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EventArgs Class

EventArgs is the base class for classes containing event data.

Namespace: System
Assembly: mscorlib (in mscorlib.dll)

<SerializableAttribute> _
<ComVisibleAttribute(True)> _
Public Class EventArgs
Dim instance As EventArgs

/** @attribute SerializableAttribute() */ 
/** @attribute ComVisibleAttribute(true) */ 
public class EventArgs
public class EventArgs
Not applicable.

This class contains no event data; it is used by events that do not pass state information to an event handler when an event is raised. If the event handler requires state information, the application must derive a class from this class to hold the data.

For example, the System.AssemblyLoadEventArgs class is used to hold the data for assembly load events, and contains a System.Reflection.Assembly that describes the loaded assembly.

For more information about events, see the EventHandler topic.

The following code sample illustrates the use of EventArgs.

In this sample, FireEventArgs is a set of event arguments derived from EventArgs, and passed to the FireEventHandler when an event is raised by calling ActivateFireAlarm.

Imports System


' FireEventArgs: a custom event inherited from EventArgs.

Public Class FireEventArgs
   Inherits EventArgs
   Public Sub New(room As String, ferocity As Integer) = room
      Me.ferocity = ferocity
   End Sub 'New
   ' The fire event will have two pieces of information-- 
   ' 1) Where the fire is, and 2) how "ferocious" it is. 
   Public room As String
   Public ferocity As Integer
End Class 'FireEventArgs
'end of class FireEventArgs

Public Class FireAlarm
   ' Events are handled with delegates, so we must establish a FireEventHandler
   ' as a delegate:

   Delegate Sub FireEventHandler(sender As Object, fe As FireEventArgs)
   ' Now, create a public event "FireEvent" whose type is our FireEventHandler delegate.

   Public Event FireEvent As FireEventHandler
   ' This will be the starting point of our event-- it will create FireEventArgs,
   ' and then raise the event, passing FireEventArgs.

   Public Sub ActivateFireAlarm(room As String, ferocity As Integer)
      Dim fireArgs As New FireEventArgs(room, ferocity)
      '  Now, raise the event by invoking the delegate. Pass in 
      '  the object that initated the event (Me) as well as FireEventArgs. 
      '  The call must match the signature of FireEventHandler.

      RaiseEvent FireEvent(Me, fireArgs)

   End Sub 'ActivateFireAlarm
End Class 'FireAlarm

' The following event will be the EventHandler.
Class FireHandlerClass
   ' Create a FireAlarm to handle and raise the fire events. 

   Public Sub New(fireAlarm As FireAlarm)
      ' Add a delegate containing the ExtinguishFire function to the class'
      ' event so that when FireAlarm is raised, it will subsequently execute 
      ' ExtinguishFire.

      AddHandler fireAlarm.FireEvent, AddressOf ExtinguishFire

   End Sub 'New
   ' This is the function to be executed when a fire event is raised.
   Sub ExtinguishFire(sender As Object, fe As FireEventArgs)
      Console.WriteLine("The ExtinguishFire function was called by {0}.", sender.ToString())
      ' Now, act in response to the event.

      If fe.ferocity < 2 Then
         Console.WriteLine("This fire in the {0} is no problem.  I'm going to pour some water on it.",
         If fe.ferocity < 5 Then
            Console.WriteLine("Using Fire Extinguisher to put out the fire in the {0}.",
            Console.WriteLine("The fire in the {0} is out of control.  I'm calling the fire department.",
         End If
      End If 'end of class FireHandlerClass
   End Sub 'ExtinguishFire
End Class 'FireHandlerClass

Public Class FireEventTest
   Public Shared Sub Main()
      ' Create an instance of the class that will be raising the event.
      Dim myFireAlarm As New FireAlarm()
      ' Create an instance of the class that will be handling the event. Note that 
      ' it receives the class that will fire the event as a parameter. 

     Dim myFireHandler As New FireHandlerClass(myFireAlarm)
      ' Now, use the FireAlarm class to raise a few events. 

      myFireAlarm.ActivateFireAlarm("Kitchen", 3)
      myFireAlarm.ActivateFireAlarm("Study", 1)
      myFireAlarm.ActivateFireAlarm("Porch", 5)
   End Sub 'Main 'end of main
End Class 'FireEventTest ' end of FireEventTest

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0

XNA Framework

Supported in: 1.0