ch5_8.cs

《c#技术内幕代码》

using System;

public delegate Boolean AccountEventHandler(object sender, double amt);
   
public class Account
{
   private double Total;
   public 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 Boolean OnPost( double amt )
   {
      if ( ah != null )
         return ah(this, amt );
      return true;
   }
   
   public void PostDebit(double amt)
   {
      // Fire the event, checking to see if the return is ok
      Boolean okToSubtract = OnPost(amt);
      if ( okToSubtract )
      {
         Console.WriteLine("Debit ok!");
         Total -= amt;
      }
      else
      {
         Console.WriteLine("Debit NOT ok! Not posting");
      }
   }
   public void PostCredit( double amt )
   {
      Total += amt;
   }
}

public class CH5_5
{
   public static Boolean CheckPost( object o, double d )
   {
      // See if it went negative
      Account a = (Account)o;
      if ( d > a.getTotal() )
         return false;
      return true;
   }
   
   public static void Main()
   {
      Account a = new Account();
      AccountEventHandler evh = new 
         AccountEventHandler( CheckPost );
      a.AddOnPost( evh );

      // Can we debit 100?
      if ( a.OnPost( 100.00 ) )	 
         a.PostDebit( 100.00 );
      else
         Console.WriteLine("Cannot debit 100.0");
   }
}