Calling Static Functions. For our next, more advanced example SimpleDelegate2. LogHandler Logger ; myClass. Calling Member Functions. In the simple example above, the Logger function merely writes the string out.
A different function might want to log the information to a file, but to do this, the function needs to know what file to write the information to SimpleDelegate3. IO; namespace Akadia. Close ; fileStream. LogHandler fl. Logger ; myClass. Process myLogger ; fl. The cool part here is that we didn't have to change the Process function; the code to all the delegate is the same regardless of whether it refers to a static or member function.
Being able to point to member functions is nice, but there are more tricks you can do with delegates. In C , delegates are multicast , which means that they can point to more than one function at a time that is, they're based off the System.
MulticastDelegate type. A multicast delegate maintains a list of functions that will all be called when the delegate is invoked. We can add back in the logging function from the first example, and call both delegates. DataReceived event. NET follows a naming pattern of ending all event data classes with EventArgs. You determine which event data class is associated with an event by looking at the delegate for the event.
The EventArgs class is the base type for all event data classes. EventArgs is also the class you use when an event does not have any data associated with it. When you create an event that is only meant to notify other classes that something happened and does not need to pass any data, include the EventArgs class as the second parameter in the delegate.
You can pass the EventArgs. Empty value when no data is provided. The EventHandler delegate includes the EventArgs class as a parameter. When you want to create a customized event data class, create a class that derives from EventArgs , and then provide any members needed to pass data that is related to the event.
Typically, you should use the same naming pattern as. NET and end your event data class name with EventArgs. The following example shows an event data class named ThresholdReachedEventArgs. It contains properties that are specific to the event being raised.
To respond to an event, you define an event handler method in the event receiver. This method must match the signature of the delegate for the event you are handling. In the event handler, you perform the actions that are required when the event is raised, such as collecting user input after the user clicks a button. To receive notifications when the event occurs, your event handler method must subscribe to the event. The method subscribes to the ThresholdReached event.
NET allows subscribers to register for event notifications either statically or dynamically. Static event handlers are in effect for the entire life of the class whose events they handle. A subscriber can handle multiple events from multiple publishers. Events are typically used to signal user actions such as button clicks or menu selections in graphical user interfaces. When an event has multiple subscribers, the event handlers are invoked synchronously when an event is raised.
To invoke events asynchronously, see Calling Synchronous Methods Asynchronously. In the. How to subscribe to and unsubscribe from events. How to publish events that conform to. NET Guidelines. How to raise base class events in derived classes.
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