ch5_5.cs

《c#技术内幕代码》

using System;


public class MyEventArgs : EventArgs 
{
   private readonly Boolean IsDebit;
   private readonly Boolean IsCredit;
   private readonly double  Amount;
   
   public MyEventArgs( Boolean IsD, double Amt )
   {
      if ( IsD )
      {
         IsDebit = true;
	 IsCredit = false;
      }
      else
      {
         IsDebit = false;
	 IsCredit = true;
      }
      Amount = Amt;
   }
   
   public Boolean IsADebit()
   {
      return IsDebit;
   }
   public Boolean IsACredit()
   {
      return IsCredit;
   }
   public double AmountOfDebitOrCredit()
   {
      return Amount;
   }
}

public delegate Boolean AccountEventHandler(object sender, MyEventArgs e);
   
public class Account
{
   private double Total;
   private AccountEventHandler ah = null;
      
         
   public Account()
   {
      Total = 0;
   }
   public double getTotal()
   {
      return Total;
   }
   public void AddOnPost(AccountEventHandler handler) 
   {
      ah = (AccountEventHandler)Delegate.Combine(ah, handler);
   }
   public void RemoveOnPost(AccountEventHandler handler) 
   {
      ah = (AccountEventHandler)Delegate.Remove(ah, handler);
   }
   public void PostDebit(double amt)
   {
      // First, create an argument list
      MyEventArgs ev = new MyEventArgs( true, amt );
      // Fire the event, checking to see if the return is ok
      Boolean okToSubtract = true;
      if ( ah != null )
         okToSubtract = ah(this, ev);
      if ( okToSubtract )
      {
         Console.WriteLine("Debit ok!");
         Total -= amt;
      }
      else
      {
         Console.WriteLine("Debit NOT ok! Not posting");
      }
   }
   public void PostCredit(double amt)
   {
      // First, create an argument list
      MyEventArgs ev = new MyEventArgs( false, amt );
      // Fire the event, checking to see if the return is ok
      Boolean okToAdd = true;
      if ( ah != null )
         okToAdd = ah(this, ev);
      if ( okToAdd )
         Total += amt;
   }
}

public class CH5_5
{
   public static Boolean CheckPost( object o, MyEventArgs arg )
   {
      if ( arg.IsADebit() )
      {
         // See if it went negative
	 Account a = (Account)o;
	 if ( arg.AmountOfDebitOrCredit() > a.getTotal() )
	    return false;
      }
      return true;
   }
   
   public static void Main()
   {
      Account a = new Account();
      AccountEventHandler evh = new 
         AccountEventHandler( CheckPost );
      a.AddOnPost( evh );
	 
      a.PostDebit( 100.00 );
      a.PostCredit( 200.00 );
      a.PostDebit( 100.00 );
   }
}