try this
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
SqlCommand cmd = new SqlCommand("SELECT Database_Name FROM Create_db", cnn);
DataTable dt = new DataTable();
Master.FindControl("TreeView1");
TreeView tv = (TreeView)Master.FindControl("TreeView1");
SqlDataAdapter da = new SqlDataAdapter(cmd);
cnn.Open();
da.Fill(dt);
cnn.Close();
var parentNode = new TreeNode("Database");
if (dt.Rows.Count > 0)
{
foreach (DataRow row in dt.Rows)
{
string value = row["Database_Name"] + "";
parentNode.ChildNodes.Add(new TreeNode(value));
}
}
tv.Nodes.Add(parentNode);
}
}