[Vb.Net]
Public Shared Function GetCheckedNodes(ByVal nodes As NodeCollection, ByVal includeChildren As Boolean) As NodeBase()
  Dim checkedNodes As ArrayList = New ArrayList
  For Each n As NodeBase In nodes
    If (n.Checked) Then
      checkedNodes.Add(n)
    End If
    If (includeChildren And n.HasChildren) Then
      checkedNodes.AddRange(GetCheckedNodes(n.Items, includeChildren))
    End If
  Next
  Return CType(checkedNodes.ToArray(GetType(NodeBase)), NodeBase())
End Function

[Vb.Net]
Dim checkedNodes as NodeBase() = GetCheckedNodes(flyGrid.Rows.Items, true);

Public Class Form1
    Private random As System.Random ' a random numbers generator
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        SetupGrid()
    End Sub
    Private Sub AddNodes(ByVal grid As FlyGrid, ByVal prefix As String, ByVal nodes As NodeCollection)
        If Me.random Is Nothing Then
            Me.random = New System.Random(grid.Images.Images.Count)
        End If
        Dim v_arrNode As Node() = New Node(10 - 1) {}
        Dim v_imageIndex As Integer = 0
        For v_i As Integer = 0 To v_arrNode.Length - 1
            v_imageIndex = Me.random.Next(0, grid.Images.Images.Count)
            Dim v_arrobj As Object() = New Object(grid.Columns.VisibleColumns.Length - 1) {}
            v_arrobj(0) = String.Concat(prefix, "Node", v_i.ToString)
            v_arrNode(v_i) = New TreeViewNode(v_arrobj, v_imageIndex)
        Next v_i
        nodes.AddRange(v_arrNode)
    End Sub
    Private Sub SetupGrid()
        grid.BeginInit()
        Try
            AddNodes(grid, "", grid.Rows.Items)
            For Each v_node As Node In grid.Rows.Items
                AddNodes(grid, "Sub", v_node.Items)
                For Each v_subnode As Node In v_node.Items
                    AddNodes(grid, "Subsub", v_subnode.Items)
                Next v_subnode
            Next v_node
            'select the first node
            grid.Selected = grid.Rows.Items(0)
        Finally
            grid.EndInit()
        End Try
    End Sub

    Private Sub btnReplaceImage1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnReplaceImage1.Click
        'this function just changes image index of node - so replaces graphic appearance of the node
        Dim selectedNode As TreeViewNode = grid.Selected
        If (selectedNode Is Nothing) Then
            MessageBox.Show("Please select a node")
        Else
            'change image on a random basis (random generator is created in AddNodes procedure)
            selectedNode.ImageIndex = ChangeImageIndex(selectedNode.ImageIndex)
            selectedNode.SelectedImageIndex = selectedNode.ImageIndex
        End If
    End Sub
    Private Function ChangeImageIndex(ByVal currentImageIndex As Integer) As Integer
        Dim newImageIndex = random.Next(grid.Images.Images.Count)
        While (newImageIndex = currentImageIndex)
            newImageIndex = random.Next(grid.Images.Images.Count)
        End While
        Return newImageIndex
    End Function

    Private Sub btnReplaceImage2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnReplaceImage2.Click
        'this fprocedure just replaces images in the image list and replaces graphic appearance of the node
        Dim selectedNode As TreeViewNode = grid.Selected
        If (selectedNode Is Nothing) Then
            MessageBox.Show("Please select a node")
        Else
            'change image on a random basis (random generator is created in AddNodes procedure)
            Dim selectedImageIndex As Integer = selectedNode.ImageIndex
            Dim newImageIndex As Integer = ChangeImageIndex(selectedImageIndex)
            Dim selImage As System.Drawing.Image = grid.Images.Images(selectedImageIndex)
            Dim newImage As System.Drawing.Image = grid.Images.Images(newImageIndex)
            'exchange images in the image list
            grid.Images.Images(selectedImageIndex) = newImage
            grid.Images.Images(newImageIndex) = selImage
            grid.InvalidateNode(selectedNode)
        End If
    End Sub
End Class