Перебор циклом всех узлов в ветви TreeView

    Private Sub TreeView1_AfterCheck(sender As Object, e As TreeViewEventArgs) Handles TreeView1.AfterCheck
 
        Dim node As TreeNode = e.Node
 
        For i=0 To node.Parent.Nodes.Count - 1
            Dim unused = node.Parent.Nodes(i).Index
 
            If e.Node.Index <> unused Then
                node.Parent.Nodes(i).Checked = False
            Else
                node.Parent.Nodes(i).Checked = True
            End If
        Next i
    End Sub

    Private Sub TreeView1_AfterCheck(sender As Object, e As TreeViewEventArgs) Handles TreeView1.AfterCheck
 
        Dim parNode As TreeNode = e.Node.Parent
 
        If parNode Is Nothing Then Exit Sub
 
        For Each node as TreeNode In parNode.Nodes
            If Not (node Is e.Node) Then
                node.Checked = False
            End If
        Next
    End Sub

Dim updateTree as boolean = false 'флаг обновления дерева
 
    Private Sub TreeView1_AfterCheck(sender As Object, e As TreeViewEventArgs) Handles TreeView1.AfterCheck
        If updateTree Then Exit Sub
 
        updateTree = True 
 
        TreeNodesUpdate(e.Node)
 
        updateTree = False
 
    End Sub
 
    Private Sub TreeNodesUpdate(currentNode As TreeNode)
 
        Dim parNode As TreeNode = currentNode.Parent
 
        If parNode Is Nothing Then Exit Sub
 
        For Each node as TreeNode In parNode.Nodes
            If Not (node Is currentNode) Then
                node.Checked = False
            End If
        Next
    End Sub

    Private Sub TreeView1_AfterCheck(sender As Object, e As TreeViewEventArgs) Handles TreeView1.AfterCheck
        If updateTree Then Exit Sub
        If e.Node.Checked = False Then e.Node.Checked = True
 
        TreeNodesUpdate(e.Node)
     End Sub