vb.net，combobox.datasource会改变所选的索引吗？

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问题描述

让我尝试用最简单的方式描述我的问题：我有combobox1和combobox2。我希望实现两件事：

Let me try to describe my problems in the simplest way: I have combobox1 and combobox2. I hope to achieve two things:

Combox1绑定到list1（字符串列表）。当用户选择list1中的项目时，将从数据库中获取list2（字符串列表），并将combobox绑定到list2。

Combox1 is bound to list1 (a list of string). When a user selects an item in list1, list2 (a list of string) will be obtained from database and combobox is bound to list2.

在combobox1中的combobox1和text2中，这两个值将显示在组合框中。因此我将DropDown设置为dropdpwnstyle到两个组合框。

So I set DropDown as dropdpwnstyle to both comboboxes.

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

当我调试时，我注意到两件事：

When I debug, I notice two things:

After Me.combobox1.SelectedText = text1， Me.combobox1.SelectedText =。但Me.combobox1.Text = text1。这是因为combobox1.SelectedIndex = -1？

After Me.combobox1.SelectedText=text1, actually, Me.combobox1.SelectedText="". But Me.combobox1.Text=text1. Is this because combobox1.SelectedIndex=-1?

Combobox1.datasource = m_list1将combobox1.selectedindex从-1更改为0.这将触发combobox.selectedIndexchange

Combobox1.datasource=m_list1 changes combobox1.selectedindex from -1 to 0. This will fire the combobox.selectedIndexchange event.

因此，上述代码的结果是实现了目标1，但目标2从未实现。

So the results of above code is that goal 1 is achieved, but goal 2 is never achieved. combobox1.selected index is always 0 and combobox2.selected index is always 0 too.

推荐答案

这里有两个代表国家和大陆的类：

Here 2 classes that represents country and continent :

'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

现在将2个组合框添加到名为cmbContinent和cmbCountry的表单中，然后将以下代码添加到您的表单：

Now add 2 comboboxs to a form named cmbContinent and cmbCountry, then add the following code to your form :

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

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