vb.net，combobox.datasource會改變選定的索引？

Question

讓我試着用最簡單的方式來描述我的問題：我有combobox1和combobox2。我希望實現兩件事情：

1. Combox1綁定到list1（字符串列表）。當用戶選擇list1中的一個項目時，將從數據庫獲取list2（一個字符串列表），並將組合框綁定到list2。

2. 如果用戶在combobox1中指定了text1並在combobox2中指定了text2，那麼無論綁定列表如何，這兩個值都將顯示在組合框中。

因此，我將DropDown設置爲dropdpwnstyle到兩個組合框。

Public Sub New(Optional ByVal text1 As String = "", Optional ByVal text2 As String = "") 
     ' This call is required by the designer. 
     InitializeComponent() 

     ' Add any initialization after the InitializeComponent() call. 
     Me.combobox1.selectedText=text1 
     Me.combobox2.selectedText=text2 
End Sub 


Private Sub Form_Load(sender As Object, e As System.EventArgs) Handles Me.Load 
    BindComboBox1() 
End Sub 

Private Sub BindComboBox1() 
     'm_list1 is a list of string 

combobox1.DataSource = m_list1 

End Sub 

Private Sub GetCombobox2() 

    'based on the selected item in combobox1, m_list2 which is a list of string is obtained 

    ComboBox2.DataSource = m_list2 
End Sub 

Private Sub combobox1_SelectedIndexChanged(sender As Object, e As System.EventArgs) Handles combobox1.SelectedIndexChanged 
     If combobox1.SelectedIndex <> -1 Then 
      GetCombobox2() 

     End If 
End Sub 

當我調試，我注意到兩兩件事：

1. Me.combobox1.SelectedText =文本1之後，實際上，Me.combobox1.SelectedText = 「」。但是Me.combobox1.Text = text1。這是因爲combobox1.SelectedIndex = -1？

2. Combobox1.datasource = m_list1將combobox1.selectedindex從-1更改爲0.這將觸發combobox.selectedIndexchange事件。

因此，上述代碼的結果是實現了目標1，但目標2從未實現。 combobox1.selected索引始終爲0，combobox2.selected索引始終爲0。

Answer 1

以下2類，代表國家和大洲：

'Coded by Amen Ayach's DataClassBuilder @25/02/2012 
Public Class CountryCls 

    Private _CountryID As Integer 
    Public Property CountryID() As Integer 
     Get 
      Return _CountryID 
     End Get 
     Set(ByVal value As Integer) 
      _CountryID = value 
     End Set 
    End Property 

    Private _CountryName As String 
    Public Property CountryName() As String 
     Get 
      Return _CountryName 
     End Get 
     Set(ByVal value As String) 
      _CountryName = value 
     End Set 
    End Property 

    Private _ContinentID As Integer 
    Public Property ContinentID() As Integer 
     Get 
      Return _ContinentID 
     End Get 
     Set(ByVal value As Integer) 
      _ContinentID = value 
     End Set 
    End Property 

End Class 


'Coded by Amen Ayach's DataClassBuilder @25/02/2012 
Public Class ContinentCls 

    Private _ContinentID As Integer 
    Public Property ContinentID() As Integer 
     Get 
      Return _ContinentID 
     End Get 
     Set(ByVal value As Integer) 
      _ContinentID = value 
     End Set 
    End Property 

    Private _ContinentName As String 
    Public Property ContinentName() As String 
     Get 
      Return _ContinentName 
     End Get 
     Set(ByVal value As String) 
      _ContinentName = value 
     End Set 
    End Property 

End Class 

現在增加2個comboboxs到一個名爲cmbContinent和cmbCountry形式，然後將下面的代碼添加到您的窗體：

Dim ContinentList As New List(Of ContinentCls) 
Dim CountryList As New List(Of CountryCls) 

Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load 

    'Initialize some fake data 
    For i = 1 To 3 
     ContinentList.Add(New ContinentCls With {.ContinentID = i, .ContinentName = "Continent" + CStr(i)}) 
     For j = 1 To 5 
      CountryList.Add(New CountryCls With {.ContinentID = i, .CountryID = j, .CountryName = "Cont" + CStr(i) + " - Country" + CStr(j)}) 
     Next 
    Next 

    'Filling out ContinentCombo 
    With cmbContinent 
     .ValueMember = "ContinentID" 
     .DisplayMember = "ContinentName" 
     .DataSource = ContinentList 
    End With 

End Sub 

Private Sub cmbContinent_SelectedValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmbContinent.SelectedValueChanged 
    Try 
     'Filling out CountryCombo according to seleced ContinentCombo 
     With cmbCountry 
      .ValueMember = "CountryID" 
      .DisplayMember = "CountryName" 
      .DataSource = CountryList.Where(Function(f) f.ContinentID = cmbContinent.SelectedValue).ToList 
     End With 
    Catch ex As Exception 
     MsgBox(ex.Message) 
    End Try 
End Sub