in the event invocation list must be
