collection属性设计窗口如何显示上次添加的成员?

在datagrid控件中添加属性，名称是为Columns，类型是Collection。
设计时打开该属性添加成员，完成添加后，关闭项目后重新打开项目，再次打开属性设计窗口，发现上次添加的项目没有在成员列表中，原代码如下：
'定义一个集合myCollection
Public Class myCollection
Inherits System.Collections.CollectionBase

Default Public Property Item(ByVal index As Integer) As DataGridTextBoxColumn
Get
Return CType(List(index), DataGridTextBoxColumn)
End Get
Set(ByVal Value As DataGridTextBoxColumn)
List(index) = Value
End Set
End Property

Public Function Add(ByVal value As DataGridTextBoxColumn) As Integer
Return List.Add(value)
End Function 'Add

Public Function IndexOf(ByVal value As DataGridTextBoxColumn) As Integer
Return List.IndexOf(value)
End Function 'IndexOf


Public Sub Insert(ByVal index As Integer, ByVal value As DataGridTextBoxColumn)
List.Insert(index, value)
End Sub 'Insert


Public Sub Remove(ByVal value As DataGridTextBoxColumn)
List.Remove(value)
End Sub 'Remove


Public Function Contains(ByVal value As DataGridTextBoxColumn) As Boolean
' If value is not of type Int16, this will return false.
Return List.Contains(value)
End Function 'Contains
End Class

'定义一个myCollection类型的变量
Private theDataGridTextBoxColumn As New myCollection

'定义一个集合类型是myCollection的属性
Public Property Columns() As myCollection
Get
Return theDataGridTextBoxColumn
End Get
Set(ByVal Value As myCollection)
Dim i As Integer
theDataGridTextBoxColumn.Clear()
For i = 0 To Value.Count - 1
theDataGridTextBoxColumn.Insert(i, Value(i))
Next
'theDataGridTextBoxColumn = Value
End Set
End Property
请大虾帮我看看问题出在哪里？怎么改？