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In order to associate the event handlers with instance methods, rather than static methods, we must first create an instance of the Form1 class. We pass this instance to the Application.Run method at the end of the static Main method. We first add a handler for the ThreadExit event. We discuss the ThreadExit event in the next section. After we ve added the handler for the ThreadExit event, we add a handler for the Application object s ThreadException event. This event is invoked only when the exception occurs on the main thread. If a thread is created that causes an unhandled exception to be raised, this event will not be notified. We add an UnhandledException event handler to the current domain. This event handler will be invoked when any thread in the current domain experiences an unhandled exception. If a ThreadException event handler has been added to the Application object, it will handle any exceptions that occur on the main thread. This means that AppDomain s UnhandledException will not be invoked. Each of these event handlers do slightly different handling. The AppDomain handler is a bit more flexible in that it catches all exceptions that occur. These events should not be used in place of proper exception handling. Exceptions and exception handling were discussed in-depth in chapter 13. The ThreadExit event The ThreadExit event is similar to the ThreadException event in that it only applies to the main thread. If a thread other than the main thread exits, this event will not be raised. This event is raised when an application is terminating. The ThreadExit event is invoked after the Form Closing and Closed events are invoked. The ApplicationExit event is invoked after the ThreadExit event. The order of events during application termination is as follows: Closing, Closed, ThreadExit, ApplicationExit. The MessageLoop property Early in this chapter we briefly discussed the concept of message pumps, also known as message queues or message loops. The application object s MessageLoop property allows us to determine if a thread contains a message loop. The following instruction prints out true or false depending on whether the thread it is executed on contains a message loop:
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System.Diagnostics.Debug.WriteLine(Application.MessageLoop.ToString());
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This is useful in determining how a thread will behave. For example, if the thread does not contain a message loop, message-based timers will not work. In that circumstance, one of the other timers will be required.
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In this chapter we ve covered some of the issues related to multithreaded development in Windows Forms applications. Combining Windows Forms with multiple threads can lead to powerful applications. We discussed the problems relating to multithreaded development and also covered the use of Invoke to resolve those issues. We introduced the Graphics object and saw how it can be used to render objects onto a form. Finally, we discussed thread-related events and properties of the Application object.
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16.1 What is an apartment 267 16.2 COM interoperability 268 16.3 Summary 274
Apartments are COM constructs used to resolve concurrency control issues. Rather than forcing COM developers to use synchronization primitives, Microsoft introduced the apartment concept. This allowed for easy development of reusable components with minimal concern about concurrency control. This chapter is not intended to be a primer on COM programming. Instead, it examines the interaction of .NET with COM from a multithreaded perspective. An important thing to understand is that .NET does not use apartments for concurrency control. However, they are used when interacting with COM objects. Interaction with COM from .NET is generally referred to as interop, short for interoperability.
WHAT IS AN APARTMENT
Many developers introduction to multithreaded development involved the concept of an apartment. An apartment is based on a building metaphor. The process is comparable to a building that has one or more apartments. Restriction to an apartment is based on the type of apartment it is. The most common apartments are single and multithreaded.
Single-threaded apartment model (STA) The majority of COM objects produced are designed to execute inside an STA. The primary reason for this is that most COM objects have been developed using Visual Basic. Visual Basic produces COM objects that execute in an STA. In this text we will refer to objects that are designed and marked to execute in an STA as an STA object. Visual Basic makes it incredibly easy to produce COM objects and is partly responsible for the wide acceptance of COM. When an object is marked as an object that executes in an STA, it means that only one thread can access that object. Additionally, when that object is executing in an STA, if that object is accessed more than once, the same thread must access it each time. This allows the developers of STA objects to make use of thread local storage as a means of persisting state. Additionally, because only one thread is accessing the objects, concurrency control is no longer a concern. Since these STA objects are relatively simple, they are much easier to write than an object that executes in a multithreaded apartment (MTA). To make things a little more complex, several names for the same thing are often used. STA objects are often referred to as apartment threaded. This is somewhat misleading since every object in COM executes in an apartment. The question is how many threads can interact with an object contained within a certain apartment. If the answer is one, the apartment is STA. MTA When an apartment allows more than one thread to interact with the objects contained within it that apartment is known as an MTA. Just as STA is sometimes referred to as apartment threaded, MTA is sometimes referred to as free threaded. Objects that are marked as being free threaded will execute in an MTA. Additionally, objects can be marked as Both, meaning that they can execute in both an STA and an MTA. A process will contain at most one MTA. This means that all MTA objects within the process will execute in a shared MTA.
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