EventHandler Delegate


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

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

public delegate void EventHandler(
	Object^ sender,
	EventArgs^ e


Type: System::Object^

The source of the event.

Type: System::EventArgs^

An object that contains no event data.

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.

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
      property int Threshold;
      property DateTime TimeReached;

public ref class Counter
      int threshold;
      int total;

      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;

      event EventHandler<ThresholdReachedEventArgs^>^ ThresholdReached;

      virtual void OnThresholdReached(ThresholdReachedEventArgs^ e)
         ThresholdReached(this, e);

public ref class SampleHandler
      static void c_ThresholdReached(Object^ sender, ThresholdReachedEventArgs^ e)
         Console::WriteLine("The threshold of {0} was reached at {1}.",
                            e->Threshold,  e->TimeReached);

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");

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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