如何：在派生類中引發基類事件（C# 編程指南） · MSDN C# 編程指南 & 參考手冊 2015

# 如何：在派生類中引發基類事件（C# 編程指南）以下簡單示例演示了在基類中聲明可從派生類引發的事件的標準方法。此模式廣泛應用于 .NET Framework 類庫中的 Windows 窗體類。在創建可用作其他類的基類的類時，應考慮如下事實：事件是特殊類型的委托，只可以從聲明它們的類中調用。派生類無法直接調用基類中聲明的事件。盡管有時需要事件僅由基類引發，但在大多數情形下，應該允許派生類調用基類事件。為此，您可以在包含該事件的基類中創建一個受保護的調用方法。通過調用或重寫此調用方法，派生類便可以間接調用該事件。 | ![](https://box.kancloud.cn/2016-01-31_56adb62c1380a.jpg) 注意 | | :-- | | 不要在基類中聲明虛擬事件，也不要在派生類中重寫這些事件。C# 編譯器無法正確處理這些事件，并且無法預知的該派生的事件的用戶是否真正訂閱了基類事件。 | ``` 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 [C# 編程指南](https://msdn.microsoft.com/zh-cn/library/67ef8sbd.aspx) [事件（C# 編程指南）](https://msdn.microsoft.com/zh-cn/library/awbftdfh.aspx) [委托（C# 編程指南）](https://msdn.microsoft.com/zh-cn/library/ms173171.aspx) [訪問修飾符（C# 編程指南）](https://msdn.microsoft.com/zh-cn/library/ms173121.aspx) [在 Windows 窗體中創建事件處理程序](https://msdn.microsoft.com/zh-cn/library/dacysss4.aspx)