为什么组合框在创建时将其项目增加一倍? [英] Why is the combobox doubling its items when being created?
问题描述
我正在以编程方式创建一个组合框,如下所示:
I'm creating a combobox programmatically as follows:
var cbo = new ComboBox();
cbo.HandleCreated += (s, e) =>
{
cbo.DataSource = mylist;
};
当我执行上述操作时,组合框会显示列表中包含的项的两倍。但是,当我执行以下操作时,组合框显示正确的项目数:
When I do the above, the combobox displays double the items contained in the list. However, when I do the following, the combobox displays the correct number of items:
var cbo = new ComboBox() {
DataSource = mylist
};
为什么会这样?
推荐答案
原因
Reason
HandleCreated
仅引发一次。问题是另外一回事,这是因为ComboBox
中实现了OnHandleCreated
和数据绑定的方式。
The
HandleCreated
raises just once. the problem is something else, it's because of the way thatOnHandleCreated
and data-binding has implemented inComboBox
.
这是 ComboBox $ c $的
方法c>有效: OnHandleCreated
This is how OnHandleCreated
method of the ComboBox
works:
- 它首先引发
HandleCreated
事件。 (请记住,IsHandleCreated
在这一点上是正确的。) - 然后针对
Items
控件的集合,发送CB_ADDSTRING
本机消息将项目添加到本机组合框。
- It first raise
HandleCreated
event. (Keep in mind,IsHandleCreated
is true at this point.) - Then for each item in
Items
collection of the control, sends aCB_ADDSTRING
native message to add the item to native combo box.
这是设置 DataSource
的工作方式:
- 对于
DataSource
中的每个项目,它首先将该项目添加到项目
集合,然后检查IsHandleCreated
是否为真,发送CB_ADDSTRING
本机消息以添加该项目到本机组合框。
- For each item in
DataSource
, it first adds the item toItems
collection, then checks ifIsHandleCreated
is true, sends aCB_ADDSTRING
native message to add the item to native combo box.
因此,当您在 HandleCreated <中设置
DataSource
时/ code>事件,对于每个项目,它都会发送 CB_ADDSTRING
本机消息两次。
So when you set DataSource
in HandleCreated
event, for each item it sends CB_ADDSTRING
native message twice.
这就是为什么您在下拉菜单中两次看到项目,同时 Items.Count
显示正确计数的原因。此外,如果您单击其他项目(项目的最后一半),则会收到索引超出范围的异常。
That's why you see items twice in the drop-down and at the same time Items.Count
shows correct count. Also it you click on additional item (the last half of the items) you will receive an index out of range exception.
要解决此问题,可以使用以下任一视蛋白:
To solve the problem, you can use either of the following optoins:
-
您可以延迟通过使用
BeginInvoke
HandleCreated 事件代码>作为另一种选择,您可以依赖 VisibleChanged
事件。
As another option you can rely on VisibleChanged
event.
选项1-HandleCreated + BeginInvoke
var mylist = Enumerable.Range(1, 5).ToList();
var myvalue = 2;
var cbo = new ComboBox();
cbo.HandleCreated += (obj, args) =>
{
BeginInvoke(new Action(() =>
{
cbo.DataSource = mylist;
cbo.SelectedIndex = mylist.IndexOf(myvalue);
}));
};
this.Controls.Add(cbo);
选项2-VisibleChanged
var mylist = Enumerable.Range(1, 5).ToList();
var myvalue = 2;
var cbo = new ComboBox();
cbo.VisibleChanged+= (obj, args) =>
{
cbo.DataSource = mylist;
cbo.SelectedIndex = mylist.IndexOf(myvalue);
};
this.Controls.Add(cbo);
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