C#的类索引器

本代码创建了一个类的索引器用来访问类，就像访问数组一样

using System.Collections.Generic;
...
class Client
{
   private Dictionary<int, string> invoices 
      = new Dictionary<int, string>();
 
   public void Add (int id, string value)
      { invoices.Add (id, value); }
 
   public string this [int id]      // indexer
   {            
      get { return invoices[id]; }
      set { invoices[id] = value; }
   }
}
 
public class Test
{
   public static void Main( )
   {
      Client client = new Client();
      client.Add (1, "I005238A");
      Console.WriteLine ("Invoice: " + client[1]); // indexer access
    }	
}