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HOW TO:在衍生類別中引發基底類別事件 (C# 程式設計手冊)

下列的簡單範例會示範在基底類別 (Base Class) 內宣告事件的標準方式,讓事件也可以從衍生類別 (Derived Class) 中引發。 這個模式會在 .NET Framework 類別庫 (Class Library) 的 Windows Form 類別中廣泛使用。

當您建立可當做其他類別之基底類別的類別時,您應該考量一項事實:事件是一種委派 (Delegate) 的特殊型別,只能從宣告事件的類別中予以叫用 (Invoke)。 衍生類別無法直接叫用在基底類別中宣告的事件。 雖然有時您可能會想要使用只能由基底類別所引發的事件,但是在多數情形下,您應該啟用衍生類別來叫用基底類別事件。 若要這樣做,您可以在包裝事件的基底類別內建立保護的 (Protected) 的叫用方法; 藉由呼叫或覆寫這個叫用方法,衍生類別便能夠間接地叫用該事件。

注意事項注意事項

請勿在基底類別中宣告虛擬事件,以及進而在衍生類別中覆寫它們。C#編譯器會控制代碼這些正確而且衍生的事件訂閱者 」 實際上訂閱基底類別事件是否發生無法預期。

範例


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
 */

請參閱

參考

事件 (C# 程式設計手冊)

委派 (C# 程式設計手冊)

存取修飾詞 (C# 程式設計手冊)

概念

C# 程式設計手冊

其他資源

在 Windows Form 中建立事件處理常式