MANAGED EXCEPTION HANDLING in Visual Basic .NET

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CHAPTER 14 MANAGED EXCEPTION HANDLING
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The same handler block can t be associated with different guarded blocks: .try Label1 to Label2 catch A handler Label3 to Label4 .try Label4 to Label5 catch B handler Label3 to Label4 // Illegal!
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If the EH clause is a filter type, the filter s actual handler must immediately follow the filter block. Since the filter block must end with the endfilter instruction, this rule can be formulated as the actual handler starts with the instruction after endfilter. If a guarded block has a finally or fault handler, the same block can have no other handler. If you need other handlers, you must declare another guarded block, encompassing the original guarded block and the handler: .try { .try { .try { // Code that needs finally, catch, and fault handlers ... leave KeepGoing } finally { ... endfinally } } catch [mscorlib]System.StackOverflowException { ... leave KeepGoing } } fault { ... endfault }
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CHAPTER
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Events and Properties
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vents and properties are special metadata components that are intended to make life easier for the high-level language compilers. The most intriguing feature of events and properties is that the JIT compiler and the execution engine are completely unaware of them. Can you recall any IL instruction that deals with an event or a property That s because none exist. To understand the indifference of the JIT compiler and the execution engine toward events and properties, you need to understand the way these items are implemented.
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Events and Delegates
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The managed events I m talking about here are not synchronization elements, similar to Win32/Win64 event objects. Rather, they more closely resemble Visual Basic events and On<event> functions. Managed events provide a means to describe the asynchronous execution of methods, initiated by certain other methods. Figure 15-1 illustrates the general sequence of activities. A program unit known as the publisher, or source, of an event defines the event. We can think of this program unit as a class, for the sake of simplicity. Other program units (classes) known as subscribers, event listeners, or event sinks define the methods to be executed when the event occurs and pass this information to the event publisher. When the event publisher raises, or fires, the event by calling a special method, all the subscriber s methods associated with this event are executed. In a nutshell, to implement a managed event, we need an entity that can collect the callback methods (event handlers) from the event subscribers and execute these methods when the publisher executes a method that signifies the event. We have a type (a class) designed to do exactly that: the delegate type, which is discussed in 7. As you might remember, delegates are classes derived from the class [mscorlib] System.MulticastDelegate and play the role of politically correct type-safe function pointers in the managed world. The actual function pointer to a delegated method is passed to the delegate as an argument of its constructor, and the delegated method can subsequently be executed by calling the delegate s Invoke method. What 7 doesn t mention is that several delegates can be aggregated into one delegate. Calling the Invoke method of such an aggregated delegate invokes all the delegated methods that make up the aggregate which is exactly what we need to implement an event. The [mscorlib]System.MulticastDelegate class defines the virtual methods CombineImpl and RemoveImpl, adding a delegate (more exactly, the callback function pointer it carries) to the aggregate (more exactly, to the list of callback function pointers the aggregate carries) and
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CHAPTER 15 EVENTS AND PROPERTIES
removing a delegate from the aggregate, respectively. These methods are defined in Mscorlib.dll as follows. (I have omitted the resolution scope [mscorlib] of delegate types here because the methods are defined in the same assembly; this doesn t mean you can omit the resolution scope when you refer to these types in your assemblies.)
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