下面的简单示例演示用于在基类中声明事件，以便也可以从派生类引发它们的标准方法。 此模式广泛用于 .NET 类库中的 Windows 窗体类。
创建可以用作其他类的基类的类时，应考虑到以下事实：事件是特殊类型的委托，只能从声明它们的类中进行调用。 派生类不能直接调用在基类中声明的事件。 虽然有时可能需要只能由基类引发的事件，不过在大多数情况下，应使派生类可以调用基类事件。 为此，可以在包装事件的基类中创建受保护的调用方法。 通过调用或重写此调用方法，派生类可以间接调用事件。
不要在基类中声明虚拟事件并在派生类中重写它们。 C# 编译器不会处理这些事件，并且无法预知派生事件的订阅者是否实际上会订阅基类事件。
示例
namespace BaseClassEvents
{// Special EventArgs class to hold info about Shapes.public class ShapeEventArgs : EventArgs{public ShapeEventArgs(double area){NewArea = area;}public double NewArea { get; }}// Base class event publisherpublic abstract class Shape{protected double _area;public double Area{get => _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){// Safely raise the event for all subscribersShapeChanged?.Invoke(this, e);}}public class Circle : Shape{private double _radius;public Circle(double radius){_radius = radius;_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){_length = length;_width = width;_area = _length * _width;}public void Update(double length, double width){_length = length;_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{private readonly List<Shape> _list;public ShapeContainer(){_list = new List<Shape>();}public void AddShape(Shape shape){_list.Add(shape);// Subscribe to the base class event.shape.ShapeChanged += HandleShapeChanged;}// ...Other methods to draw, resize, etc.private void HandleShapeChanged(object sender, ShapeEventArgs e){if (sender is Shape shape){// Diagnostic message for demonstration purposes.Console.WriteLine($"Received event. Shape area is now {e.NewArea}");// Redraw the shape here.shape.Draw();}}}class Test{static void Main(){//Create the event publishers and subscribervar circle = new Circle(54);var rectangle = new Rectangle(12, 9);var container = new ShapeContainer();// Add the shapes to the container.container.AddShape(circle);container.AddShape(rectangle);// Cause some events to be raised.circle.Update(57);rectangle.Update(7, 7);// Keep the console window open in debug mode.Console.WriteLine("Press any key to continue...");Console.ReadKey();}}
}
/* Output:Received event. Shape area is now 10201.86Drawing a circleReceived event. Shape area is now 49Drawing a rectangle*/
