¡¼ How to implement events in class
 
The following procedures describe how to implement an event in a class. 
The first procedure implements an event that does not have associated data; 
it uses the classes System.EventArgs and System.EventHandler for the event data and delegate handler. 

The second procedure implements an event with custom data; 
it defines custom classes for the event data and the event delegate handler.

For a complete sample that illustrates raising and handling events, see How to: Raise and Consume Events.

To implement an event without event-specific data

1. Define a public event member in your class. Set the type of the event member to a System.EventHandler delegate. 

public class Countdown 
{
    ...
    public event EventHandler CountdownCompleted;   
}


2. Provide a protected method in your class that raises the event. 
Name the method OnEventName. 
Raise the event within the method. 

public class Countdown 
{
    ...
    public event EventHandler CountdownCompleted;   
    protected virtual void OnCountdownCompleted(EventArgs e)
    {
        if (CountdownCompleted != null)
            CountdownCompleted(this, e);
    }
}

3. Determine when to raise the event in your class. 
Call OnEventName to raise the event.
 
public class Countdown 
{
    ...
    public void Decrement
    {
        internalCounter = internalCounter - 1;
        if (internalCounter == 0)
            OnCountdownCompleted(new EventArgs());
    }
}


To implement an event with event-specific data

1. Define a class that provides data for the event. 
Name the class EventNameArgs, derive the class from System.EventArgs, and add any event-specific members. 


public class AlarmEventArgs : EventArgs 
{
   private readonly int nrings = 0;
   private readonly bool snoozePressed = false;
   
   //Properties.
   public string AlarmText {  
      ...
   }
   public int NumRings {
      ...
   }
   public bool SnoozePressed{
      ...
   }
}

2. Declare a delegate for the event. 
Name the delegate EventNameEventHandler.

public delegate void AlarmEventHandler(object sender, AlarmEventArgs e);


3. Define a public event member named EventName in your class. 
Set the type of the event member to the event delegate type.

public class AlarmClock 
{
    ...
    public event AlarmEventHandler Alarm;
}


4. Define a protected method in your class that raises the event. 
Name the method OnEventName. Raise the event within the method. 

public class AlarmClock 
{
    ...
    public event AlarmHandler Alarm;
    protected virtual void OnAlarm(AlarmEventArgs e)
    {
      if (Alarm != null) 
          Alarm(this, e); 
    }
}


5. Determine when to raise the event in your class. 
Call OnEventName to raise the event and pass in the event-specific data using EventNameEventArgs. 

Public Class AlarmClock
{
    ...
    public void Start()
    {
        ...
        System.Threading.Thread.Sleep(300);
        AlarmEventArgs e = new AlarmEventArgs(false, 0);
        OnAlarm(e);
    }
}


 






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