blocks|key|1452895|text|Text是一种引用类型，因此如果直接赋值给myComboBox变量的ComboBox属性，它将更新控件。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1452896|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

ComboBox is a reference type, so if you assign directly to the Text property of the myComboBox variable, it will update the control.

blocks|key|153216|text|事件处理程序中的sender参数(通常)将包含对引发事件的对象的引用。如果是ComboBox控件的SelectedIndexChanged事件，则更改其SelectedIndex属性的将是ComboBox。因此，在上面的代码示例中，myComboBox引用的是引发事件的ComboBox。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|153217|需要说明的是:如果您在ComboBox控件的下拉列表中选择一项，以便它引发SelectedIndexChanged事件，则事件处理程序的发送方参数将是该ComboBox，而不是它的副本。对于所有引用类型都是如此。|153218|如果它是一个值类型，引发事件，那将是一个完全不同的故事，但这是非常罕见的情况(当涉及到窗体上的控件时，它从来都不是这样的)。|153219|entityMap^0|8|6|12|8|1D|K|24|D|2M|8|38|A|3R|8|0|B|8|11|K|24|8|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]|$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|17|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|18|8|@]|D|@]|E|$]]]|K|$]]

The <code>sender</code> parameter in an event handler will (typically) contain a reference to the object that raised the event. In the case of the <code>SelectedIndexChanged</code> event of a <code>ComboBox</code> control, it will be the <code>ComboBox</code> that had its <code>SelectedIndex</code> property changed. So in your code sample above <code>myComboBox</code> is referring the <code>ComboBox</code> that raised the event.

To clarify: if you select an item in the drop-down list of a <code>ComboBox</code> control, so that it raises the <code>SelectedIndexChanged</code> event, the sender parameter of the event handler will be that same <code>ComboBox</code>, not a copy of it. This is true for all reference types.

Had it been a value type raising the event it would have been a completely different story, but that is a very rare case (it it is never the case when it comes to controls on a form).

blocks|key|1180553|text|这应该是可行的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1180554|+Dim+ctl+As+Control
+Dim+cmb+As+ComboBox

+For+Each+ctl+In+Me.Controls
+++If+sender+Is+ctl+Then
+++++cmb+=+ctl
+++++cmb.Text+=+"string"
+++++End+If
+++Next+ctl|code-block|syntax|javascript|1180555|您可以在cmd.Text+=+"+string“的位置添加特定的字符串赋值代码。通过分配cmb，您可以使用特定的组合框属性，而不是通用控件属性。|1180556|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

This should work:

<pre><code> Dim ctl As Control
 Dim cmb As ComboBox

 For Each ctl In Me.Controls
 If sender Is ctl Then
 cmb = ctl
 cmb.Text = "string"
 End If
 Next ctl
</code></pre>

You can add the specific string assignment code in place of cmd.Text = "string". Assigning cmb allows you to use the specific combobox properties rather than the generic control properties.

blocks|key|1454777|text|为了回答最初的问题，为什么你会得到一个空对象引用异常，你正在搜索的控件是窗体的直接子级吗？它是位于窗体的面板中还是其他容器中？如果是这样的话，我不认为您会在窗体的Controls集合中找到该控件。所以这一行：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1454778|Me.Controls(myComboBox.Name).Text|code-block|syntax|javascript|1454779|只搜索窗体的直接子级，而不递归地搜索这些控件的子级。|1454780|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

To answer the original question of why you were getting a null object reference exception, is the Control you are searching for a direct child of the Form? Does it sit in a panel or some other container on the form? If so then I don't think you will find that control in the Form's Controls collection. So this line:

<pre><code>Me.Controls(myComboBox.Name).Text
</code></pre>

Only searches through the forms direct children and not recursively through the children of those controls.

blocks|key|4397346|text|如果我使用下面的代码，我将得到相同的err:+object+reference+not+set+to+a+Object的instance。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4397347|Me.Controls(TBName).DataBindings.Add("Text",+dt,+dt.Columns(colindex).ColumnName)|code-block|syntax|javascript|4397348|Christian+Pena的建议适合我的情况，所以正确的代码应该是：|4397349|Me.TCAll.TabPages(0).Controls(TBName).DataBindings.Add("Text",+dt,+dt.Columns(colindex).ColumnName)|4397350|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

If I using the following code, I will get the same err: Object reference not set to an instance of an object.

<pre><code>Me.Controls(TBName).DataBindings.Add("Text", dt, dt.Columns(colindex).ColumnName)
</code></pre>

Sugestion from Christian Pena fit my case, so the right code should be:

<pre><code>Me.TCAll.TabPages(0).Controls(TBName).DataBindings.Add("Text", dt, dt.Columns(colindex).ColumnName)
</code></pre>

I have a sub that handles when 14 ComboBoxes have their Index changed. I am able to cast the sender of the event, and obtain properties from there. However, after that, I want to be able to change the properties of the actual sender, rather than the cast one. How would I do this?

Current code:

<pre><code>Private Sub ComboBoxIndexChange(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged, ComboBox2.SelectedIndexChanged [etc]
 Dim myComboBox As ComboBox = sender

 Select Case myComboBox.Text
 Case "Will"
 Me.Controls(myComboBox.Name).Text = "555-555-555"
 Case "Bob"
 Me.Controls(myComboBox.Name).Text = "555-124-1234"
 [etc]
 End Select
 End Sub
End Class
</code></pre>

As you can see, I am currently trying to use

<pre><code>Me.Controls(myComboBox.Name).Text
</code></pre>

But I get the error: Object reference not set to an instance of an object.

What can I do?

Change properties of an unknown object in VB.NET

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我有一个sub，当14个ComboBoxes的索引发生变化时进行处理。我能够强制转换事件的发送者，并从那里获取属性。但是，在此之后，我希望能够更改实际发送者的属性，而不是强制转换发送者的属性。我该怎么做呢？当前代码：Private Sub ComboBoxIndexChange(ByVal sender As Syst...

问在VB.NET中更改未知对象的属性
EN

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