Vb.net 在绑定组合框中设置空值时的奇怪行为

我准备了一个小项目来演示我的问题。它由两个具有父子关系的数据表组成。我使用vb.net 2010和framework 4.0。
-启动新的WinForm项目。
-在Form1 designer上，拖动一个BindingNavigator、一个按钮和两个组合框。
-在代码隐藏中，复制以下代码：

Public Class Form1
 Private dsMain As DataSet
 Private WithEvents bndSource As New BindingSource

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

  'Create tables
  Dim dtContacts As New DataTable("Contacts")
  Dim col As DataColumn = dtContacts.Columns.Add("IDContact", GetType(Integer))
  col.AllowDBNull = False
  col.AutoIncrement = True
  col.Unique = True
  dtContacts.Columns.Add("FullName")
  col = dtContacts.Columns.Add("IDCountry", GetType(Integer))
  col.AllowDBNull = True
  dsMain.Tables.Add(dtContacts)

  Dim dtCountries As New DataTable("Countries")
  col = dtCountries.Columns.Add("IDCountry", GetType(Integer))
  col.AllowDBNull = False
  col.AutoIncrement = True
  col.Unique = True
  dtCountries.Columns.Add("A2ISO")
  dtCountries.Columns.Add("CountryName")
  dsMain.Tables.Add(dtCountries)

  'Add relation
  Dim rel As New DataRelation("rel", dtCountries.Columns("IDCountry"), dtContacts.Columns("IDCountry"))
  dsMain.Relations.Add(rel)

  'Populate parent table
  dtCountries.Rows.Add(1, "AF", "Afghanistan")
  dtCountries.Rows.Add(2, "AL", "Albania")
  dtCountries.Rows.Add(3, "DZ", "Algeria")

  'Populate child table
  dtContacts.Rows.Add(1, "First Contact", 3)
  dtContacts.Rows.Add(2, "Second Contact", 1)

  'Set bindings
  bndSource.DataSource = dtContacts.DefaultView
  BindingNavigator1.BindingSource = bndSource

  ComboBox1.DataSource = dtCountries
  ComboBox1.DisplayMember = "A2ISO"
  ComboBox1.ValueMember = "IDCountry"
  ComboBox1.DataBindings.Add("SelectedValue", bndSource, "IDCountry")
  ComboBox2.DataSource = dtCountries
  ComboBox2.DisplayMember = "CountryName"
  ComboBox2.ValueMember = "IDCountry"
  ComboBox2.DataBindings.Add("SelectedValue", bndSource, "IDCountry")

  dsMain.AcceptChanges()
 End Sub

 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
  ComboBox1.Text = Nothing 'Set IDCountry to DBNull, same as ComboBox1.SelectedIndex = -1
  ComboBox2.Text = Nothing
 End Sub
End Class

-运行项目。
-尝试来回导航，并更改组合框中的项目：全部正确。
-现在在Contacts表中设置IDCountry=Null，这是允许的。要实现此目的，请单击按钮1。
-在ComboBox1中，如果您选择与单击按钮之前相同的项目，您会看到行为不再正确：Combox2不会相应地使用ComboBox1更新，但仍然为空。
-如果您选择了其他项目，则从现在起，所有项目都将再次正确。
这是.net数据绑定中的错误吗？如果是，是否有解决办法？

---

提前感谢。

这是我找到的解决方法。我没有找到我观察到的行为的原因，但以下代码对我有效：

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged  
 If ComboBox1.SelectedIndex < ComboBox2.Items.Count Then ComboBox2.SelectedIndex = ComboBox1.SelectedIndex  
End Sub

Private Sub ComboBox2_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox2.SelectedIndexChanged
 If ComboBox2.SelectedIndex < ComboBox1.Items.Count Then ComboBox1.SelectedIndex = ComboBox2.SelectedIndex
End Sub  

Private子Combox1\u SelectedIndexChanged（ByVal发送方作为System.Object，ByVal e作为System.EventArgs）处理Combox1.SelectedIndexChanged
如果ComboBox1.SelectedIndex

它只是使ComboBox所选索引相等。
无论如何，谢谢你