Btw I had a quick question, how can I make sure all the nodes are in collapse state when the tree initially loads?
Nodes are collapsed by default. If node.Expanded is true or ExpandLevel of a parent node is positive - then the node is expanded. Otherwise it is collapsed.
If you need to reset all nodes state after postback, then you need to get all nodes FlyTreeView.FindAll() and set every node's Expanded to null (or even to false to override ExpandLevel setting).
Also one last thing , I want to make sure that after PostBack occurs, I want that the node which I had selected before postback, only its parent node remains open and all other sibling will close? is that possible with the code you sent me?
As you can see from my code, the collapseSiblingNodes function does not collapse parent nodes, only sibling nodes of parent nodes.