12: Delegates and Lambda Expressions in Visual C#

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12: Delegates and Lambda Expressions
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SUMMARY
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This chapter began with a discussion of delegates and their use as references to methods or callbacks It introduced a powerful concept for passing a set of instructions to call in a different location, rather than immediately, when the instructions are coded Following on the heels of a brief look at the C# 20 concept of anonymous methods, the chapter introduced the C# 30 concept of lambda expressions, a syntax which supersedes (although doesn t eliminate) the C# 20 anonymous method syntax Regardless of the syntax, these constructs allow programmers to assign a set of instructions to a variable directly, without defining an explicit method that contains the instructions This provides significant flexibility for programming instructions dynamically within the method a powerful concept that greatly simplifies the programming of collections through an API known as LINQ, which stands for Language Integrated Query Finally, the chapter ended with the concept of expression trees, and how they compile into data that represents a lambda expression, rather than the delegate implementation itself This is a key feature that enables such libraries as LINQ to SQL and LINQ to XML, libraries that interpret the expression tree and use it within contexts other than CIL The term lambda expression encompasses both statement lambda and expression lambda In other words, both statement lambdas and expression lambdas are types of lambda expressions One thing that the chapter mentioned but did not elaborate on was multicast delegates The next chapter investigates multicast delegates in detail and explains how they enable the publish-subscribe pattern with events
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you saw how to store a single method inside an instance of a delegate type and invoke that method via the delegate Delegates comprise the building blocks of a larger pattern called publishsubscribe The use of delegates and their support for publish-subscribe patterns is the focus of this chapter Virtually everything described within this chapter is possible to do using delegates alone However, the event constructs that this chapter focuses on providing additional encapsulation, making the publish-subscribe pattern easier to implement and less error-prone
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N THE PRECEDING CHAPTER,
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Customizing the Event Implementation
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2 Events
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Generics and Delegates
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In the preceding chapter, all delegates were for a single callback (a multiplicity of one) However, a single delegate variable can reference a series
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13: Events
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of delegates in which each successive one points to a succeeding delegate in the form of a chain, sometimes known as a multicast delegate With a multicast delegate, you can invoke a method chain via a single method object, create variables that refer to a method s chain, and pass the delegates as parameters to pass methods The C# implementation of multicast delegates is a common pattern that would otherwise require significant manual code Known as the observer or publish-subscribe pattern, it represents scenarios where notifications of single events, such as a change in object state, are broadcast to multiple subscribers
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Consider a temperature control example, where a heater and a cooler are hooked up to the same thermostat In order for a unit to turn on and off appropriately, you notify the unit of changes in temperature One thermostat publishes temperature changes to multiple subscribers the heating and cooling units The next section investigates the code1
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Defining Subscriber Methods Begin by defining the Heater and Cooler objects (see Listing 131)
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Listing 131: Heater and Cooler Event Subscriber Implementations
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class Cooler { public Cooler(float temperature) { Temperature = temperature; } public float Temperature { get{return _Temperature;} set{_Temperature = value;} } private float _Temperature;
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1 In this example, I use the term thermostat because people more commonly think of it in the context of heating and cooling systems Technically, however, thermometer would be more appropriate
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public void OnTemperatureChanged(float newTemperature) { if (newTemperature > Temperature) { SystemConsoleWriteLine("Cooler: On"); } else { SystemConsoleWriteLine("Cooler: Off"); } } } class Heater { public Heater(float temperature) { Temperature = temperature; } public float Temperature { get{return _Temperature;} set{_Temperature = value;} } private float _Temperature; public void OnTemperatureChanged(float newTemperature) { if (newTemperature < Temperature) { SystemConsoleWriteLine("Heater: On"); } else { SystemConsoleWriteLine("Heater: Off"); } } }
The two classes are essentially identical, with the exception of the temperature comparison (In fact, you could eliminate one of the classes if you used a delegate as a method pointer for comparison within the