Flatten Treeview Nodes collection to List without recursion - vb.net

Question

Hopefully this is not a repeat question. I looked and found similar questions but not sufficient in this case.
I have many tree view controls, and can traverse the Nodes recursively for various reasons.
However, I often need to traverse the Nodes as if they are in a List.
I would like to make a function that creates a Generic.List(of TreeNode) from the Nodes collection, without recursion if at all possible.
(Without recursion purely for the exercise of doing it without recursion - I understand it may not be possible)
This function would save alot of time with repeated use across a massive solution, where the coders could use a simple For Each paradigm to traverse the Nodes.
I have seen a technique to 'flatten' the Nodes collection using C#, which uses LINQ and recursion, but I am not sure that the syntax can be converted to VB.NET cleanly. So if there are any clever VB functions out there can that I can mold to this task - would be very helpful.
There are many similar questions and very informative answers on SO, like this one:
Enumerating Collections that are not inherently IEnumerable?
...which highlights stack overflow errors in very deep trees using some algorithms. I hope that a method that does not use recursion will not suffer from Stack overflow errors - however, I am prepared that it might be long and clumsy and slow.
I am also prepared for the answer that 'It is not possible to do this without recursion'. However, I would like to confirm or deny this claim using the power of SO (this forum)

Answer 1

It's possible, and not very hard at all....
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
TreeView1.ExpandAll()
For Each TN As TreeNode In TreeView1.NodesToListWithoutRecursionBecauseWhyNot(TraverseType.BreadthFirst)
Debug.Print(TN.Text)
Next
End Sub
End Class
Public Module Extensions
Public Enum TraverseType
BreadthFirst
DepthFirst
End Enum
<Runtime.CompilerServices.Extension()> _
Public Function NodesToListWithoutRecursionBecauseWhyNot(ByVal TV As TreeView, Optional ByVal Traverse As TraverseType = TraverseType.DepthFirst) As List(Of TreeNode)
Dim nodes As New List(Of TreeNode)
Select Case Traverse
Case TraverseType.BreadthFirst
Dim Q As New Queue(Of TreeNode)
For Each TN As TreeNode In TV.Nodes
Q.Enqueue(TN)
Next
While Q.Count > 0
Dim TN As TreeNode = Q.Dequeue
nodes.Add(TN)
For Each subTN As TreeNode In TN.Nodes
Q.Enqueue(subTN)
Next
End While
Case TraverseType.DepthFirst
Dim L As New List(Of TreeNode)
For Each TN As TreeNode In TV.Nodes
L.Add(TN)
Next
While L.Count > 0
Dim TN As TreeNode = L.Item(0)
L.RemoveAt(0)
nodes.Add(TN)
For i As Integer = TN.Nodes.Count - 1 To 0 Step -1
L.Insert(0, TN.Nodes(i))
Next
End While
End Select
Return nodes
End Function
End Module

Answer 2

Just add the nodes to the list but at the same time keep the position of the last node you processed. A node is considered process when its immediate children are added to the list.
Public Function GetAllNodes(ByVal topNode As TreeNode)
Dim allNodes As New List(Of TreeNode)
Dim lastProcessPosition As Integer = 0
allNodes.Add(topNode)
Do While lastProcessPosition < allNodes.Count
allNodes.AddRange(allNodes(lastProcessPosition).Nodes)
lastProcessPosition += 1
Loop
Return allNodes
End Function
If you don't have a top node then just substitute the parameter for a list of nodes to start with.
Public Function GetAllNodes(ByVal topNodes As TreeNodeCollection)
Dim allNodes As New List(Of TreeNode)
Dim lastProcessPosition As Integer = 0
allNodes.AddRange(topNodes)
Do While lastProcessPosition < allNodes.Count
allNodes.AddRange(allNodes(lastProcessPosition).Nodes)
lastProcessPosition += 1
Loop
Return allNodes
End Function
I'm not sure if a check for Nothing must be done on the Nodes property before using it.
Note: I was able to remove this for loop and replace it with AddRange
'For Each node As TreeNode In allNodes(lastProcessPosition).Nodes
' allNodes.Add(node)
'Next