介绍.NET中的委派（一）

//
using System;
using System.WinForms;	// 在beta2版本中为：System.Windows.Forms
using System.IO;

class Set {
   private Object[] items;

   public Set(Int32 numItems) {
      items = new Object[numItems];
      for (Int32 i = 0; i < numItems; i++)
         items[i] = i;
   }

   // 定义 Feedback，类型为delegate
   // (注意: 这个类型在Set类中是嵌套的)
   public delegate void Feedback(
      Object value, Int32 item, Int32 numItems);

   public void ProcessItems(Feedback feedback) {
      for (Int32 item = 0; item < items.Length; item++) {
         if (feedback != null) {
            // 一旦指定了回调，便调用它们
            feedback(items[item], item + 1, items.Length);
         }
      }
   }
}


class App {
   [STAThreadAttribute]
   static void Main() {
      StaticCallbacks();
      InstanceCallbacks();
   }

   static void StaticCallbacks() {
      // 创建一个Set对象，其中有五个项目
      Set setOfItems = new Set(5);

      // 处理项目，feedback=null
      setOfItems.ProcessItems(null);
      Console.WriteLine();

      // 处理项目，feedback=Console
      setOfItems.ProcessItems(new Set.Feedback(App.FeedbackToConsole));
      Console.WriteLine();

      // 处理项目，feedback =MsgBox
      setOfItems.ProcessItems(new Set.Feedback(App.FeedbackToMsgBox));
      Console.WriteLine();

      // 处理项目，feedback = console AND MsgBox
      Set.Feedback fb = null;
      fb += new Set.Feedback(App.FeedbackToConsole);
      fb += new Set.Feedback(App.FeedbackToMsgBox);
      setOfItems.ProcessItems(fb);
      Console.WriteLine();
   }

   static void FeedbackToConsole(
      Object value, Int32 item, Int32 numItems) {
      Console.WriteLine("Processing item {0} of {1}: {2}.", 
         item, numItems, value);
   }

   static void FeedbackToMsgBox(
      Object value, Int32 item, Int32 numItems) {
      MessageBox.Show(String.Format("Processing item {0} of {1}: {2}.",
        item, numItems, value));
   }

   static void InstanceCallbacks() {
      //创建一个Set对象，其中有五个元素
      Set setOfItems = new Set(5);

      // 处理项目，feedback=File
      App appobj = new App();
      setOfItems.ProcessItems(new Set.Feedback(appobj.FeedbackToFile));
      Console.WriteLine();
   }

   void FeedbackToFile(
      Object value, Int32 item, Int32 numItems) {

      StreamWriter sw = new StreamWriter("Status", true);
      sw.WriteLine("Processing item {0} of {1}: {2}.", 
         item, numItems, value);
      sw.Close();
   }
}
//

public delegate void Feedback(
Object value, Int32 item, Int32 numItems);

//
public class Feedback : System.MulticastDelegate {
   // 构造器
   public Feedback(Object target, Int32 methodPtr);

   // 方法与源代码描述的原型相同
   public void virtual Invoke(
      Object value, Int32 item, Int32 numItems);

   // 方法允许被异步回调，后继文章将讨论这些方法
   public virtual IAsyncResult BeginInvoke(
      Object value, Int32 item, Int32 numItems, 
      AsyncCallback callback, Object object);
   public virtual void EndInvoke(IAsyncResult result);
}
//

//
Boolean DelegateRefersToInstanceMethodOfType(
   MulticastDelegate d, Type type) {

   return((d.Target != null) && d.Target.GetType == type);
}
//

//
Boolean DelegateRefersToMethodOfName(
   MulticastDelegate d, String methodName) {

   return(d.Method.Name == methodName);
}
//

//
public void ProcessItems(Feedback feedback) {
   for (Int32 item = 1; item <= items.Length; item++) {
      if (feedback != null) {
         // 如果指定任何回调，则调用它们
         feedback(items[item], item, items.Length);
      }
   }
}
//

feedback(items[item], item, items.Length);

feedback.Invoke(items[item], item, items.Length);