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Delegato EventHandler

 

Data di pubblicazione: ottobre 2016

Rappresenta il metodo di gestione di un evento che non dispone di dati associati.

Spazio dei nomi:   System
Assembly:  mscorlib (in mscorlib.dll)

[SerializableAttribute]
[ComVisibleAttribute(true)]
public delegate void EventHandler(
	object sender,
	EventArgs e
)

Parametri

sender
Type: System.Object

L'origine dell’evento.

e
Type: System.EventArgs

Oggetto che non contiene dati dell'evento.

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 T:System.Object and refers to the instance that raises the event. Its second parameter is derived from type T:System.EventArgs and holds the event data. If the event does not generate event data, the second parameter is simply the value of the F:System.EventArgs.Empty field. Otherwise, the second parameter is a type derived from T:System.EventArgs and supplies any fields or properties needed to hold the event data.

The T:System.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 T:System.EventHandler`1 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 T:System.EventHandler delegate. The method assigned to the T:System.EventHandler delegate is called in the OnThresholdReached method.

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; }
    }
}

Universal Windows Platform
Disponibile da 8
.NET Framework
Disponibile da 1.1
Libreria di classi portabile
Supportato in: piattaforme .NET portabili
Silverlight
Disponibile da 2.0
Windows Phone Silverlight
Disponibile da 7.0
Windows Phone
Disponibile da 8.1
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