1316: Declaring a Generic Delegate Type in C#.NET

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Listing 1316: Declaring a Generic Delegate Type
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public delegate void EventHandler<T>(object sender, T e) where T : EventArgs;
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When you use EventHandler<T>, each class that requires a particular sender-EventArgs pattern need not declare its own delegate definition Instead, they can all share the same one, changing the thermostat example as shown in Listing 1317
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Listing 1317: Using Generics with Delegates
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public class Thermostat { public class TemperatureArgs: SystemEventArgs
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{ public TemperatureArgs( float newTemperature ) { NewTemperature = newTemperature; } public float NewTemperature { get{return _newTemperature;} set{_newTemperature = value;} } private float _newTemperature; } // TemperatureChangeHandler no longer needed // public delegate void TemperatureChangeHandler( // object sender, TemperatureArgs newTemperature); // Define the event publisher without using // TemperatureChangeHandler public event EventHandler<TemperatureArgs> OnTemperatureChange; public float CurrentTemperature { } private float _CurrentTemperature; }
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Listing 1317 assumes, of course, that EventHandler<T> is defined somewhere In fact, SystemEventHandler<T>, as just declared, is included in the 20 Framework Class Library Therefore, in the majority of circumstances when using events in C# 20 or later, it is not necessary to declare a custom delegate data type Note that SystemEventHandler<T> restricts T to derive from EventArgs using a constraint, exactly what was necessary to correspond with the general convention for the event declaration of C# 10
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ADVANCED TOPIC Event Internals Events restrict external classes from doing anything other than adding subscribing methods to the publisher via the += operator and then unsubscribing
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13: Events
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using the -= operator In addition, they restrict classes, other than the containing class, from invoking the event To do this the C# compiler takes the public delegate variable with its event keyword modifier and declares the delegate as private In addition, it adds a couple of methods and two special event blocks Essentially, the event keyword is a C# shortcut for generating the appropriate encapsulation logic Consider the example in the event declaration shown in Listing 1318
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Listing 1318: Declaring the OnTemperatureChange Event
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public class Thermostat { // Define the delegate data type public delegate void TemperatureChangeHandler( object sender, TemperatureArgs newTemperature); public event TemperatureChangeHandler OnTemperatureChange }
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When the C# compiler encounters the event keyword, it generates CIL code equivalent to the C# code shown in Listing 1319
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Listing 1319: C# Equivalent of the Event CIL Code Generated by the Compiler
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public class Thermostat { // Define the delegate data type public delegate void TemperatureChangeHandler( object sender, TemperatureArgs newTemperature); // Declaring the delegate field to save the // list of subscribers private TemperatureChangeHandler OnTemperatureChange; public void add_OnTemperatureChange( TemperatureChangeHandler handler) { SystemDelegateCombine(OnTemperatureChange, handler); } public void remove_OnTemperatureChange( TemperatureChangeHandler handler) { SystemDelegateRemove(OnTemperatureChange, handler); }
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public event TemperatureChangeHandler OnTemperatureChange { add { add_OnTemperatureChange(value) } remove { remove_OnTemperatureChange(value) } } }
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In other words, the code shown in Listing 1318 is the C# shorthand the compiler uses to trigger the code expansion shown in Listing 1319 The C# compiler first takes the original event definition and defines a private delegate variable in its place By doing this, the delegate becomes unavailable to any external class, even to classes derived from it Next, the C# compiler defines two methods, add_OnTemperatureChange() and remove_OnTemperatureChange(), where the OnTemperatureChange suffix is taken from the original name of the event These methods are responsible for implementing the += and -= assignment operators, respectively As Listing 1319 shows, these methods are implemented using the static SystemDelegateCombine() and SystemDelegateRemove() methods, discussed earlier in the chapter The first parameter passed to each of these methods is the private TemperatureChangeHandler delegate instance, OnTemperatureChange Perhaps the most curious part of the code generated from the event keyword is the last part The syntax is very similar to that of a property s getter and setter methods except that the methods are add and remove The add block takes care of handling the += operator on the event by passing the call to add_OnTemperatureChange() In a similar manner, the remove block operator handles the -= operator by passing the call on to remove_OnTemperatureChange It is important to notice the similarities between this code and the code generated for a property Readers will recall that the C# implementation of a property is to create get_<propertyname> and set_<propertyname> and then to pass calls to the get and set blocks on to these methods Clearly, the event syntax is very similar
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