我创建一个自定义的控件，继承自组合框。我要当一个项目被添加到组合框来执行自己的检查，以检测。不要紧，如果是C＃或Vb.NET，但我不知道该怎么做。

I am creating a custom control that inherits from ComboBox. I need to detect when a Item is added to the ComboBox to perform my own checks. It doesn't matter if it's C# or Vb.NET, but I don't know how to do it.

我尝试了所有的东西，我在互联网上找到，包括这个线程，但在答案的链接是离线，我没猜我应该怎么办。

I tried all the things I found across the internet, including this thread, but the link in the answer is offline and I didn't manage to guess what should I do.

例如，该code在Vb.net：

For example, this code in Vb.net:

Public Sub SomeFunc() Handles Items.CollectionChanged
    '....
End Sub

它说，产品属性没有被定义的 WithEvents就。

It says that Items property is not defined WithEvents.

控制没有使用的BindingSource。我需要控制时，该产品被添加到执行自定义操作。项目直接添加到 .Items 财产：

The control is not using a BindingSource. I need the control to perform a custom action when an Item is added. Items are added directly into the .Items property with:

customComboBox.Items.Add("item");

能不能做到？

Can it be done?

推荐答案

我认为最好的办法是将监听本机的 ComboBox的邮件：

I think the best approach would be to listen for the native ComboBox messages:

不要被这个词所迷惑的 STRING 的，他们都当添加，插入或删除项激活。因此，当列表被清除。

Don't be fooled by the word STRING, they are all fired whenever you add, insert or delete an item. So when the list is cleared.

Public Class UIComboBox
    Inherits ComboBox

    Private Sub NotifyAdded(index As Integer)
    End Sub

    Private Sub NotifyCleared()
    End Sub

    Private Sub NotifyInserted(index As Integer)
    End Sub

    Private Sub NotifyRemoved(index As Integer)
    End Sub

    Protected Overrides Sub WndProc(ByRef m As Message)
        Select Case m.Msg
            Case CB_ADDSTRING
                MyBase.WndProc(m)
                Dim index As Integer = (Me.Items.Count - 1)
                Me.NotifyAdded(index)
                Exit Select
            Case CB_DELETESTRING
                MyBase.WndProc(m)
                Dim index As Integer = m.WParam.ToInt32()
                Me.NotifyRemoved(index)
                Exit Select
            Case CB_INSERTSTRING
                MyBase.WndProc(m)
                Dim index As Integer = m.WParam.ToInt32()
                Me.NotifyAdded(If((index > -1), index, (Me.Items.Count - 1)))
                Exit Select
            Case CB_RESETCONTENT
                MyBase.WndProc(m)
                Me.NotifyCleared()
                Exit Select
            Case Else
                MyBase.WndProc(m)
                Exit Select
        End Select
    End Sub

    Private Const CB_ADDSTRING As Integer = &H143
    Private Const CB_DELETESTRING As Integer = &H144
    Private Const CB_INSERTSTRING As Integer = 330
    Private Const CB_RESETCONTENT As Integer = &H14B

End Class