通过

如何在派生类中引发基类事件（C# 编程指南）

  • 项目

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。

注意

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 publisher
    public 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 subscribers
            ShapeChanged?.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 subscriber
            var 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 10201.86
        Drawing a circle
        Received event. Shape area is now 49
        Drawing a rectangle
 */

另请参阅