获取DataGridViewComboboxColumn SelectedValue（VB.Net） [英] Get DataGridViewComboboxColumn SelectedValue (VB.Net)

问题描述

我需要在DataGridView中获取ComboBox的选定值。我有部分工作，但是如果更改网格中的另一个ComboBox，则会收到 Null Reference Exception 。这是我的代码：

I need to get the selected value of a ComboBox in a DataGridView. I have it partially working, but I get a Null Reference Exception if I change a another ComboBox in the grid. Here's my code:

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

在第一次更改ComboBox时此方法正常，但是如果更改了另一个ComboBox，则生成Null引用异常。任何想法为什么会这样？注意：我在MSDN的讨论表上找到了大部分代码。

This works fine the first time the ComboBox is changed, but generates a Null Reference Exception if another ComboBox is changed. Any ideas why this is happening? Note: I found most this code on MSDN's discussion forms.

谢谢！

Peter

推荐答案

最好避免在不必要时使用全局变量。

It's best to avoid global variables when they are unnecessary.

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

You just need to test for whether comboBox is nothing before trying to access a property of comboBox:

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，这个SelectedIndexChanged事件会同时被调用新的组合框。我怀疑当调用旧的 comboBox 时，发件人为null / Nothing，因为其值正在更改。也许。但是无论发生什么，null都是null。在尝试访问其任何属性之前，只需测试它是否不为空即可。

It seems to me that when the comboBox is set from an old value to the new value, that this SelectedIndexChanged event gets called for both the old and new comboboxes. I suspect that when it gets called for the old comboBox, the sender is null/Nothing because its value is getting changed. Maybe. But no matter what it is happening, a null is a null. Just test that it's not null before you try to access any of its properties.

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