Sub TreeNumList()
        TextBoxTreeNum.Clear()
        For Each n As TreeNode In TreeViewTask.Nodes
            SubTreeNumList(n, 0, CStr(n.Index + 1))
        Next
End Sub

Sub SubTreeNumList(ByVal node As TreeNode, ByVal level As Int32, ByVal Num As String)
        TextBoxTreeNum.Text += Num + Environment.NewLine
        For Each n As TreeNode In node.Nodes
            Num += "." + CStr(n.Index + 1)
            SubTreeNumList(n, (level + 1), Num)
        Next
End Sub

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2.1
2.1.2
2.1.2.3

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2.1
2.2
2.3