VB.NET手动填充ComboBox后SelectedValue返回Nothing的问题求助

我需要将手动填充的ComboBox选项（数据为"C|Credit,D|Debit"）中选中项的C或D值存入数据库。在Form_Load中调用以下代码填充ComboBox：

FillCombosWithValueMDisplayM(m01OpnCrDr, "C|Credit,D|Debit")

但调用CollectFormData函数收集窗体控件数据时，该ComboBox的SelectedValue返回Nothing。

填充ComboBox的公共子过程 #

Sub FillCombosWithValueMDisplayM(CtrlCombo As ComboBox, CommaSepVDPair As String)
       
        Dim pairs() = CommaSepVDPair.Split(",")
        For Each pair As String In pairs
            Dim KeyVal() = pair.Split("|")
            Dim newItem As New With {.Value = KeyVal(0).ToString, .Display = KeyVal(1).ToString}
            CtrlCombo.Items.Add(newItem)
        Next

        CtrlCombo.ValueMember = "Value"
        CtrlCombo.DisplayMember = "Display"

End Sub

收集窗体数据的公共函数（部分代码） #

Function CollectFormData(frm As Form, tbl As DataTable)
        'Function CollectFormData(frm As Form, FieldList As List(Of String)) As Dictionary(Of String, Object)
        Dim FormDataDict As New Dictionary(Of String, Object)
        Dim CtlType As Type
        Dim CtrlArray As Control()
        Dim Ctrl As Control
        Dim fldname As String
        Dim FldType As OleDb.OleDbType
        Dim CtlValuePlaceHolder As Object

        For Each row As DataRow In tbl.Rows
            fldname = row("COLUMN_NAME")
            FldType = row("DATA_TYPE").ToString

            If fldname = "ID" Then
                'do nothing
            Else
                CtrlArray = frm.Controls.Find(fldname, True)

                If CtrlArray.Count > 0 Then
                    Ctrl = CtrlArray(0)
                    CtlType = Ctrl.GetType

                    Select Case CtlType
                        Case GetType(TextBox)
                            CtlValuePlaceHolder = CNulls(TryCast(Ctrl, TextBox).Text, FldType)
                            FormDataDict.Add(fldname, CtlValuePlaceHolder)

                        Case GetType(ComboBox) '(here combobox m01OpnCrDr returns Nothing)
                            'CtlValuePlaceHolder = CNulls(TryCast(Ctrl, ComboBox).SelectedValue, FldType)
                            Dim selectedValue As Object = TryCast(Ctrl, ComboBox).SelectedValue

                            CtlValuePlaceHolder = CNulls(selectedValue, FldType)
                            FormDataDict.Add(fldname, CtlValuePlaceHolder)

                        Case GetType(C1.Win.Calendar.C1DateEdit)
                            CtlValuePlaceHolder = CNulls(TryCast(Ctrl, C1.Win.Calendar.C1DateEdit).Value, FldType)
                            FormDataDict.Add(fldname, CtlValuePlaceHolder)

                        Case GetType(CheckBox)
                            If TryCast(Ctrl, CheckBox).Checked Then
                                CtlValuePlaceHolder = True
                            Else
                                CtlValuePlaceHolder = False
                            End If
                            FormDataDict.Add(fldname, CtlValuePlaceHolder)

                        Case GetType(DateTimePicker)

                            CtlValuePlaceHolder = CNulls(TryCast(Ctrl, DateTimePicker).Value, FldType)
                            FormDataDict.Add(fldname, CtlValuePlaceHolder)

                        Case GetType(C1.Win.Calendar.C1DateEdit)

                            CtlValuePlaceHolder = CNulls(TryCast(Ctrl, C1.Win.Calendar.C1DateEdit).Value, FldType)
                            FormDataDict.Add(fldname, CtlValuePlaceHolder)


                        Case GetType(MaskedTextBox)
                            CtlValuePlaceHolder = CNulls(TryCast(Ctrl, MaskedTextBox).Text, FldType)
                            FormDataDict.Add(fldname, CtlValuePlaceHolder)

                    End Select

                End If
            End If
        Next

        Return FormDataDict

End Function

问题原因 #

填充ComboBox时使用了匿名类型（New With {.Value = ..., .Display = ...}），WinForms的ComboBox无法正确识别匿名类型的属性来绑定ValueMember，导致SelectedValue始终返回Nothing。匿名类型是编译器内部生成的私有类型，ComboBox的绑定机制无法正常解析其属性。

解决方案 #

方法一：使用强类型类替代匿名类型（推荐） #

定义一个公开的类来存储ComboBox的选项数据：

Public Class ComboItem
    Public Property Value As String
    Public Property Display As String
End Class

修改填充ComboBox的子过程，使用该类创建选项：

Sub FillCombosWithValueMDisplayM(CtrlCombo As ComboBox, CommaSepVDPair As String)
    Dim pairs() = CommaSepVDPair.Split(",")
    For Each pair As String In pairs
        Dim KeyVal() = pair.Split("|")
        Dim newItem As New ComboItem() With {
            .Value = KeyVal(0).ToString,
            .Display = KeyVal(1).ToString
        }
        CtrlCombo.Items.Add(newItem)
    Next

    CtrlCombo.ValueMember = "Value"
    CtrlCombo.DisplayMember = "Display"
End Sub

这样ComboBox可以正确识别ValueMember和DisplayMember，SelectedValue就能正常返回选中项的Value值。

方法二：通过SelectedItem反射获取属性值 #

如果不想修改填充逻辑，可以在CollectFormData函数中，通过反射从SelectedItem中提取Value属性：

Case GetType(ComboBox)
    Dim cbo = TryCast(Ctrl, ComboBox)
    Dim selectedValue As Object = Nothing
    If cbo.SelectedItem IsNot Nothing Then
        ' 反射获取匿名类型的Value属性值
        selectedValue = cbo.SelectedItem.GetType().GetProperty("Value").GetValue(cbo.SelectedItem, Nothing)
    End If
    CtlValuePlaceHolder = CNulls(selectedValue, FldType)
    FormDataDict.Add(fldname, CtlValuePlaceHolder)

这种方法不需要修改填充逻辑，但反射会带来一定的性能开销，且代码维护性不如强类型方案。

内容的提问来源于stack exchange，提问作者Hemu