获取 DataGridViewComboboxColumn SelectedValue (VB.Net)

Question

我需要获取 DataGridView 中 ComboBox 的选定值。我已经部分工作了，但是如果我更改网格中的另一个组合框，我会收到空引用异常。这是我的代码：

Private Sub dgvSampleList_EditingControlShowing(ByVal sender As Object, ByVal e As DataGridViewEditingControlShowingEventArgs) Handles dgvSampleList.EditingControlShowing
    Dim comboBox As ComboBox = CType(e.Control, ComboBox)

    If (comboBox IsNot Nothing) Then
        'Remove an existing event-handler
        RemoveHandler comboBox.SelectedIndexChanged, New EventHandler(AddressOf ComboBox_SelectedIndexChanged)

        'Add the event handler. 
        AddHandler comboBox.SelectedIndexChanged, New EventHandler(AddressOf ComboBox_SelectedIndexChanged)
    End If
End Sub

Private Sub ComboBox_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim comboBox As ComboBox = CType(sender, ComboBox)
    'Display selected value
    MsgBox("ProgramID: " & comboBox.SelectedValue.ToString)
End Sub

第一次更改组合框时效果很好，但如果更改另一个组合框，则会生成空引用异常。有什么想法为什么会发生这种情况吗？注意：我在 MSDN 的讨论表单上找到了大部分此类代码。

谢谢！

彼得

Answer 1

不必要时最好避免使用全局变量。

在尝试访问comboBox的属性之前，您只需要测试comboBox是否什么都没有：

Private Sub ComboBox_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim comboBox As ComboBox = CType(sender, ComboBox)
    'Display selected value
    If comboBox IsNot Nothing Then
        MsgBox("ProgramID: " & comboBox.SelectedValue.ToString)
    End If
End Sub

在我看来，当comboBox从旧值设置为新值时值，为旧的和新的组合框调用此 SelectedIndexChanged 事件。我怀疑当它被旧的 comboBox 调用时，发送者为 null/Nothing，因为它的值正在改变。或许。但无论发生什么，空就是空。在尝试访问它的任何属性之前，只需测试它是否不为 null。

Answer 2

尝试检查 comboBox.SelectedItem.ToString 而不是 comboBox.SelectedValue.ToString

希望有帮助。

Answer 3

我有同样的问题。通过对代码进行一些小的更改来解决。

声明一个全局变量

Dim comboBoxCol As New DataGridViewComboBoxColumn
Dim gol As Integer = 0



 Dim comboBox As ComboBox
    Private Sub dgvSampleList_EditingControlShowing(ByVal sender As Object, ByVal e As DataGridViewEditingControlShowingEventArgs) Handles DGVItems.EditingControlShowing
        comboBox = CType(e.Control, ComboBox)

        If (comboBox IsNot Nothing) Then

            'Add the event handler.  
            AddHandler comboBox.SelectedIndexChanged, New EventHandler(AddressOf ComboBox_SelectedIndexChanged)
            gol = 1
            'AddHandler comboBox.SelectionChangeCommitted, New EventHandler(AddressOf ComboBox_SelectedIndexChanged)
        End If
    End Sub

    Private Sub ComboBox_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
        comboBox = CType(sender, ComboBox)
        If gol = 1 Then
            Dim item As String = comboBox.Text
            MsgBox(item)
            gol = 0
        End If
  End Sub