在为集合类型依赖项属性分配值时，将创建单例

Question

我正在尝试在包含控件的所有子项的List（Of）类型的WPF中创建依赖项对象。尽管采取了here概述的预防措施，但在为属性分配值时似乎创建了单例。当我分配一个值时，会添加一个值乘以该类的实例数。

这是我的代码：

Public Class Period
Inherits ListBox

'-------- PROPERTIES --------'
Private Property Subjects() As List(Of Subject)
    Get
        Return Me.GetValue(SubjectsProperty)
    End Get
    Set(value As List(Of Subject))
        Me.SetValue(SubjectsProperty, value)
    End Set
End Property

Private Shared ReadOnly SubjectsProperty As DependencyProperty = DependencyProperty.Register("Subjects", GetType(List(Of Subject)), GetType(Period), New FrameworkPropertyMetadata(New List(Of Subject))) 

Public Sub New()
            MyBase.New()
            Me.Subjects = New List(Of Subject) 'Reason: stop it from adding values to defult value collection. See:http://msdn.microsoft.com/en-us/library/aa970563.aspx
End Sub

此子例程用于在添加子项时刷新属性：

 Private Sub RefreshSubjectsProperty()
    Dim ListOfSubjects As New List(Of Subject)
    Dim Subject As New Subject
    Dim Entry As New PeriodEntry

    For Each Entry In LogicalTreeHelper.GetChildren(Me)
        Subject.SubjectName = Entry.SubjectName
        Subject.Teacher = Entry.Teacher
        Subject.Students = Entry.Students

        ListOfSubjects.Add(Subject)
    Next

    Me.Subjects = ListOfSubjects
End Sub