delegate void WorkStarted();
delegate void WorkProgressing();
delegate int WorkCompleted();

class Worker {
public void DoWork() {
Console.WriteLine("Worker: work started");
if( started != null ) started();

Console.WriteLine("Worker: work progressing");
if( progressing != null ) progressing();

Console.WriteLine("Worker: work completed");
if( completed != null ) {

foreach( WorkCompleted wc in completed.GetInvocationList() ) {
wc.BeginInvoke(new AsyncCallback(WorkGraded), wc);
}
}
}

void WorkGraded(IAsyncResult res) {
WorkCompleted wc = (WorkCompleted)res.AsyncState;
int grade = wc.EndInvoke(res);
Console.WriteLine("Worker grade= " + grade);
}

public event WorkStarted started;
public event WorkProgressing progressing;
public event WorkCompleted completed;
}

class Boss {
public int WorkCompleted() {
System.Threading.Thread.Sleep(3000);
Console.WriteLine("Better"); return 6; /* 10分以内 */
}
}

class Universe {
static void WorkerStartedWork() {
Console.WriteLine("Universe notices worker starting work");
}

static int WorkerCompletedWork() {
System.Threading.Thread.Sleep(4000);
Console.WriteLine("Universe is pleased with worker''s work");
return 7;
}

static void Main() {
Worker peter = new Worker();
Boss boss = new Boss();
peter.completed += new WorkCompleted(boss.WorkCompleted);
peter.started += new WorkStarted(Universe.WorkerStartedWork);
peter.completed += new WorkCompleted(Universe.WorkerCompletedWork);
peter.DoWork();

Console.WriteLine("Main: worker completed work");
Console.ReadLine();
}
}

Peter知道异步获取结果会带来一些问题。由于异步触发事件，所以目标方法有可能执行于另一个线程中，就像Peter的“目标方法何时完成”的通知那样。然而，Peter熟悉第14章“多线程用户界面”，因此，他知道在构建WinForms应用程序时如何去处理此类问题。

从此，他们一直过得都很快乐。