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If you ve provided an event handler and have attached it to an event, then when that event is raised, the handler is called. WPF, however, adds a new kind of event object called a routed event, which sends the event not just to the element to which the event handler is attached but also to other elements in the tree. The events built into the WPF controls and elements use this new type of event. To distinguish standard .NET events from routed events, standard events are called CLR events. In this chapter, you ll learn how to use WPF s built-in routed events. You can also create your own custom routed events but since most WPF programmers won t commonly do this, we won t cover that topic. WPF uses three different routing strategies for its routed events: The Direct routing strategy is the most like standard CLR event behavior. When an event is raised on an element, WPF checks for an event handler only on the element that raised the event. The Bubbling routing strategy first checks the element on which the event was raised and then routes the event upward through the tree, all the way to the top element. On each successive element the event is raised, and if that element has a handler for it, the handler is invoked. The Tunneling routing strategy starts at the top of the tree and works its way down, raising the event on each element until it gets to the element on which the event was initially raised. By convention, the names of the built-in, WPF tunneling events start with the prefix Preview.

By parallelizing an application, you can greatly speed it up (or slow it down if you do it badly!). It is worth considering the following: It is a shame that the ability to utilize all available processing power on a machine (for example, dormant GPUs) was not included in this release. Many developers feel the Concurrency and Coordination Runtime (CCR) should have been included in this release. The CCR assists with creating loosely coupled asynchronous applications and was originally included with Microsoft Robotics Studio (it has since been separated out). At the time of writing, the CCR is not free for commercial usage. For more info on CCR, please refer to: http://msdn. microsoft.com/en-gb/library/bb648752.aspx. Looking toward the future, is it possible that a future version of Task Manager could allow you to distribute work across multiple machines paving the way for grid computing libraries within .NET Multicore shift will mean that existing pricing/licensing models need to be reconsidered.

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GUI programming is event driven, which means that while the program is running, it can be interrupted at any time by events such as button clicks, key presses, or system timers. When this happens, the program needs to handle the event and then continue on its course. Clearly, this asynchronous handling of program events is the perfect situation to use C# events. Windows GUI programming uses events so extensively that there is a standard .NET Framework pattern for using them, which you are strongly encouraged to follow. The foundation of the standard pattern for event usage is the EventHandler delegate type, which is declared in the System namespace. The declaration of the EventHandler delegate type is shown in the following code: The first parameter is meant to hold a reference to the object that raised the event. It is of type object and can, therefore, match any instance of any type. The second parameter is meant to hold state information of whatever type is appropriate for the application. The return type is void.

Any element involved in the tunneling or bubbling process can signal that the routing shouldn t go any further. We ll cover this topic shortly. WPF s nested content model created the need for routed events. Consider, for example, the content of the window shown in Figure 9-3. It consists of an Image inside a Border inside another Border. If the user clicks the Image inside the window, which of the objects should handle the MouseLeftButtonUp event Should it be handled by the Image, by one of the two Borders, or by the Window object As you ll shortly see, the answer depends on the application and how the application conceptually groups the three components. Routed events add a great deal more built-in flexibility for handling events.

public delegate void EventHandler(object sender, EventArgs e);

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