C#

public class Checks;
{
  public Checks(int RegisterID, int CatID);
  {
    :
    :
  }
}

public class Register
{
  public Register(int size)
  {
    :
    :
  }
  
  public object this[int RegisterID, int CatID]
  {
   get
      {
      string key = Convert.ToString(RegisterID);
      key += Convert.ToString(CatID);
      return KeyToObject(key);
      }

   set
      {
         :
         :
      }
  }

  protected object KeyToObject(string key)
  {
   for(int n = 0; n < _count; ++n)
          {
           Checks trankey = (Checks)_items[n];
           if(key == trankey.key)
                  return trankey;
           }
   return null;
   }
}

class Class1
{
  static void Main(string[] args)
  {
     Register Pay = new Register (1);
     Checks temp = new Checks (5,3);
     temp.Description = "Vons";
     temp.Amount = 30.50m;
     Register [5,3]= temp;
     temp = new Checks(6,2);
     temp.Description = "Home Depot";
     temp.Amount = 126.75m;
     Register [6,2]= temp;
     temp = Register [5,3];   // throws the error
  }
}