.NET Framework Class Library
EventHandler Delegate

Represents the method that will handle an event that has no event data.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)
Syntax
<[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_0_0_0%]> _
<[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_0_0_1%](True)> _
Public Delegate Sub EventHandler ( _
	sender As [%$TOPIC/xhb70ccc_en-us_VS_110_1_0_0_0_2%], _
	e As [%$TOPIC/xhb70ccc_en-us_VS_110_1_0_0_0_3%] _
)
[[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_1_0_0%]]
[[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_1_0_1%](true)]
public delegate void EventHandler(
	[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_1_0_2%] sender,
	[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_1_0_3%] e
)
[[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_2_0_0%]]
[[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_2_0_1%](true)]
public delegate void EventHandler(
	[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_2_0_2%]^ sender, 
	[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_2_0_3%]^ e
)
[<[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_3_0_0%]>]
[<[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_3_0_1%](true)>]
type EventHandler = 
    delegate of  
        sender:[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_3_0_2%] * 
        e:[%$TOPIC/xhb70ccc_en-us_VS_110_1_0_3_0_3%] -> unit

Parameters

sender
Type: SystemObject

The source of the event.

e
Type: SystemEventArgs

An object that contains no event data.

Remarks

The event model in the .NET Framework is based on having an event delegate that connects an event with its handler. To raise an event, two elements are needed:

  • A delegate that identifies the method that provides the response to the event.

  • Optionally, a class that holds the event data, if the event provides data.

The delegate is a type that defines a signature, that is, the return value type and parameter list types for a method. You can use the delegate type to declare a variable that can refer to any method with the same signature as the delegate.

The standard signature of an event handler delegate defines a method that does not return a value. This method's first parameter is of type Object and refers to the instance that raises the event. Its second parameter is derived from type EventArgs and holds the event data. If the event does not generate event data, the second parameter is simply the value of the EventArgsEmpty field. Otherwise, the second parameter is a type derived from EventArgs and supplies any fields or properties needed to hold the event data.

The EventHandler delegate is a predefined delegate that specifically represents an event handler method for an event that does not generate data. If your event does generate data, you must use the generic EventHandlerTEventArgs delegate class.

To associate the event with the method that will handle the event, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate.

For more information about event handler delegates, see Handling and Raising Events.

Examples

The following example shows an event named ThresholdReached that is associated with an EventHandler delegate. The method assigned to the EventHandler delegate is called in the OnThresholdReached method.

Module Module1

    Sub Main()
        Dim c As Counter = New Counter(New Random().Next(10))
        AddHandler c.ThresholdReached, AddressOf c_ThresholdReached

        Console.WriteLine("press 'a' key to increase total")
        While Console.ReadKey(True).KeyChar = "a"
            Console.WriteLine("adding one")
            c.Add(1)
        End While 
    End Sub 

    Sub c_ThresholdReached(sender As Object, e As ThresholdReachedEventArgs)
        Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached)
        Environment.Exit(0)
    End Sub 
End Module 

Class Counter
    Private threshold As Integer 
    Private total As Integer 

    Public Sub New(passedThreshold As Integer)
        threshold = passedThreshold
    End Sub 

    Public Sub Add(x As Integer)
        total = total + x
        If (total >= threshold) Then 
            Dim args As ThresholdReachedEventArgs = New ThresholdReachedEventArgs()
            args.Threshold = threshold
            args.TimeReached = DateTime.Now
            OnThresholdReached(args)
        End If 
    End Sub 

    Protected Overridable Sub OnThresholdReached(e As ThresholdReachedEventArgs)
        RaiseEvent ThresholdReached(Me, e)
    End Sub 

    Public Event ThresholdReached As EventHandler(Of ThresholdReachedEventArgs)
End Class 

Class ThresholdReachedEventArgs
    Inherits EventArgs

    Public Property Threshold As Integer 
    Public Property TimeReached As DateTime
End Class
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; }
    }
}
Version Information

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8
Platforms

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.