大家好，好久不来这里了。此段代码的错误提示该如何处理？

Option Strict On
Structure CUSTOMER
Dim COMPANY As String
Dim MANAGER As String
Dim ADDERSS As String
Dim CITY As String
Dim COUNTRY As String
Dim CUSTOMERSINCE As Date
Dim BALANCE As Decimal
End Structure

Public Class Form1
'该数组保存了10个客户的数据
Private CUSTOMERS(9) As CUSTOMER

Private CUST As CUSTOMER
Private CURRENTINDEX As Integer

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

End Sub
'该按钮调用GETCUSTOMER()函数来获取下一个客户的数据,并利用SHOWCUSTOMER()子程序把客
'户的数据显示在窗体上的标签里,然后增加变量CURRENTINDEX的值,让它指向当前客户的索引
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
If CURRENTINDEX = COUNTCUSTOMERS() Then CURRENTINDEX = 0

End Sub
'返回CUSTOMERS数组里的记录数量
Function COUNTCUSTOMERS() As Integer
Return (CUSTOMERS.Length)
End Function
Function GETCUSTOMER(ByVal IDX As Integer) As CUSTOMER

End Function
End Class

红色部分提示：
错误 1 “GETCUSTOMER”不能通过 class“Form1”在项目外部公开类型“CUSTOMER”。 C:\Documents and Settings\Administrator\Local Settings\Application Data\Temporary Projects\types项目（包括自定义数据类型）\Form1.vb 32 51 types项目（包括自定义数据类型）

这是书上的例子，该怎么办？