An event can be raised only from the declaration space in which it is declared. Therefore, a class cannot raise events from any other class, even one from which it is derived.
事件只能在它被声明的声明空间（类）中使用。所以不能从任何其他类引发，即使该类是事件所在类的继承类。
那我们如何才可以引发基类中的事件，给个实例大家一看就明白了。
namespace BaseClassEvents
{
using System;
using System.Collections.Generic; // Special EventArgs class to hold info about Shapes.
public class ShapeEventArgs : EventArgs
{
private double newArea; public ShapeEventArgs(double d)
{
newArea = d;
}
public double NewArea
{
get { return newArea; }
}
} // Base class event publisher
public abstract class Shape
{
protected double area; public double Area
{
get { return area; }
set { area = value; }
}
// The event. Note that by using the generic EventHandler<T> event type
// we do not need to declare a separate delegate type.
public event EventHandler<ShapeEventArgs> ShapeChanged; public abstract void Draw(); //The event-invoking method that derived classes can override.
protected virtual void OnShapeChanged(ShapeEventArgs e)
{
// Make a temporary copy of the event to avoid possibility of
// a race condition if the last subscriber unsubscribes
// immediately after the null check and before the event is raised.
EventHandler<ShapeEventArgs> handler = ShapeChanged;
if (handler != null)
{
handler(this, e);
}
}
} public class Circle : Shape
{
private double radius;
public Circle(double d)
{
radius = d;
area = 3.14 * radius * radius;
}
public void Update(double d)
{
radius = d;
area = 3.14 * radius * radius;
OnShapeChanged(new ShapeEventArgs(area));
}
protected override void OnShapeChanged(ShapeEventArgs e)
{
// Do any circle-specific processing here. // Call the base class event invocation method.
base.OnShapeChanged(e);
}
public override void Draw()
{
Console.WriteLine("Drawing a circle");
}
} public class Rectangle : Shape
{
private double length;
private double width;
public Rectangle(double length, double width)
{
this.length = length;
this.width = width;
area = length * width;
}
public void Update(double length, double width)
{
this.length = length;
this.width = width;
area = length * width;
OnShapeChanged(new ShapeEventArgs(area));
}
protected override void OnShapeChanged(ShapeEventArgs e)
{
// Do any rectangle-specific processing here. // Call the base class event invocation method.
base.OnShapeChanged(e);
}
public override void Draw()
{
Console.WriteLine("Drawing a rectangle");
} } // Represents the surface on which the shapes are drawn
// Subscribes to shape events so that it knows
// when to redraw a shape.
public class ShapeContainer
{
List<Shape> _list; public ShapeContainer()
{
_list = new List<Shape>();
} public void AddShape(Shape s)
{
_list.Add(s);
// Subscribe to the base class event.
s.ShapeChanged += HandleShapeChanged;
} // ...Other methods to draw, resize, etc. private void HandleShapeChanged(object sender, ShapeEventArgs e)
{
Shape s = (Shape)sender; // Diagnostic message for demonstration purposes.
Console.WriteLine("Received event. Shape area is now {0}", e.NewArea); // Redraw the shape here.
s.Draw();
}
} class Test
{ static void Main(string[] args)
{
//Create the event publishers and subscriber
Circle c1 = new Circle(54);
Rectangle r1 = new Rectangle(12, 9);
ShapeContainer sc = new ShapeContainer(); // Add the shapes to the container.
sc.AddShape(c1);
sc.AddShape(r1); // Cause some events to be raised.
c1.Update(57);
r1.Update(7, 7); // Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}
}
/* Output:
Received event. Shape area is now 10201.86
Drawing a circle
Received event. Shape area is now 49
Drawing a rectangle
*/
参考
How to: Raise Base Class Events in Derived Classes (C# Programming Guide)