C#学习基本概念之事件（四）---EventHandler委托（事件处理）

Represents the method that will handle an event that has no event data.
命名空间:
System
程序集: mscorlib（位于 mscorlib.dll）

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

参数

sender

Type: System.Object
The source of the event.

e

Type: System.EventArgs
An object that contains no event data.

备注
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.

示例
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; }
}
}

备注：https://msdn.microsoft.com/zh-cn/library/system.eventhandler.aspx