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Explicitly Implementing an Event
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The System.Windows.Forms.Control type defines about 70 events . If the Control type implemented the events by allowing the compiler to implicitly generate the add and remove accessor methods and delegate fields, every Control object would have 70 delegate fields in it just for the events! Since most programmers care about just a few events, an enormous amount of memory would be wasted for each object created from a Control-derived type . By the way, the ASP .NET System.Web.UI.Control and the Windows Presentation Foundation (WPF) System.Windows.UIElement type also offer many events that most programmers do not use . In this section, I discuss how the C# compiler allows a class developer to explicitly implement an event, allowing the developer to control how the add and remove methods manipulate the callback delegates . I m going to demonstrate how explicitly implementing an event can be used to efficiently implement a class that offers many events . However, there are certainly other scenarios where you might want to explicitly implement a type s event . To efficiently store event delegates, each object that exposes events will maintain a collection (usually a dictionary) with some sort of event identifier as the key and a delegate list as the value . When a new object is constructed, this collection is empty . When interest in an event is registered, the event s identifier is looked up in the collection . If the event identifier is there, the new delegate is combined with the list of delegates for this event . If the event identifier isn t in the collection, the event identifier is added with the delegate . When the object needs to raise an event, the event identifier is looked up in the collection . If the collection doesn t have an entry for the event identifier, nothing has registered interest in the event and no delegates need to be called back . If the event identifier is in the collection, the delegate list associated with the event identifier is invoked . Implementing this design pattern is the responsibility of the developer who is designing the type that defines the events; the developer using the type has no idea how the events are implemented internally .
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Here is an example of how you could accomplish this pattern . First, I implemented an EventSet class that represents a collection of events and each event s delegate list as follows:
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using System; using System.Collections.Generic; // This class exists to provide a bit more type safety and // code maintainability when using EventSet public sealed class EventKey : Object { } public sealed class EventSet { // The private dictionary used to maintain EventKey -> Delegate mappings private readonly Dictionary<EventKey, Delegate> m_events = newDictionary<EventKey, Delegate>(); // Adds an EventKey -> Delegate mapping if it doesn't exist or // combines a delegate to an existing EventKey public void Add(EventKey eventKey, Delegate handler) { Monitor.Enter(m_events); Delegate d; m_events.TryGetValue(eventKey, out d); m_events[eventKey] = Delegate.Combine(d, handler); Monitor.Exit(m_events); } // Removes a delegate from an EventKey (if it exists) and // removes the EventKey -> Delegate mapping the last delegate is removed public void Remove(EventKey eventKey, Delegate handler) { Monitor.Enter(m_events); // Call TryGetValue to ensure that an exception is not thrown if // attempting to remove a delegate from an EventKey not in the set Delegate d; if (m_events.TryGetValue(eventKey, out d)) { d = Delegate.Remove(d, handler); // If a delegate remains, set the new head else remove the EventKey if (d != null) m_events[eventKey] = d; else m_events.Remove(eventKey); } Monitor.Exit(m_events); } // Raises the event for the indicated EventKey public void Raise(EventKey eventKey, Object sender, EventArgs e) { // Don't throw an exception if the EventKey is not in the set Delegate d; Monitor.Enter(m_events); m_events.TryGetValue(eventKey, out d); Monitor.Exit(m_events); if (d // // // // != null) { Because the dictionary can contain several different delegate types, it is impossible to construct a type-safe call to the delegate at compile time. So, I call the System.Delegate type's DynamicInvoke method, passing it the callback method's parameters as an array of
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