Sequential Invocation in Visual C#

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Sequential Invocation
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The process of notifying both heater and cooler appears in Figure 132
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Figure 132 Delegate Invocation Sequence Diagram
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Although you coded only a single call to OnTemperatureChange(), the call is broadcast to both subscribers so that from that one call, both cooler and heater are notified of the change in temperature If you added more subscribers, they too would be notified by OnTemperatureChange() Although a single call, OnTemperatureChange(), caused the notification of each subscriber, they are still called sequentially, not simultaneously, because a single delegate can point to another delegate that can, in turn, point to additional delegates
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Advanced Topic: Multicast Delegate Internals
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To understand how events work, you need to revisit the first examination of the SystemDelegate type internals Recall that the delegate keyword is an alias for a type derived from SystemMulticastDelegate In turn, SystemMulticastDelegate is derived from SystemDelegate, which, for its part, comprises an object reference and a method pointer (of type SystemReflectionMethodInfo ) When you create a delegate, the compiler automatically employs the SystemMulticastDelegate type rather than the SystemDelegate type The MulticastDelegate class includes an object reference and method pointer, just like its Delegate base class, but it also contains a reference to another SystemMulticastDelegate object When you add a method to a multicast delegate, the MulticastDelegate class creates a new instance of the delegate type, stores the object reference and the method pointer for the added method into the new instance, and adds the new delegate instance as the next item in a list of delegate instances In effect, the MulticastDelegate class maintains a linked list of Delegate objects Conceptually, you can represent the thermostat example as shown in Figure 133
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Figure 133 Multicast Delegates Chained Together
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When invoking the multicast, each delegate instance in the linked list is called sequentially Generally, delegates are called in the order they were added, but this behavior is not specified within the CLI specification, and furthermore, it can be overridden Therefore, programmers should not depend on an invocation order
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Error Handling
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Error handling makes awareness of the sequential notification critical If one subscriber throws an exception, later subscribers in the chain do not receive the notification Consider, for example, if you changed the Heater's OnTemperatureChanged() method so that it threw an exception, as shown in Listing 1323
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Listing 1323 OnTemperatureChanged() Throwing an Exception
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class Heater { public void OnTemperatureChanged(float newTemperature) { throw new NotImplementedException(); } }
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Figure 134 shows an updated sequence diagram Even though cooler subscribed to receive messages, the heater exception terminates the chain and prevents the cooler object from receiving
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notification
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Figure 134 Delegate Invocation with Exception Sequence Diagram
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To avoid this problem so that all subscribers receive notification, regardless of the behavior of earlier subscribers, you must manually enumerate through the list of subscribers and call them individually Listing 1324 shows the updates required in the CurrentTemperature property The results appear in Output 134
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Listing 1324 Handling Exceptions from Subscribers
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public class Thermostat { // Define the delegate data type public delegate void TemperatureChangeHandler( float newTemperature); // Define the event publisher public event TemperatureChangeHandler OnTemperatureChange; public float CurrentTemperature { get{return _CurrentTemperature;} set { if (value != CurrentTemperature) { _CurrentTemperature = value; if(OnTemperatureChange != null) { foreach( TemperatureChangeHandler handler in OnTemperatureChangeGetInvocationList() ) { try
{ handler(value); } catch(Exception exception) { ConsoleWriteLine(exceptionMessage); } } } } } } private float _CurrentTemperature; }
Output 134
Enter temperature: 45 The method or operation is not implemented Cooler: Off
This listing demonstrates that you can retrieve a list of subscribers from a delegate's GetInvocationList() method Enumerating over each item in this list returns the individual subscribers If you then place each invocation of a subscriber within a try/catch block, you can handle any error conditions before continuing with the enumeration loop In this sample, even though heaterOnTemperatureChanged() throws an exception, cooler still receives notification of the temperature change
Method Returns and Pass-By-Reference
There is another scenario where it is useful to iterate over the delegate invocation list instead of simply activating a notification directly This scenario relates to delegates that either don't return void or have ref or out parameters In the thermostat example so far, the OnTemperatureHandler delegate had a return type of void Furthermore, it did not include any parameters that were ref or out type parameters, parameters that return data to the caller This is important because an invocation of a delegate potentially triggers notification to multiple subscribers If the subscribers return a value, it is ambiguous which subscriber's return value would be used If you changed OnTemperatureHandler to return an enumeration value, indicating whether the device was on because of the temperature change, the new delegate would look like Listing 1325