获取 DataGridViewComboboxColumn SelectedValue (VB.Net)答案

【问题标题】：Get DataGridViewComboboxColumn SelectedValue (VB.Net)获取 DataGridViewComboboxColumn SelectedValue (VB.Net)
【发布时间】：2011-06-13 04:29:35
【问题描述】：

我需要在 DataGridView 中获取 ComboBox 的选定值。我让它部分工作，但如果我更改网格中的另一个 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 时可以正常工作，但如果更改另一个 ComboBox，则会生成空引用异常。任何想法为什么会发生这种情况？注意：我在 MSDN 的讨论表单中找到了大部分代码。

谢谢！

彼得

【问题讨论】：

标签： vb.net reference null selectedvalue datagridviewcombobox

【解决方案1】：

全局变量在不必要时最好避免使用。

您只需要在尝试访问comboBox 的属性之前测试comboBox 是否为空：

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，新旧组合框都会调用此 SelectedIndexChanged 事件。我怀疑当它被调用旧的 comboBox 时，发件人是 null/Nothing，因为它的值正在改变。也许。但无论发生什么，空值就是空值。在尝试访问它的任何属性之前，只需测试它不为空即可。

【讨论】：

【解决方案2】：

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

希望对您有所帮助。

【讨论】：

【解决方案3】：

我也有同样的问题。通过对代码进行小的更改进行整理。

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

【讨论】：