当用户选中或取消选中选中组合框中的复选框时，我想防止下拉列表关闭。
我复制了一些 Microsoft 代码来创建一个选中的组合框。由于它不是开箱即用的，我做了一些定制。
这是我的代码：
Imports System.ComponentModel
Imports System.Collections.ObjectModel
Public Class CheckedCombobox
Inherits ComboBox
Public Event ItemCheck(ByVal sender As Object, ByVal e As System.Windows.Forms.ItemCheckEventArgs)
<Browsable(False)> _
Public Overloads ReadOnly Property Items() As ComboBox.ObjectCollection
Get
Return MyBase.Items
End Get
End Property
Private WithEvents _ItemCollection As New ObservableCollection(Of String)
Public Property ItemCollection As ObservableCollection(Of String)
Get
Return _ItemCollection
End Get
Set(value As ObservableCollection(Of String))
_ItemCollection = value
End Set
End Property
Private _ItemDictionary As New Dictionary(Of String, Boolean)
Public ReadOnly Property ItemDictionary As Dictionary(Of String, Boolean)
Get
Return _ItemDictionary
End Get
End Property
Public ReadOnly Property CheckedItemCollection As List(Of String)
Get
Return New List(Of String)(From item In ItemDictionary Where item.Value = True Select item.Key)
End Get
End Property
Public ReadOnly Property UnCheckedItemCollection As List(Of String)
Get
Return New List(Of String)(From item In ItemDictionary Where item.Value = False Select item.Key)
End Get
End Property
Public Sub setCheckState(ByVal key As String, ByVal checkstate As Boolean)
_ItemDictionary(key) = checkstate
End Sub
Public Function getCheckState(ByVal key As String)
Return (_ItemDictionary(key))
End Function
Public Sub New()
Me.DrawMode = Windows.Forms.DrawMode.OwnerDrawVariable
End Sub
Protected Overrides Sub OnCreateControl()
MyBase.OnCreateControl()
For Each item In ItemCollection
_ItemDictionary.Add(item, False)
Next
End Sub
Private Sub ItemsChanged(ByVal sender As Object, ByVal e As System.Collections.Specialized.NotifyCollectionChangedEventArgs) Handles _ItemCollection.CollectionChanged
Select Case e.Action
Case Specialized.NotifyCollectionChangedAction.Add
If e.NewStartingIndex = ItemDictionary.Count Then
_ItemDictionary.Add(e.NewItems(0), False)
MyBase.Items.Add(e.NewItems(0))
End If
Case Specialized.NotifyCollectionChangedAction.Remove
_ItemDictionary.Remove(MyBase.Items(e.OldStartingIndex))
MyBase.Items.RemoveAt(e.OldStartingIndex)
Case Specialized.NotifyCollectionChangedAction.Move
Dim _item As Object = MyBase.Items(e.OldStartingIndex)
MyBase.Items.RemoveAt(e.OldStartingIndex)
MyBase.Items.Insert(e.NewStartingIndex, _item)
Case Specialized.NotifyCollectionChangedAction.Replace
Throw New Exception("Not implemented yet!")
Case Specialized.NotifyCollectionChangedAction.Reset
Dim _checkeditems As New List(Of String)(CheckedItemCollection)
MyBase.Items.Clear()
MyBase.Items.AddRange(_ItemCollection.ToArray)
_ItemDictionary.Clear()
For Each item In _ItemCollection
_ItemDictionary.Add(item, _checkeditems.Contains(item))
Next
End Select
Me.Invalidate()
End Sub
Protected Overrides Sub OnDrawItem(ByVal e As System.Windows.Forms.DrawItemEventArgs)
e.DrawBackground()
Dim p As Point = e.Bounds.Location
If e.Index >= 0 Then
p.Offset(1, 1)
If getCheckState(MyBase.Items(e.Index)) Then
CheckBoxRenderer.DrawCheckBox(e.Graphics, p, VisualStyles.CheckBoxState.CheckedNormal)
Else
CheckBoxRenderer.DrawCheckBox(e.Graphics, p, VisualStyles.CheckBoxState.UncheckedNormal)
End If
p.Offset(12, 0)
e.Graphics.DrawString(MyBase.GetItemText(Me.Items(e.Index)), e.Font, New SolidBrush(e.ForeColor), p.X, p.Y)
End If
If e.State = DrawItemState.Selected Then
e.DrawFocusRectangle()
End If
MyBase.OnDrawItem(e)
End Sub
Private Sub checkedChanged(ByVal index As Integer)
Dim checked As Boolean = _ItemDictionary(MyBase.Items.Item(index))
If checked Then
_ItemDictionary(MyBase.Items.Item(index)) = False
RaiseEvent ItemCheck(Me, New ItemCheckEventArgs(index, CheckState.Unchecked, CheckState.Checked))
Else
_ItemDictionary(MyBase.Items.Item(index)) = True
RaiseEvent ItemCheck(Me, New ItemCheckEventArgs(index, CheckState.Checked, CheckState.Unchecked))
End If
Me.Invalidate()
End Sub
Private n As nWindow = Nothing
Private Const WM_CTLCOLORLISTBOX As Integer = &H134
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
MyBase.WndProc(m)
If m.Msg = WM_CTLCOLORLISTBOX Then
If n Is Nothing Then
n = New nWindow(Me)
n.AssignHandle(m.LParam)
AddHandler n.checkedChanged, AddressOf checkedChanged
End If
End If
End Sub
Private Sub CheckedCombobox_Click(sender As Object, e As System.EventArgs) Handles Me.SelectedIndexChanged
Debugger.Break()
End Sub
End Class
Public Class nWindow
Inherits NativeWindow
Private Const WM_LBUTTONDOWN As Integer = &H201
Private _combobox As CheckedCombobox
Public Event checkedChanged(ByVal index As Integer)
Public Sub New(ByVal cb As CheckedCombobox)
_combobox = cb
End Sub
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
If m.Msg = WM_LBUTTONDOWN Then
Dim itemHeight As Integer = _combobox.ItemHeight
If New Point(m.LParam.ToInt32).Y \ itemHeight <= _combobox.Items.Count - 1 And New Point(m.LParam.ToInt32).Y \ itemHeight >= 0 Then
If New Point(m.LParam.ToInt32).X >= 1 And New Point(m.LParam.ToInt32).X <= 11 Then
RaiseEvent checkedChanged(_combobox.SelectedIndex)
End If
End If
End If
MyBase.WndProc(m)
End Sub
End Class