EventHandler Delegate
Definition
Important
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Represents the method that will handle an event that has no event data.
public delegate void EventHandler(System::Object ^ sender, EventArgs ^ e);public delegate void EventHandler(object sender, EventArgs e);public delegate void EventHandler(object? sender, EventArgs e);[System.Serializable]
public delegate void EventHandler(object sender, EventArgs e);[System.Serializable]
[System.Runtime.InteropServices.ComVisible(true)]
public delegate void EventHandler(object sender, EventArgs e);type EventHandler = delegate of obj * EventArgs -> unit[<System.Serializable>]
type EventHandler = delegate of obj * EventArgs -> unit[<System.Serializable>]
[<System.Runtime.InteropServices.ComVisible(true)>]
type EventHandler = delegate of obj * EventArgs -> unitPublic Delegate Sub EventHandler(sender As Object, e As EventArgs)Parameters
- sender
- Object
The source of the event.
An object that contains no event data.
- Attributes
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.
using namespace System;
public ref class ThresholdReachedEventArgs : public EventArgs
{
public:
property int Threshold;
property DateTime TimeReached;
};
public ref class Counter
{
private:
int threshold;
int total;
public:
Counter() {};
Counter(int passedThreshold)
{
threshold = passedThreshold;
}
void Add(int x)
{
total += x;
if (total >= threshold) {
ThresholdReachedEventArgs^ args = gcnew ThresholdReachedEventArgs();
args->Threshold = threshold;
args->TimeReached = DateTime::Now;
OnThresholdReached(args);
}
}
event EventHandler<ThresholdReachedEventArgs^>^ ThresholdReached;
protected:
virtual void OnThresholdReached(ThresholdReachedEventArgs^ e)
{
ThresholdReached(this, e);
}
};
public ref class SampleHandler
{
public:
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);
}
};
void main()
{
Counter^ c = gcnew Counter((gcnew Random())->Next(10));
c->ThresholdReached += gcnew EventHandler<ThresholdReachedEventArgs^>(SampleHandler::c_ThresholdReached);
Console::WriteLine("press 'a' key to increase total");
while (Console::ReadKey(true).KeyChar == 'a') {
Console::WriteLine("adding one");
c->Add(1);
}
}
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; }
}
}
open System
type ThresholdReachedEventArgs(threshold, timeReached) =
inherit EventArgs()
member _.Threshold = threshold
member _.TimeReached = timeReached
type Counter(threshold) =
let mutable total = 0
let thresholdReached = Event<_>()
member this.Add(x) =
total <- total + x
if total >= threshold then
let args = ThresholdReachedEventArgs(threshold, DateTime.Now)
thresholdReached.Trigger(this, args)
[<CLIEvent>]
member _.ThresholdReached = thresholdReached.Publish
let c_ThresholdReached(sender, e: ThresholdReachedEventArgs) =
printfn $"The threshold of {e.Threshold} was reached at {e.TimeReached}."
exit 0
let c = Counter(Random().Next 10)
c.ThresholdReached.Add c_ThresholdReached
printfn "press 'a' key to increase total"
while Console.ReadKey(true).KeyChar = 'a' do
printfn "adding one"
c.Add 1
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
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 EventArgs.Empty 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 EventHandler<TEventArgs> 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.
Extension Methods
| GetMethodInfo(Delegate) |
Gets an object that represents the method represented by the specified delegate. |
