Fill ID in a tree view

Hi guys, i am having a problem regarding a tree view control. I am very new to C# and stuck on how to add ID to the corresponding tree nodes. I am not sure whether this lines of code holds ID or if not then what should i include. I want to get the ID of the selected node. How do i proceed please let me know. My code is given below -

Thanks in advance.

private void PopulateTreeView()
        {
            treeDepartments.Nodes.Clear();
            String strConn = "Server =server;Database =DB;Integrated Security = True;";
            SqlConnection conn = new SqlConnection(strConn);
            SqlDataAdapter da = new SqlDataAdapter("Select * from tSubDepartments", conn);
            SqlDataAdapter daCategories = new SqlDataAdapter("Select * from tDepartments", conn);
            da.Fill(ds, "tSubDepartments");
            daCategories.Fill(ds, "tDepartments");
            ds.Relations.Add("Dept_SubDept", ds.Tables["tDepartments"].Columns["dpCode"], ds.Tables["tSubDepartments"].Columns["dpCode"]);
            foreach (DataRow dr in ds.Tables["tDepartments"].Rows)
            {
                int id = Convert.ToInt32(dr["dpCode"]);
                TreeNode tn = new TreeNode(dr["dpName"].ToString());
                foreach (DataRow drChild in dr.GetChildRows("Dept_SubDept"))
                {
                    tn.Nodes.Add(drChild["sdName"].ToString());
                }
                treeDepartments.Nodes.Add(tn); 
            }
        }

---

Apurba