vb.net，combobox.datasource 会改变选定的索引吗？

Question

让我试着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：

1. Combox1 绑定到 list1（字符串列表）。当用户选择 list1 中的一个项目时，将从数据库中获取 list2（字符串列表）并将组合框绑定到 list2。

2. 如果用户在 combobox1 中指定 text1，在 combobox2 中指定 text2，则这两个值将显示在组合框中，无论绑定列表如何。

所以我将 DropDown 设置为两个组合框的 dropdpwnstyle。

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：

1. 在 Me.combobox1.SelectedText=text1 之后，实际上是 Me.combobox1.SelectedText=""。但是 Me.combobox1.Text=text1。这是因为 combobox1.SelectedIndex=-1 吗？

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

所以上面代码的结果是目标 1 实现了，但目标 2 永远没有实现。 combobox1.selected 索引始终为 0，combobox2.selected 索引也始终为 0。

Answer 1

这里有 2 个代表国家和大陆的类：

'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中：

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