Events

1. Handling and Raising Events
2. How to: Raise and Consume Events
3. How to: Handle Multiple Events Using Event Properties

https://msdn.microsoft.com/en-us/library/edzehd2t(v=vs.110).aspx

Handling and Raising Events

Events in the .NET Framework are based on the delegate model. The delegate model follows the observer design pattern, which enables a subscriber to register with, and receive notifications from, a provider. An event sender pushes a notification that an event has happened, and an event receiver receives that notification and defines a response to it. This article describes the major components of the delegate model, how to consume events in applications, and how to implement events in your code.

For information about handling events in Windows 8.x Store apps, see Events and routed events overview (Windows store apps).

Events

An event is a message sent by an object to signal the occurrence of an action. The action could be caused by user interaction, such as a button click, or it could be raised by some other program logic, such as changing a property’s value. The object that raises the event is called the event sender. The event sender doesn't know which object or method will receive (handle) the events it raises. The event is typically a member of the event sender; for example, the Click event is a member of the Button class, and the PropertyChanged event is a member of the class that implements the INotifyPropertyChanged interface.

To define an event, you use the event (in C#) or Event (in Visual Basic) keyword in the signature of your event class, and specify the type of delegate for the event. Delegates are described in the next section.

Typically, to raise an event, you add a method that is marked as protected and virtual (in C#) or Protected and Overridable (in Visual Basic). Name this method OnEventName; for example, OnDataReceived. The method should take one parameter that specifies an event data object. You provide this method to enable derived classes to override the logic for raising the event. A derived class should always call the OnEventName method of the base class to ensure that registered delegates receive the event.

The following example shows how to declare an event named ThresholdReached. The event is associated with the EventHandler delegate and raised in a method named OnThresholdReached.

C#

VB

    class Counter
    {
        public event EventHandler ThresholdReached;

        protected virtual void OnThresholdReached(EventArgs e)
        {
            EventHandler handler = ThresholdReached;
            if (handler != null)
            {
                handler(this, e);
            }
        }

        // provide remaining implementation for the class
    }

Delegates

A delegate is a type that holds a reference to a method. A delegate is declared with a signature that shows the return type and parameters for the methods it references, and can hold references only to methods that match its signature. A delegate is thus equivalent to a type-safe function pointer or a callback. A delegate declaration is sufficient to define a delegate class.

Delegates have many uses in the .NET Framework. In the context of events, a delegate is an intermediary (or pointer-like mechanism) between the event source and the code that handles the event. You associate a delegate with an event by including the delegate type in the event declaration, as shown in the example in the previous section. For more information about delegates, see the Delegate class.

The .NET Framework provides the EventHandler and EventHandler<TEventArgs> delegates to support most event scenarios. Use the EventHandler delegate for all events that do not include event data. Use the EventHandler<TEventArgs> delegate for events that include data about the event. These delegates have no return type value and take two parameters (an object for the source of the event and an object for event data).

Delegates are multicast, which means that they can hold references to more than one event-handling method. For details, see the Delegate reference page. Delegates provide flexibility and fine-grained control in event handling. A delegate acts as an event dispatcher for the class that raises the event by maintaining a list of registered event handlers for the event.

For scenarios where the EventHandler and EventHandler<TEventArgs> delegates do not work, you can define a delegate. Scenarios that require you to define a delegate are very rare, such as when you must work with code that does not recognize generics. You mark a delegate with the delegate in (C#) and Delegate (in Visual Basic) keyword in the declaration. The following example shows how to declare a delegate named ThresholdReachedEventHandler.

C#

VB

    public delegate void ThresholdReachedEventHandler(ThresholdReachedEventArgs e);

Event Data

Data that is associated with an event can be provided through an event data class. The .NET Framework provides many event data classes that you can use in your applications. For example, the SerialDataReceivedEventArgs class is the event data class for the SerialPort.DataReceived event. The .NET Framework 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. For example, the SerialDataReceivedEventHandler delegate includes the SerialDataReceivedEventArgs class as one of its parameters.

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 the .NET Framework 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.

C#

VB

    public class ThresholdReachedEventArgs : EventArgs
    {
        public int Threshold { get; set; }
        public DateTime TimeReached { get; set; }
    }

Event Handlers

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 following example shows an event handler method named c_ThresholdReached that matches the signature for the EventHandler delegate. The method subscribes to the ThresholdReached event.

C#

VB

    class Program
    {
        static void Main(string[] args)
        {
            Counter c = new Counter();
            c.ThresholdReached += c_ThresholdReached;

            // provide remaining implementation for the class
        }

        static void c_ThresholdReached(object sender, EventArgs e)
        {
            Console.WriteLine("The threshold was reached.");
        }
    }

Static and Dynamic Event Handlers

The .NET Framework 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. Dynamic event handlers are explicitly activated and deactivated during program execution, usually in response to some conditional program logic. For example, they can be used if event notifications are needed only under certain conditions or if an application provides multiple event handlers and run-time conditions define the appropriate one to use. The example in the previous section shows how to dynamically add an event handler. For more information, see Events and Events.

Raising Multiple Events

If your class raises multiple events, the compiler generates one field per event delegate instance. If the number of events is large, the storage cost of one field per delegate may not be acceptable. For those situations, the .NET Framework provides event properties that you can use with another data structure of your choice to store event delegates.

Event properties consist of event declarations accompanied by event accessors. Event accessors are methods that you define to add or remove event delegate instances from the storage data structure. Note that event properties are slower than event fields, because each event delegate must be retrieved before it can be invoked. The trade-off is between memory and speed. If your class defines many events that are infrequently raised, you will want to implement event properties. For more information, see How to: Handle Multiple Events Using Event Properties.

Related Topics

Title	Description
How to: Raise and Consume Events	Contains examples of raising and consuming events.
How to: Handle Multiple Events Using Event Properties	Shows how to use event properties to handle multiple events.
Observer Design Pattern	Describes the design pattern that enables a subscriber to register with, and receive notifications from, a provider.
How to: Consume Events in a Web Forms Application	Shows how to handle an event that is raised by a Web Forms control.

How to: Raise and Consume Events

The examples in this topic show how to work with events. They include examples of the EventHandler delegate, the EventHandler<TEventArgs> delegate, and a custom delegate, to illustrate events with and without data.

The examples use concepts described in the Events article.

Example

The first example shows how to raise and consume an event that doesn't have data. It contains a class named Counter that has an event named ThresholdReached. This event is raised when a counter value equals or exceeds a threshold value. The EventHandler delegate is associated with the event, because no event data is provided.

C#

VB

using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Counter c = new Counter(new Random().Next(10));
            c.ThresholdReached += c_ThresholdReached;

            Console.WriteLine("press 'a' key to increase total");
            while (Console.ReadKey(true).KeyChar == 'a')
            {
                Console.WriteLine("adding one");
                c.Add(1);
            }
        }

        static void c_ThresholdReached(object sender, EventArgs e)
        {
            Console.WriteLine("The threshold was reached.");
            Environment.Exit(0); 
        }
    }

    class Counter
    {
        private int threshold;
        private int total;

        public Counter(int passedThreshold)
        {
            threshold = passedThreshold;
        }

        public void Add(int x)
        {
            total += x;
            if (total >= threshold)
            {
                OnThresholdReached(EventArgs.Empty);
            }
        }

        protected virtual void OnThresholdReached(EventArgs e)
        {
            EventHandler handler = ThresholdReached;
            if (handler != null)
            {
                handler(this, e);
            }
        }

        public event EventHandler ThresholdReached;
    }
}

Example

The next example shows how to raise and consume an event that provides data. The EventHandler<TEventArgs> delegate is associated with the event, and an instance of a custom event data object is provided.

C#

C++

VB

using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Counter c = new Counter(new Random().Next(10));
            c.ThresholdReached += c_ThresholdReached;

            Console.WriteLine("press 'a' key to increase total");
            while (Console.ReadKey(true).KeyChar == 'a')
            {
                Console.WriteLine("adding one");
                c.Add(1);
            }
        }

        static void c_ThresholdReached(object sender, ThresholdReachedEventArgs e)
        {
            Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold,  e.TimeReached);
            Environment.Exit(0);
        }
    }

    class Counter
    {
        private int threshold;
        private int total;

        public Counter(int passedThreshold)
        {
            threshold = passedThreshold;
        }

        public void Add(int x)
        {
            total += x;
            if (total >= threshold)
            {
                ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();
                args.Threshold = threshold;
                args.TimeReached = DateTime.Now;
                OnThresholdReached(args);
            }
        }

        protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
        {
            EventHandler<ThresholdReachedEventArgs> handler = ThresholdReached;
            if (handler != null)
            {
                handler(this, e);
            }
        }

        public event EventHandler<ThresholdReachedEventArgs> ThresholdReached;
    }

    public class ThresholdReachedEventArgs : EventArgs
    {
        public int Threshold { get; set; }
        public DateTime TimeReached { get; set; }
    }
}

Example

The next example shows how to declare a delegate for an event. The delegate is named ThresholdReachedEventHandler. This is just an illustration. Typically, you do not have to declare a delegate for an event, because you can use either the EventHandler or the EventHandler<TEventArgs> delegate. You should declare a delegate only in rare scenarios, such as making your class available to legacy code that cannot use generics.

C#

VB

using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Counter c = new Counter(new Random().Next(10));
            c.ThresholdReached += c_ThresholdReached;

            Console.WriteLine("press 'a' key to increase total");
            while (Console.ReadKey(true).KeyChar == 'a')
            {
                Console.WriteLine("adding one");
                c.Add(1);
            }
        }

        static void c_ThresholdReached(Object sender, ThresholdReachedEventArgs e)
        {
            Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached);
            Environment.Exit(0);
        }
    }

    class Counter
    {
        private int threshold;
        private int total;

        public Counter(int passedThreshold)
        {
            threshold = passedThreshold;
        }

        public void Add(int x)
        {
            total += x;
            if (total >= threshold)
            {
                ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();
                args.Threshold = threshold;
                args.TimeReached = DateTime.Now;
                OnThresholdReached(args);
            }
        }

        protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
        {
            ThresholdReachedEventHandler handler = ThresholdReached;
            if (handler != null)
            {
                handler(this, e);
            }
        }

        public event ThresholdReachedEventHandler ThresholdReached;
    }

    public class ThresholdReachedEventArgs : EventArgs
    {
        public int Threshold { get; set; }
        public DateTime TimeReached { get; set; }
    }

    public delegate void ThresholdReachedEventHandler(Object sender, ThresholdReachedEventArgs e);
}

How to: Handle Multiple Events Using Event Properties

To use event properties, you define the event properties in the class that raises the events, and then set the delegates for the event properties in classes that handle the events. To implement multiple event properties in a class, the class must internally store and maintain the delegate defined for each event. A typical approach is to implement a delegate collection that is indexed by an event key.

To store the delegates for each event, you can use the EventHandlerList class, or implement your own collection. The collection class must provide methods for setting, accessing, and retrieving the event handler delegate based on the event key. For example, you could use a Hashtable class, or derive a custom class from the DictionaryBase class. The implementation details of the delegate collection do not need to be exposed outside your class.

Each event property within the class defines an add accessor method and a remove accessor method. The add accessor for an event property adds the input delegate instance to the delegate collection. The remove accessor for an event property removes the input delegate instance from the delegate collection. The event property accessors use the predefined key for the event property to add and remove instances from the delegate collection.

To handle multiple events using event properties

1. Define a delegate collection within the class that raises the events.

2. Define a key for each event.

3. Define the event properties in the class that raises the events.

4. Use the delegate collection to implement the add and remove accessor methods for the event properties.

5. Use the public event properties to add and remove event handler delegates in the classes that handle the events.

Example

The following C# example implements the event properties MouseDown and MouseUp, using an EventHandlerList to store each event's delegate. The keywords of the event property constructs are in bold type.

Note
Event properties are not supported in Visual Basic 2005.

C#

C++

VB

// The class SampleControl defines two event properties, MouseUp and MouseDown.
class SampleControl : Component
{
    // :
    // Define other control methods and properties.
    // :

    // Define the delegate collection.
    protected EventHandlerList listEventDelegates = new EventHandlerList();

    // Define a unique key for each event.
    static readonly object mouseDownEventKey = new object();
    static readonly object mouseUpEventKey = new object();

    // Define the MouseDown event property.
    public event MouseEventHandler MouseDown
    {
        // Add the input delegate to the collection.
        add
        {
            listEventDelegates.AddHandler(mouseDownEventKey, value);
        }
        // Remove the input delegate from the collection.
        remove
        {
            listEventDelegates.RemoveHandler(mouseDownEventKey, value);
        }
    }

    // Raise the event with the delegate specified by mouseDownEventKey
    private void OnMouseDown(MouseEventArgs e)
    {
        MouseEventHandler mouseEventDelegate =
            (MouseEventHandler)listEventDelegates[mouseDownEventKey];
        mouseEventDelegate(this, e);
    }

    // Define the MouseUp event property.
    public event MouseEventHandler MouseUp
    {
        // Add the input delegate to the collection.
        add
        {
            listEventDelegates.AddHandler(mouseUpEventKey, value);
        }
        // Remove the input delegate from the collection.
        remove
        {
            listEventDelegates.RemoveHandler(mouseUpEventKey, value);
        }
    }

    // Raise the event with the delegate specified by mouseUpEventKey
    private void OnMouseUp(MouseEventArgs e)
    {
        MouseEventHandler mouseEventDelegate =
            (MouseEventHandler)listEventDelegates[mouseUpEventKey];
        mouseEventDelegate(this, e);
    }
}

How to: Consume Events in a Web Forms Application

A common scenario in ASP.NET Web Forms applications is to populate a webpage with controls, and then perform a specific action based on which control the user clicks. For example, a System.Web.UI.WebControls.Button control raises an event when the user clicks it in the webpage. By handling the event, your application can perform the appropriate application logic for that button click.

To handle a button click event on a webpage

1. Create a ASP.NET Web Forms page (webpage) that has a Button control with the OnClick value set to the name of method that you will define in the next step.

   XML

   <asp:Button ID="Button1" runat="server" Text="Click Me" OnClick="Button1_Click" />  
   
   

2. Define an event handler that matches the Click event delegate signature and that has the name you defined for the OnClick value.

   C#

   VB

   protected void Button1_Click(object sender, EventArgs e)  
   {  
       // perform action  
   }  
   
   

   The Click event uses the EventHandler class for the delegate type and the EventArgs class for the event data. The ASP.NET page framework automatically generates code that creates an instance of EventHandler and adds this delegate instance to the Click event of the Button instance.

3. In the event handler method that you defined in step 2, add code to perform any actions that are required when the event occurs.

Observer Design Pattern

The observer design pattern enables a subscriber to register with and receive notifications from a provider. It is suitable for any scenario that requires push-based notification. The pattern defines a provider (also known as a subject or an observable) and zero, one, or more observers. Observers register with the provider, and whenever a predefined condition, event, or state change occurs, the provider automatically notifies all observers by calling one of their methods. In this method call, the provider can also provide current state information to observers. In the .NET Framework, the observer design pattern is applied by implementing the generic System.IObservable<T> and System.IObserver<T> interfaces. The generic type parameter represents the type that provides notification information.

Applying the Pattern

The observer design pattern is suitable for distributed push-based notifications, because it supports a clean separation between two different components or application layers, such as a data source (business logic) layer and a user interface (display) layer. The pattern can be implemented whenever a provider uses callbacks to supply its clients with current information.

Implementing the pattern requires that you provide the following:

• A provider or subject, which is the object that sends notifications to observers. A provider is a class or structure that implements the IObservable<T> interface. The provider must implement a single method, IObservable<T>.Subscribe, which is called by observers that wish to receive notifications from the provider.

• An observer, which is an object that receives notifications from a provider. An observer is a class or structure that implements the IObserver<T> interface. The observer must implement three methods, all of which are called by the provider:

  • IObserver<T>.OnNext, which supplies the observer with new or current information.

  • IObserver<T>.OnError, which informs the observer that an error has occurred.

  • IObserver<T>.OnCompleted, which indicates that the provider has finished sending notifications.

• A mechanism that allows the provider to keep track of observers. Typically, the provider uses a container object, such as a System.Collections.Generic.List<T> object, to hold references to the IObserver<T> implementations that have subscribed to notifications. Using a storage container for this purpose enables the provider to handle zero to an unlimited number of observers. The order in which observers receive notifications is not defined; the provider is free to use any method to determine the order.

• An IDisposable implementation that enables the provider to remove observers when notification is complete. Observers receive a reference to the IDisposable implementation from the Subscribe method, so they can also call the IDisposable.Dispose method to unsubscribe before the provider has finished sending notifications.

• An object that contains the data that the provider sends to its observers. The type of this object corresponds to the generic type parameter of the IObservable<T> and IObserver<T> interfaces. Although this object can be the same as the IObservable<T> implementation, most commonly it is a separate type.

Note
In addition to implementing the observer design pattern, you may be interested in exploring libraries that are built using the IObservable<T> and IObserver<T> interfaces. For example, Reactive Extensions for .NET (Rx) consist of a set of extension methods and LINQ standard sequence operators to support asynchronous programming.

Implementing the Pattern

The following example uses the observer design pattern to implement an airport baggage claim information system. A BaggageInfo class provides information about arriving flights and the carousels where baggage from each flight is available for pickup. It is shown in the following example.

C#

VB

using System;
using System.Collections.Generic;

public class BaggageInfo
{
   private int flightNo;
   private string origin;
   private int location;

   internal BaggageInfo(int flight, string from, int carousel)
   {
      this.flightNo = flight;
      this.origin = from;
      this.location = carousel;
   }

   public int FlightNumber {
      get { return this.flightNo; }
   }

   public string From {
      get { return this.origin; }
   }

   public int Carousel {
      get { return this.location; }
   }
}

A BaggageHandler class is responsible for receiving information about arriving flights and baggage claim carousels. Internally, it maintains two collections:

• observers - A collection of clients that will receive updated information.

• flights - A collection of flights and their assigned carousels.

Both collections are represented by generic List<T> objects that are instantiated in the BaggageHandler class constructor. The source code for the BaggageHandler class is shown in the following example.

C#

VB

public class BaggageHandler : IObservable<BaggageInfo>
{
   private List<IObserver<BaggageInfo>> observers;
   private List<BaggageInfo> flights;

   public BaggageHandler()
   {
      observers = new List<IObserver<BaggageInfo>>();
      flights = new List<BaggageInfo>();
   }

   public IDisposable Subscribe(IObserver<BaggageInfo> observer)
   {
      // Check whether observer is already registered. If not, add it
      if (! observers.Contains(observer)) {
         observers.Add(observer);
         // Provide observer with existing data.
         foreach (var item in flights)
            observer.OnNext(item);
      }
      return new Unsubscriber<BaggageInfo>(observers, observer);
   }

   // Called to indicate all baggage is now unloaded.
   public void BaggageStatus(int flightNo)
   {
      BaggageStatus(flightNo, String.Empty, 0);
   }

   public void BaggageStatus(int flightNo, string from, int carousel)
   {
      var info = new BaggageInfo(flightNo, from, carousel);

      // Carousel is assigned, so add new info object to list.
      if (carousel > 0 && ! flights.Contains(info)) {
         flights.Add(info);
         foreach (var observer in observers)
            observer.OnNext(info);
      }
      else if (carousel == 0) {
         // Baggage claim for flight is done
         var flightsToRemove = new List<BaggageInfo>();
         foreach (var flight in flights) {
            if (info.FlightNumber == flight.FlightNumber) {
               flightsToRemove.Add(flight);
               foreach (var observer in observers)
                  observer.OnNext(info);
            }
         }
         foreach (var flightToRemove in flightsToRemove)
            flights.Remove(flightToRemove);

         flightsToRemove.Clear();
      }
   }

   public void LastBaggageClaimed()
   {
      foreach (var observer in observers)
         observer.OnCompleted();

      observers.Clear();
   }
}

Clients that wish to receive updated information call the BaggageHandler.Subscribe method. If the client has not previously subscribed to notifications, a reference to the client's IObserver<T> implementation is added to the observers collection.

The overloaded BaggageHandler.BaggageStatus method can be called to indicate that baggage from a flight either is being unloaded or is no longer being unloaded. In the first case, the method is passed a flight number, the airport from which the flight originated, and the carousel where baggage is being unloaded. In the second case, the method is passed only a flight number. For baggage that is being unloaded, the method checks whether the BaggageInfo information passed to the method exists in the flights collection. If it does not, the method adds the information and calls each observer's OnNext method. For flights whose baggage is no longer being unloaded, the method checks whether information on that flight is stored in the flights collection. If it is, the method calls each observer's OnNext method and removes the BaggageInfo object from the flights collection.

When the last flight of the day has landed and its baggage has been processed, the BaggageHandler.LastBaggageClaimed method is called. This method calls each observer's OnCompleted method to indicate that all notifications have completed, and then clears the observers collection.

The provider's Subscribe method returns an IDisposable implementation that enables observers to stop receiving notifications before the OnCompleted method is called. The source code for this Unsubscriber(Of BaggageInfo) class is shown in the following example. When the class is instantiated in the BaggageHandler.Subscribe method, it is passed a reference to the observers collection and a reference to the observer that is added to the collection. These references are assigned to local variables. When the object's Dispose method is called, it checks whether the observer still exists in the observerscollection, and, if it does, removes the observer.

C#

VB

internal class Unsubscriber<BaggageInfo> : IDisposable
{
   private List<IObserver<BaggageInfo>> _observers;
   private IObserver<BaggageInfo> _observer;

   internal Unsubscriber(List<IObserver<BaggageInfo>> observers, IObserver<BaggageInfo> observer)
   {
      this._observers = observers;
      this._observer = observer;
   }

   public void Dispose() 
   {
      if (_observers.Contains(_observer))
         _observers.Remove(_observer);
   }
}

The following example provides an IObserver<T> implementation named ArrivalsMonitor, which is a base class that displays baggage claim information. The information is displayed alphabetically, by the name of the originating city. The methods of ArrivalsMonitor are marked as overridable (in Visual Basic) or virtual (in C#), so they can all be overridden by a derived class.

C#

VB

using System;
using System.Collections.Generic;

public class ArrivalsMonitor : IObserver<BaggageInfo>
{
   private string name;
   private List<string> flightInfos = new List<string>();
   private IDisposable cancellation;
   private string fmt = "{0,-20} {1,5}  {2, 3}";

   public ArrivalsMonitor(string name)
   {
      if (String.IsNullOrEmpty(name))
         throw new ArgumentNullException("The observer must be assigned a name.");

      this.name = name;
   }

   public virtual void Subscribe(BaggageHandler provider)
   {
      cancellation = provider.Subscribe(this);
   }

   public virtual void Unsubscribe()
   {
      cancellation.Dispose();
      flightInfos.Clear();
   }

   public virtual void OnCompleted() 
   {
      flightInfos.Clear();
   }

   // No implementation needed: Method is not called by the BaggageHandler class.
   public virtual void OnError(Exception e)
   {
      // No implementation.
   }

   // Update information.
   public virtual void OnNext(BaggageInfo info) 
   {
      bool updated = false;

      // Flight has unloaded its baggage; remove from the monitor.
      if (info.Carousel == 0) {
         var flightsToRemove = new List<string>();
         string flightNo = String.Format("{0,5}", info.FlightNumber);
         
         foreach (var flightInfo in flightInfos) {
            if (flightInfo.Substring(21, 5).Equals(flightNo)) {
               flightsToRemove.Add(flightInfo);
               updated = true;
            }
         }
         foreach (var flightToRemove in flightsToRemove)
            flightInfos.Remove(flightToRemove);

         flightsToRemove.Clear();
      }
      else {
         // Add flight if it does not exist in the collection.
         string flightInfo = String.Format(fmt, info.From, info.FlightNumber, info.Carousel);
         if (! flightInfos.Contains(flightInfo)) {
            flightInfos.Add(flightInfo);
            updated = true;
         }
      }
      if (updated) {
         flightInfos.Sort();
         Console.WriteLine("Arrivals information from {0}", this.name);
         foreach (var flightInfo in flightInfos)
            Console.WriteLine(flightInfo);

         Console.WriteLine();
      }
   }
}

The ArrivalsMonitor class includes the Subscribe and Unsubscribe methods. The Subscribe method enables the class to save the IDisposable implementation returned by the call to Subscribe to a private variable. The Unsubscribe method enables the class to unsubscribe from notifications by calling the provider's Dispose implementation. ArrivalsMonitor also provides implementations of the OnNext, OnError, and OnCompleted methods. Only the OnNext implementation contains a significant amount of code. The method works with a private, sorted, generic List<T> object that maintains information about the airports of origin for arriving flights and the carousels on which their baggage is available. If the BaggageHandler class reports a new flight arrival, the OnNext method implementation adds information about that flight to the list. If the BaggageHandler class reports that the flight's baggage has been unloaded, the OnNext method removes that flight from the list. Whenever a change is made, the list is sorted and displayed to the console.

The following example contains the application entry point that instantiates the BaggageHandler class as well as two instances of the ArrivalsMonitorclass, and uses the BaggageHandler.BaggageStatus method to add and remove information about arriving flights. In each case, the observers receive updates and correctly display baggage claim information.

C#

VB

using System;
using System.Collections.Generic;

public class Example
{
   public static void Main()
   {
      BaggageHandler provider = new BaggageHandler();
      ArrivalsMonitor observer1 = new ArrivalsMonitor("BaggageClaimMonitor1");
      ArrivalsMonitor observer2 = new ArrivalsMonitor("SecurityExit");

      provider.BaggageStatus(712, "Detroit", 3);
      observer1.Subscribe(provider);
      provider.BaggageStatus(712, "Kalamazoo", 3);
      provider.BaggageStatus(400, "New York-Kennedy", 1);
      provider.BaggageStatus(712, "Detroit", 3);
      observer2.Subscribe(provider);
      provider.BaggageStatus(511, "San Francisco", 2);
      provider.BaggageStatus(712);
      observer2.Unsubscribe();
      provider.BaggageStatus(400);
      provider.LastBaggageClaimed();
   }
}
// The example displays the following output:
//      Arrivals information from BaggageClaimMonitor1
//      Detroit                712    3
//
//      Arrivals information from BaggageClaimMonitor1
//      Detroit                712    3
//      Kalamazoo              712    3
//
//      Arrivals information from BaggageClaimMonitor1
//      Detroit                712    3
//      Kalamazoo              712    3
//      New York-Kennedy       400    1
//
//      Arrivals information from SecurityExit
//      Detroit                712    3
//
//      Arrivals information from SecurityExit
//      Detroit                712    3
//      Kalamazoo              712    3
//
//      Arrivals information from SecurityExit
//      Detroit                712    3
//      Kalamazoo              712    3
//      New York-Kennedy       400    1
//
//      Arrivals information from BaggageClaimMonitor1
//      Detroit                712    3
//      Kalamazoo              712    3
//      New York-Kennedy       400    1
//      San Francisco          511    2
//
//      Arrivals information from SecurityExit
//      Detroit                712    3
//      Kalamazoo              712    3
//      New York-Kennedy       400    1
//      San Francisco          511    2
//
//      Arrivals information from BaggageClaimMonitor1
//      New York-Kennedy       400    1
//      San Francisco          511    2
//
//      Arrivals information from SecurityExit
//      New York-Kennedy       400    1
//      San Francisco          511    2
//
//      Arrivals information from BaggageClaimMonitor1
//      San Francisco          511    2

Related Topics

Title	Description
Observer Design Pattern Best Practices	Describes best practices to adopt when developing applications that implement the observer design pattern.
How to: Implement a Provider	Provides a step-by-step implementation of a provider for a temperature monitoring application.
How to: Implement an Observer	Provides a step-by-step implementation of an observer for a temperature monitoring application.