获取DataGridViewComboboxColumn SelectedValue(VB.Net)

我需要在DataGridView中获取ComboBox的选定值.我有部分工作,但如果我在网格中更改另一个ComboBox,我会得到一个Null Reference Exception.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

这在第一次更改ComboBox时工作正常,但如果另一个ComboBox发生更改,则会生成Null Reference Exception.任何想法为什么会这样？注意：我在MSDN的讨论表中找到了大部分此代码.

谢谢！

彼得

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