Foreground code:

<asp:TreeView ID="TreeView1" runat="server" ExpandDepth="0" OnTreeNodeExpanded="TreeView1_TreeNodeExpanded" ShowLines="True">

Code-behind:

protected void TreeView1_TreeNodeExpanded(object sender, TreeNodeEventArgs e)
         {
              TreeNodeCollection ts = null;
             if (e.Node.Parent == null)
             {
                 ts = ((TreeView)sender).Nodes;
             }
             else
                 ts = e.Node.Parent.ChildNodes;
             foreach (TreeNode node in ts)
             {
                 if (node != e.Node)
                 {
                     node.Collapse();
                 }
             }
         }

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