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AutoResetEvent can be problematic. There isn t necessarily any corre-

spondence between the number of times you signal it and the number of times it releases threads. If you signal it twice in a row when no threads are waiting, it doesn t keep count it s just as signaled after the second signal as it was after the first one. This can lead to bugs where you occasionally miss signals, and your code can grind to a halt. Approach with caution.

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Somewhere in our application, we ll build this presentation model. It may be hydrated with the results of a simple database query, like a flat report. Or it may be calculated and projected from another set of interesting data. It s common to have a class whose sole responsibility is to formulate the presentation model. Doing the work of building a presentation model in application code is better than doing that work in the view. The view is convoluted enough as it is, and it s focused on HTML and style. A separate class that creates the presentation model can be easily tested, programmed, and maintained. It s also best not to create the presentation model in the controller. The controller is busy deciding which view to render and coordinating these other efforts. Listing 2.2 offers a simplistic look at how a controller might send the presentation model to the view.

These types are wrappers around the underlying Windows event synchronization primitive, so as with Mutex, you can use events for cross-process coordination. And of course, this also means that you incur the cost of getting the OS scheduler involved. .NET 4 introduces an alternative called ManualResetEventSlim. This will use busywaiting techniques similar to a spinlock for short waits. So just like the Monitor, it gets the scheduler involved only when a wait is necessary. Therefore, unless you really need the extra features available from the nonslim version (e.g., you need cross-process synchronization) the ManualResetEventSlim class is a better choice than ManualResetE vent if you re using .NET 4 or later.

.NET 4 introduces the CountdownEvent class, which provides a handy solution to a fairly common problem: knowing when you re done. Remember back in Example 16-6, we ran into an issue with the thread pool. We queued up a couple of pieces of work, but we had no way of knowing when they were done. One solution to that would be to use the Task Parallel Library, which we ll get to shortly, but an alternative would have been to use the CountdownEvent class.

public ViewResult Index() { IEnumerable<CustomerSummary> summaries = _customerSummaries.GetAll(); return View(summaries); }

if you know how many pieces of work there will be up front, you can pass that number into the constructor). For each piece of work that completes you call Signal. And if you need to wait for all outstanding work to complete (e.g., before your program exits), just call Wait.

In this sense, a content type is the node s class, defining the characteristics of the node and the operations it can perform The article content type, for example, has several characteristics: a title, an image, a body, and category tags When an article node is created, the content type tells Drupal what to do with it, such as publish it to the front page, allow comments, set a custom URL path, create a revision, and so on Modules are used to tell Drupal what to do with a node Many of Drupal s most popular and powerful contributed modules are based upon content types The Ubercart e-commerce module is a great example When enabled, the module creates a content type called Product with a number of fields, including a product description, price, image, shipping weight, and more.

The System.Collections.Concurrent namespace provides various collection classes that are designed to be used in multithreaded environments. They look a little different from the normal collection classes because they are designed to be used without needing any locking, which means they can t offer features that rely on things staying consistent from one moment to the next. Numerical indexing is out, for example, because the number of items in the collection may change, as we saw when trying to use List<T> in a multithreaded fashion in Example 16-11. So these are not thread-safe versions of normal collection classes they are collections whose APIs are designed to support multithreaded use without the caller needing to use locks.

BlockingCollection is not just a multithreaded collection; it also offers associated co-

Once the CustomerSummary objects have been created, the controller passes them into the View() method, which transferring the objects to the view B. There s a special mechanism for sharing the model in ASP.NET MVC 2, and we ll cover it next.