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How to remove all but checked nodes and their parents in a treeview?
private void RecursiveCheckNodesDown(TreeNodeCollection tree, bool checkedFlag)
    {
        if (tree != null)
            foreach (TreeNode node in tree)
                RecursiveCheckNodesDown(node.Nodes, node.Checked = checkedFlag);
    }

    private void RecursiveCheckNodesUp(TreeNode node, bool checkedFlag)
    {
        if( node != null )
            RecursiveCheckNodesUp(node.Parent, node.Checked = checkedFlag);
    }

    private void SomeTreeBeginUpdate()
    {
        SomeTree.BeginUpdate();
        SomeTree.AfterCheck -= SomeTree_AfterCheck;
    }

    private void SomeTreeEndUpdate()
    {
        SomeTree.AfterCheck += SomeTree_AfterCheck;
        SomeTree.EndUpdate();
    }

    private void SomeTree_AfterCheck(object sender, TreeViewEventArgs e)
    {
        SomeTreeBeginUpdate();
        RecursiveCheckNodesDown(e.Node.Nodes, e.Node.Checked);
        if( e.Node.Checked )
            RecursiveCheckNodesUp(e.Node.Parent, e.Node.Checked);
        SomeTreeEndUpdate();
    }

    this.SomeTree.AfterCheck += new System.Windows.Forms.TreeViewEventHandler(this.SomeTree_AfterCheck);

//pseudocode
bool prune(treenode t)
{
   bool mayNotDeleteThisNode=t.isChecked;
   foreach(treenode c in t.childnodes)
   {
      if(prune(c)) mayNotDeleteThisNode=true;
   }

   if(!mayNotDeleteThisNode)
   {
      delete(t)
   }

   return mayNotDeleteThisNode;
}