How to: Implement Events in Your Class

How to: Implement Events in Your Cl...

Collapse All

Language Filter

This page is specific to

Microsoft Visual Studio 2008/.NET Framework 3.5

Other versions are also available for the following:

.NET Framework Developer's Guide

How to: Implement Events in Your 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

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;   
}

Visual Basic

Public Class Countdown
    ...
    Public Event CountdownCompleted As EventHandler
End Class

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);
    }
}

Visual Basic

Public Class Countdown
   ...
   Public Event CountdownCompleted As EventHandler
   Protected Overridable Sub OnCountdownCompleted(e As EventArgs)
      RaiseEvent CountdownCompleted(Me, e)
   End Sub
End Class

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());
    }
}

Visual Basic

Public Class Countdown
    ...
    Public Function Decrement
        InternalCounter = internalCounter - 1
        If internalCounter = 0
            OnCountdownComplete(New EventArgs())
        End If
    End Function
End Class

To implement an event with event-specific data

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;
   
   //Constructor.
   public AlarmEventArgs(bool snoozePressed, int nrings) 
   {
      this.snoozePressed = snoozePressed;
      this.nrings = nrings;
   }
   
   //Properties.
   public string AlarmText {  
      ...
   }
   public int NumRings {
      ...
   }
   public bool SnoozePressed{
      ...
   }
}

Visual Basic

Public Class AlarmEventArgs
   Inherits EventArgs
   Private nrings As Integer = 0
   Private _snoozePressed As Boolean = False
   
   'Constructor.
   Public Sub New(ByVal snoozePressed As Boolean, ByVal nrings As Integer) 
      Me.snoozePressed = snoozePressed
      Me.nrings = nrings
   End Sub
   
   'Properties.
   Public ReadOnly Property AlarmText() As String
      ...
   End Property 
   
   Public ReadOnly Property NumRings() As Integer
      ...
   End Property 
   
   Public ReadOnly Property SnoozePressed() As Boolean
      ...
   End Property
End Class

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

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

Visual Basic

Public Delegate Sub AlarmEventHandler(sender As Object, e As AlarmEventArgs)

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;
}

Visual Basic

Public Class AlarmClock
    ...
    Public Event Alarm As AlarmEventHandler
End Class

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); 
    }
}

Visual Basic

Public Class AlarmClock
    ...
    Public Event Alarm As AlarmEventHandler
    Protected Overridable Sub OnAlarm(e As AlarmEventArgs)
        RaiseEvent Alarm(Me, e)
    End Sub
End Class

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);
    }
}

Visual Basic

Public Class AlarmClock
    ...
    Public Function Start
        ...
        System.Threading.Thread.Sleep(300)
        Dim e As AlarmEventArgs = New AlarmEventArgs(False, 0)
        OnAlarm(e)
    End Function
End Class

Concepts

Handling and Raising Events

Events and Delegates

Raising an Event

How to: Raise and Consume Events

Product Families

Resources

About Microsoft

Popular Places

Popular Searches

Popular Downloads

To implement an event without event-specific data

To implement an event with event-specific data

Concepts