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When the user clicks the plus button next to a Node that has child Nodes, the NodeExpand event fires. Use the RadTreeNodeEventArgs.Node property to get or set the clicked-on Node properties. The Node ExpandMode property must be set to ServerSide, ServerSideCallback or WebService for this event to fire.

protected void RadTreeView1_NodeExpand(object sender, Telerik.Web.UI.RadTreeNodeEventArgs e)
    for (int i = 1; i <= 10; i++)
        e.Node.Nodes.Add(new Telerik.Web.UI.RadTreeNode("Child Node " + i.ToString()));
    e.Node.ExpandMode = Telerik.Web.UI.TreeNodeExpandMode.ClientSide;
Protected Sub RadTreeView1_NodeExpand(ByVal sender As Object, ByVal e As Telerik.Web.UI.RadTreeNodeEventArgs)
    Dim i As Integer = 1
    While i <= 10
        e.Node.Nodes.Add(New Telerik.Web.UI.RadTreeNode("Child Node " + i.ToString()))
        System.Math.Max(System.Threading.Interlocked.Increment(i), i - 1)
    End While
    e.Node.ExpandMode = Telerik.Web.UI.TreeNodeExpandMode.ClientSide
End Sub

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