[VB.NET]MDI Thumbnail Preview

Public Class ThumbnailDialog

Private _mdiParent As Form
Private _thumbnailPool As Dictionary(Of Form, Image)

Private Property m_MDIParent As Form
    Get
        Return _mdiParent
    End Get
    Set(ByVal value As Form)
        _mdiParent = value
    End Set
End Property

Private ReadOnly Property m_ThumbnailPool As Dictionary(Of Form, Image)
    Get
        If _thumbnailPool Is Nothing Then
            _thumbnailPool = New Dictionary(Of Form, Image)
        End If
        Return _thumbnailPool
    End Get
End Property

Public ReadOnly Property SelectedChildForm As Form
    Get
        Return If(ListBox1.SelectedIndex = -1, Nothing, m_MDIParent.MdiChildren(ListBox1.SelectedIndex))
    End Get
End Property

Private Sub ThumbnailDialog_KeyUp(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles Me.KeyUp
    If Not e.Control Then
        DialogResult = If(ListBox1.SelectedIndex = -1, DialogResult.Cancel, DialogResult.OK)
        Me.Close()
    End If
End Sub

Protected Overrides Function ProcessCmdKey(ByRef msg As System.Windows.Forms.Message, ByVal keyData As System.Windows.Forms.Keys) As Boolean
    If keyData = (Keys.Control Or Keys.Tab) Then
        If ListBox1.Items.Count > 1 Then
            ListBox1.SelectedIndex = (ListBox1.SelectedIndex + 1) Mod ListBox1.Items.Count
        End If
        Return True
    End If
    Return MyBase.ProcessCmdKey(msg, keyData)
End Function

Private Sub ThumbnailDialog_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Me.KeyPreview = True
    ListBox1.DisplayMember = "Text"
    ListBox1.DataSource = m_MDIParent.MdiChildren
    Dim childForm As Form = m_MDIParent.ActiveMdiChild
    If childForm Is Nothing Then
        Return
    End If
    ListBox1.SelectedItem = childForm
End Sub

Public Sub New(ByVal mdiParent As Form)

    ' This call is required by the designer.
    InitializeComponent()

    ' Add any initialization after the InitializeComponent() call.
    m_MDIParent = mdiParent
    PictureBox1.SizeMode = PictureBoxSizeMode.Zoom
End Sub

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
    Dim childForm As Form = m_MDIParent.MdiChildren(ListBox1.SelectedIndex)
    Dim thumbnailImage As Image = If(m_ThumbnailPool.ContainsKey(childForm), m_ThumbnailPool(childForm), Nothing)
    If thumbnailImage Is Nothing Then
        thumbnailImage = New Bitmap(childForm.Width, childForm.Height)
        m_ThumbnailPool.Add(childForm, thumbnailImage)
        childForm.DrawToBitmap(thumbnailImage, New Rectangle(0, 0, thumbnailImage.Width, thumbnailImage.Height))
    End If
    PictureBox1.Image = thumbnailImage
End Sub

End Class

• VB.NET