Approaches to Web Development for Bioinformatics

Medical Computing > Web Development for Bioinformatics > Application Development Languages > C# > Events

Developing Applications with C#

Events

An event is a mechanism for a class to be notified when something it is interested occurs. For example, continuing the long running search example above, an event could be the discovery of a match in that search. Events are also used in C# by user interface frameworks for modelling user interface events like button clicks. Events make use of delegate types to enable this. Events are a pattern that can increase the modularity of code by decoupling the event source from the event handler.

When one class notifies other classes that an event has occurred it is said to have raised an event and is called the source of the event. Events are declared using the event keyword and can be either instance or static fields of a class. You may declared accessors for events in a similar way to get and set accessors for properties. However, the accessors for events are add and remove. If you do not declared accessors for an event the C# compiler will automatically generate them.

Here is a variation on the long running search example above using events. This time the event is when the search is finished. The class can be tested from the command line.

// A program to demonstrate use of events in C#

// A delegate type for hooking up notifications.
public delegate void SearchFinishedEventHandler(Search sender);

// The class that is the source of events
public class Search
{

// Put the results of the search in some object here
private string searchResults;

// The seach finished event
public event SearchFinishedEventHandler Finished;

// Invoke the Finished event
private void OnFinished()
{

if (Finished != null)
{

Finished(this);

}

}

// Do the search
public void Start()
{

System.Console.WriteLine("Search.Start");
// .. a long time passes
searchResults = "a match";
OnFinished();

}

// Return the search results to the calling code
public string getSearchResults() {

return searchResults;

}

}

// Exercise the SearchEvent class
class SearchEventTest
{

// The handler for the search finished event
private static void MySearchFinishedEventHandler(Search search)
{

System.Console.WriteLine("Collecting results: " + search.getSearchResults());

}

// Test it out from the command line
static void Main()
{

// Create the search object
Search search = new Search();

// Add our own event handler
search.Finished += new SearchFinishedEventHandler(MySearchFinishedEventHandler);

// Execute the search
search.Start();

}

The key difference compared with the delegate example is that the callback did not need to be passed in as a search method argument. This allows any number of event handlers to be added and even removed later. For example, one event handler may alert the user that the search is completed and another event handler may start the next search. Adding the event handler is done in the program with the += operator. Removing an event handler can be done with the -= operator. The add and remove event accessors are automatically generated by the C# compiler in this example. The output from this program is

>SearchEvent
Search.Start
Collecting results: a match

To create an event that corresponds to the .NET Framework guidelines. These guidelines require that the event take two parameters. The first parameter should be the source of the event. The second parameter should derive from the System.EventArgs class. EventHandler is useful for events that do not require any more information.

C# interfaces may also declare events. The implementing class should declare the same event with the public access modifier. Even though the event is declared public outside code will only be able to access it via the += and -= accessors.

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