how to load child nodes of tree view when parent node is selected

I have  web page and this web page has one tree view control under an update panel.
now right now i am loading all the nodes at once. but this makes the page really slow as there are tons of records in the database from where tree view is picking data.

I want to know how can i load the child nodes only when the parent node is selected.

ziorrinfotechAsked:
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tiagosalgadoCommented:
Ok, so check this code and adapt it to your own propose...
<asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel runat="server" ID="up1">
        <ContentTemplate>
            <asp:TreeView ID="TreeView1" runat="server" 
                onselectednodechanged="TreeView1_SelectedNodeChanged">
            </asp:TreeView>    
        </ContentTemplate>
    </asp:UpdatePanel>
 
======================= CODE BEHIND =========================
 
protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                GetParentNodes();
            }
        }
 
        protected void GetParentNodes()
        {
            using (SqlConnection conn = new SqlConnection("YOUR_CONNECTIONSTRING_HERE"))
            {
                
                using (SqlCommand cmd = new SqlCommand("SELECT * FROM ParentNodes",conn))
                {
                    conn.Open();
                    SqlDataReader dr = cmd.ExecuteReader();
                    while (dr.Read())
                    {
                        TreeView1.Nodes.Add(new TreeNode(dr["node"].ToString(), dr["id"].ToString()));
                    }
                    
                }
            }
        }
 
        protected void GetChildNodesFromParent(TreeNode ParentNode)
        {
            using (SqlConnection conn = new SqlConnection("YOUR_CONNECTIONSTRING_HERE"))
            {
 
                using (SqlCommand cmd = new SqlCommand("SELECT * FROM ChildNodes WHERE parent_id = @id", conn))
                {
                    cmd.Parameters.Add("@id", SqlDbType.Int, 4).Value = int.Parse(ParentNode.Value);
                    conn.Open();
                    SqlDataReader dr = cmd.ExecuteReader();
                    while (dr.Read())
                    {
                        TreeView1.SelectedNode.ChildNodes.Add(new TreeNode(dr["node"].ToString(), dr["id"].ToString()));
                    }
 
                }
            }
        }
 
        protected void TreeView1_SelectedNodeChanged(object sender, EventArgs e)
        {
            GetChildNodesFromParent(TreeView1.SelectedNode);
        }

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ziorrinfotechAuthor Commented:
this code works, but what is happening now is when the parent nodes are loaded initially, there is no expand icon next to it. i want to have an expand icon next to the parent node so that the users know that there are child nodes below the particular parent node, if any.
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