我有一个带有两个组合框的 Windows 窗体。每个组合框的SelectedValue属性都是绑定到简单 DTO 上的属性的数据。每个组合框的选项都来自模型对象列表。我只需要表单上的控件来更新 DTO；我不需要以编程方式修改任何 DTO 的属性并查看正在更新的相应控件 - 即，我只需要单向（控件 -> 源）数据绑定即可工作。
当用户更改第一个组合框的值时，第二个组合框的选项将完全改变。但是，我在此设置中遇到了两个问题，我无法弄清楚它们发生的原因或如何解决它们：
- 每当更改第一个组合框时，数据绑定框架就会生成并吞下 NRE（我可以在 Visual Studio IDE 的即时窗口中看到它抛出），这提示我某些设置不正确。更改第二个组合框或任何其他不相关的数据绑定控件（组合框或其他）不会生成 NRE。
- 同样，每当第一个组合框发生变化时，在生成上述 NRE 后，第二个组合框加载成功，但第一个组合框的选定索引重置为 -1。我怀疑这是因为数据绑定的“推送”事件触发以更新控件，并且由于某种原因，支持第一个组合框的 DTO 属性的值被重置为 NULL/Nothing。
有谁知道为什么会发生这些事情？我模拟了我的问题，它展示了上述两个问题。我还添加了第三个组合框，它与前两个中的任何一个都没有关系，就像一个完整性检查以表明一个不依赖于另一个组合框的组合框可以正常工作。
此代码复制了问题 - 粘贴为 Visual Basic Windows 窗体项目（3.5 框架）的默认 Form1 类的代码。
Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Windows.Forms
Public Class Form1
Inherits System.Windows.Forms.Form
'Form overrides dispose to clean up the component list.
<System.Diagnostics.DebuggerNonUserCode()> _
Protected Overrides Sub Dispose(ByVal disposing As Boolean)
Try
If disposing AndAlso components IsNot Nothing Then
components.Dispose()
End If
Finally
MyBase.Dispose(disposing)
End Try
End Sub
'Required by the Windows Form Designer
Private components As System.ComponentModel.IContainer
'NOTE: The following procedure is required by the Windows Form Designer
'It can be modified using the Windows Form Designer.
'Do not modify it using the code editor.
<System.Diagnostics.DebuggerStepThrough()> _
Private Sub InitializeComponent()
Me.cboA = New System.Windows.Forms.ComboBox()
Me.cboB = New System.Windows.Forms.ComboBox()
Me.cboC = New System.Windows.Forms.ComboBox()
Me.SuspendLayout()
'
'cboA
'
Me.cboA.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList
Me.cboA.FormattingEnabled = True
Me.cboA.Location = New System.Drawing.Point(120, 25)
Me.cboA.Name = "cboA"
Me.cboA.Size = New System.Drawing.Size(121, 21)
Me.cboA.TabIndex = 0
'
'cboB
'
Me.cboB.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList
Me.cboB.FormattingEnabled = True
Me.cboB.Location = New System.Drawing.Point(120, 77)
Me.cboB.Name = "cboB"
Me.cboB.Size = New System.Drawing.Size(121, 21)
Me.cboB.TabIndex = 1
'
'cboC
'
Me.cboC.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList
Me.cboC.FormattingEnabled = True
Me.cboC.Location = New System.Drawing.Point(120, 132)
Me.cboC.Name = "cboC"
Me.cboC.Size = New System.Drawing.Size(121, 21)
Me.cboC.TabIndex = 2
'
'Form1
'
Me.AutoScaleDimensions = New System.Drawing.SizeF(6.0!, 13.0!)
Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
Me.ClientSize = New System.Drawing.Size(284, 262)
Me.Controls.Add(Me.cboC)
Me.Controls.Add(Me.cboB)
Me.Controls.Add(Me.cboA)
Me.Name = "Form1"
Me.Text = "Form1"
Me.ResumeLayout(False)
End Sub
Friend WithEvents cboA As System.Windows.Forms.ComboBox
Friend WithEvents cboB As System.Windows.Forms.ComboBox
Friend WithEvents cboC As System.Windows.Forms.ComboBox
Private _DataObject As MyDataObject
Private _IsInitialized As Boolean = False
Public Sub New()
' This call is required by the Windows Form Designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
_DataObject = New MyDataObject()
BindControls()
End Sub
Private Sub BindControls()
LoadComboA(cboA)
Dim cmbABinding As New Binding("SelectedValue", _DataObject, "ValueA", True, DataSourceUpdateMode.OnPropertyChanged)
cboA.DataBindings.Add(cmbABinding)
Dim cmbBBinding As New Binding("SelectedValue", _DataObject, "ValueB", True, DataSourceUpdateMode.OnPropertyChanged)
cboB.DataBindings.Add(cmbBBinding)
LoadComboC(cboC)
Dim cmbCBinding As New Binding("SelectedValue", _DataObject, "ValueC", True, DataSourceUpdateMode.OnPropertyChanged)
cboC.DataBindings.Add(cmbCBinding)
End Sub
Protected Overrides Sub OnLoad(ByVal e As System.EventArgs)
MyBase.OnLoad(e)
_IsInitialized = True
cboA.SelectedIndex = 0
cboC.SelectedIndex = 0
End Sub
Private Sub ComboA_SelectedValueChanged(ByVal sender As Object, ByVal e As EventArgs) Handles cboA.SelectedValueChanged
If _IsInitialized Then
LoadComboB(cboB, cboA.SelectedValue.ToString())
cboB.SelectedIndex = 0
End If
End Sub
Private Sub LoadComboA(ByVal cmbBox As ComboBox)
Dim someData As New Dictionary(Of String, String)()
someData.Add("Value1", "Text 1")
someData.Add("Value2", "Text 2")
someData.Add("Value3", "Text 3")
cmbBox.DataSource = someData.ToList()
cmbBox.DisplayMember = "Value"
cmbBox.ValueMember = "Key"
End Sub
Private Sub LoadComboB(ByVal cmbBox As ComboBox, ByVal selector As String)
Dim someSubData As New Dictionary(Of String, String)()
Select Case selector
Case "Value1"
someSubData.Add("SubValue1", "Value1 - Sub Text 1")
someSubData.Add("SubValue2", "Value1 - Sub Text 2")
someSubData.Add("SubValue3", "Value1 - Sub Text 3")
Case "Value2"
someSubData.Add("SubValue4", "Value2 - Sub Text 4")
someSubData.Add("SubValue5", "Value2 - Sub Text 5")
someSubData.Add("SubValue6", "Value2 - Sub Text 6")
Case "Value3"
someSubData.Add("SubValue7", "Value3 - Sub Text 7")
someSubData.Add("SubValue8", "Value3 - Sub Text 8")
someSubData.Add("SubValue9", "Value3 - Sub Text 9")
End Select
cmbBox.DataSource = someSubData.ToList()
cmbBox.DisplayMember = "Value"
cmbBox.ValueMember = "Key"
End Sub
Private Sub LoadComboC(ByVal cmbBox As ComboBox)
Dim someData As New Dictionary(Of String, String)()
someData.Add("Value100", "Text 100")
someData.Add("Value101", "Text 101")
cmbBox.DataSource = someData.ToList()
cmbBox.DisplayMember = "Value"
cmbBox.ValueMember = "Key"
End Sub
End Class
Public Class MyDataObject ' DTO class
Private _ValueA As String
Public Property ValueA() As String
Get
Return _ValueA
End Get
Set(ByVal value As String)
_ValueA = value
End Set
End Property
Private _ValueB As String
Public Property ValueB() As String
Get
Return _ValueB
End Get
Set(ByVal value As String)
_ValueB = value
End Set
End Property
Private _ValueC As String
Public Property ValueC() As String
Get
Return _ValueC
End Get
Set(ByVal value As String)
_ValueC = value
End Set
End Property
End Class