RadTreeView for ASP.NET AJAX
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RadControls for ASP.NET AJAX

This article shows hot to expand only one Node per Level with JavaScript. Hook to the OnClientNodeClicking event of the TreeView and handle it in the following way:

CopyJavaScript
<script type="text/javascript">
    function OnClientNodeClicking(sender, eventArgs) {
        var node = eventArgs.get_node();

        node.toggle();

        if (node.get_nodes().get_count()) {
            CollapseSiblings(node);
        }
    }

    function CollapseSiblings(node) {
        var parent = node.get_parent();
        var siblings = parent.get_nodes();
        var siblingsCount = siblings.get_count();

        for (var nodeIndex = 0; nodeIndex < siblingsCount; nodeIndex++)
         {
            var siblingNode = siblings.getNode(nodeIndex);

            if ((siblingNode != node) && (siblingNode.get_expanded())) 
            {
                siblingNode.collapse();
                return;
            }
        }
    }
</script>