13: Events in Visual C#.NET

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13: Events
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the field declaration This simple change provides all the encapsulation needed By adding the event keyword, you prevent use of the assignment operator on a public delegate field (for example, thermostatOnTemperatureChange = coolerOnTemperatureChanged) In addition, only the containing class is able to invoke the delegate that triggers the publication to all subscribers (for example, disallowing thermostatOnTemperatureChange(42) from outside the class) In other words, the event keyword provides the needed encapsulation that prevents any external class from publishing an event or unsubscribing previous subscribers they did not add This resolves the two issues with plain delegates and is one of the key reasons for the event keyword in C#
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Coding Conventions All you need to do to gain the desired functionality is to take the original delegate variable declaration, change it to a field, and add the event keyword With these two changes, you provide the necessary encapsulation and all other functionality remains the same However, an additional change occurs in the delegate declaration in the code in Listing 1314 To follow standard C# coding conventions, you changed OnTemperatureChangeHandler so that the single temperature parameter was replaced with two new parameters, sender and temperatureArgs This change is not something that the C# compiler will enforce, but passing two parameters of these types is the norm for declaring a delegate intended for an event The first parameter, sender, should contain an instance of the class that invoked the delegate This is especially helpful if the same subscriber method registers with multiple events for example, if the heaterOnTemperatureChanged event subscribes to two different Thermostat instances In such a scenario, either Thermostat instance can trigger a call to heaterOnTemperatureChanged In order to determine which instance of Thermostat triggered the event, you use the sender parameter from inside HeaterOnTemperatureChanged() The second parameter, temperatureArgs, is of type ThermostatTemperatureArgs Using a nested class is appropriate because it conforms to the same scope as the OnTemperatureChangeHandler delegate itself The
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important part about TemperatureArgs, at least as far as the coding convention goes, is that it derives from SystemEventArgs The only significant property on SystemEventArgs is Empty and it is used to indicate that there is no event data When you derive TemperatureArgs from SystemEventArgs, however, you add an additional property, NewTemperature, as a means to pass the temperature from the thermostat to the subscribers To summarize the coding convention for events: The first argument, sender, is of type object and it contains a reference to the object that invoked the delegate The second argument is of type SystemEventArgs or something that derives from SystemEventArgs but contains additional data about the event You invoke the delegate exactly as before, except for the additional parameters Listing 1315 shows an example
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Listing 1315: Firing the Event Notification
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public class Thermostat { public float CurrentTemperature { get{return _CurrentTemperature;} set { if (value != CurrentTemperature) { _CurrentTemperature = value; // If there are any subscribers // then notify them of changes in // temperature if(OnTemperatureChange != null) { // Call subscribers OnTemperatureChange( this, new TemperatureArgs(value) ); } } } } private float _CurrentTemperature; }
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You usually specify the sender using the container class (this) because that is the only class that can invoke the delegate for events
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13: Events
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In this example, the subscriber could cast the sender parameter to Thermostat and access the current temperature that way, as well as via the TemperatureArgs instance However, the current temperature on the Thermostat instance may change via a different thread In the case of events that occur due to state changes, passing the previous value along with the new value is a frequent pattern used to control what state transitions are allowable
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Generics and Delegates The preceding section mentioned that the typical pattern for defining delegate data is to specify the first parameter, sender, of type object and the second parameter, eventArgs, to be a type deriving from SystemEventArgs One of the more cumbersome aspects of delegates in C# 10 is that you have to declare a new delegate type whenever the parameters on the handler change Every creation of a new derivation from SystemEventArgs (a relatively common occurrence) required the declaration of a new delegate data type that uses the new EventArgs derived type For example, in order to use TemperatureArgs within the event notification code in Listing 1315, it is necessary to declare the delegate type TemperatureChangeHandler that has TemperatureArgs as a parameter With generics, you can use the same delegate data type in many locations with a host of different parameter types, and remain strongly typed Consider the delegate declaration example shown in Listing 1316
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