public void RegisterNext(IExpenseHander next)
 {
     _next = next;
 }
 public void RegisterNext(IExpenseHander next)
 {
     throw new InvalidOperationException("End of chain handler must be the end of chain");
 }
 public ExpenseHandler(IExpenseApprower approver)
 {
     _approver = approver;
     _next     = EndOfChainExpenseHandler.Instance;
 }