Public Class MainForm
 
    Dim trv As New TreeView With {.Parent = Me, .Height = 200}
 
    Dim WithEvents btnSubDialog As New Button With {.Parent = Me, .Text = _
      "Subform", .Top = 210}
 
    Private Sub MainForm_Load(sender As System.Object, e As System.EventArgs) _
      Handles MyBase.Load
        'Testdaten
        For i As Integer = 1 To 10
            Dim tn As New TreeNode("a" & i.ToString)
            Dim tnn As TreeNode = tn.Nodes.Add(tn.Text & "SN1")
            tnn.Nodes.Add(tnn.Text & " E1")
            tn.Nodes.Add(tn.Text & "SN2")
            trv.Nodes.Add(tn)
        Next i
    End Sub
 
 
    Private Sub btnSubDialog_Click(sender As Object,_
        e As System.EventArgs) Handles btnSubDialog.Click
 
        Using mySubForm As New SubForm
 
            mySubForm.ParentTrv = trv
            mySubForm.ShowDialog()
        End Using
    End Sub
End Class
 
 
Public Class SubForm
 
    Public WriteOnly Property ParentTrv As TreeView
        Set(value As TreeView)
            Treeview_CloneNodes(value, trv)
        End Set
    End Property
 
    Private trv As New TreeView With {.Parent = Me}
 
 
    Private Sub Treeview_CloneNodes(ByVal trv As TreeView, trv2 As TreeView)
 
        trv2.Nodes.Clear()
 
        For i As Integer = 0 To trv.Nodes.Count - 1
            Dim tnc As TreeNode = CType(trv.Nodes(i).Clone, TreeNode)
            trv2.Nodes.Add(tnc)
        Next i
 
    End Sub
 
End Class