13: Events in Visual C#

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13: Events
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Another important characteristic to note about the generated CIL code is that the CIL equivalent of the event keyword remains in the CIL In other words, an event is something the CIL code recognizes explicitly; it is not just a C# construct By keeping an equivalent event keyword in the CIL code, all languages and editors are able to provide special functionality because they can recognize the event as a special class member
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Customizing the Event Implementation You can customize the code for += and -= that the compiler generates Consider, for example, changing the scope of the OnTemperatureChange delegate so it is protected rather than private This, of course, would allow classes derived from Thermostat to access the delegate directly instead of being limited to the same restrictions as external classes To enable this, C# allows the same property as the syntax shown in Listing 1317 In other words, C# allows you to define custom add and remove blocks to provide implementation for each aspect of the event encapsulation Listing 1320 provides an example
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Listing 1320: Custom add and remove Handlers
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public class Thermostat { public class TemperatureArgs: SystemEventArgs { } // Define the delegate data type public delegate void TemperatureChangeHandler( object sender, TemperatureArgs newTemperature);
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// Define the event publisher public event TemperatureChangeHandler OnTemperatureChange { add { SystemDelegateCombine(value, _OnTemperatureChange); } remove { SystemDelegateRemove(_OnTemperatureChange, value); } } protected TemperatureChangeHandler _OnTemperatureChange;
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public float CurrentTemperature { } private float _CurrentTemperature; }
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In this case, the delegate that stores each subscriber, _OnTemperatureChange, was changed to protected In addition, implementation of the add block switches around the delegate storage so that the last delegate added to the chain is the first delegate to receive a notification
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Now that you have described events, it is worth mentioning that in general, method pointers are the only cases where it is advisable to work with a delegate variable outside the context of an event In other words, given the additional encapsulation features of an event and the ability to customize the implementation when necessary, the best practice is always to use events for the observer pattern It may take a little practice to be able to code events from scratch without sample code However, they are a critical foundation to the asynchronous, multithreaded coding of later chapters
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Collection Interfaces with Standard Query Operators
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added in C# 30 were in the area of collections Extension methods and lambda expressions enabled a far superior API for working with collections In fact, in earlier editions of this book, the chapter on collections came immediately after the chapter on generics and just before the one on delegates However, lambda expressions make such a significant impact on collection APIs that it is no longer possible to cover collections without first covering delegates (the basis of lambda expressions) Now that you have a solid foundation on lambda expressions from the preceding chapter, we can delve into the details of collections, a topic that in this edition spans three chapters
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HE MOST SIGNIFICANT FEATURES
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Standard Query Operators
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Collection Interfaces with Standard Query Operators 3
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Collection Initializers
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Implicit Local Variables
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14: Collection Interfaces with Standard Query Operators
To begin, this chapter introduces anonymous types and collection initializers, topics which I covered only briefly in a few Advanced Topic sections in 5 Next, this chapter covers the various collection interfaces and how they relate to each other This is the basis for understanding collections, so readers should cover the material with diligence The section on collection interfaces includes coverage of the IEnumerable<T> extension methods that were added C# 30, which provides the foundation on which standard query operators are implemented another C# 30 feature discussed in the chapter There are two categories of collection-related classes and interfaces: those that support generics and those that don t This chapter primarily discusses the generic collection interfaces You should use collection classes that don t support generics only when writing components that need to interoperate with earlier versions of the runtime This is because everything that was available in the nongeneric form has a generic replacement that is strongly typed For Essential C# 20, I called out both the generic and the nongeneric versions of classes and interfaces However, now that we are at C# 30, I leave out discussion of the nongeneric types, which were virtually deprecated in favor of their generic equivalents Although the concepts still apply to both forms, I will not explicitly call out the names of the nongeneric versions