how to display the information in gridview when item selected in dropdown list?ASP

 protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string sql = "Select * from Categories where CategoryName=" + DropDownList1.SelectedItem.Text;
        SqlConnection conn = new SqlConnection();
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["LisamCn"].ToString(); ;
        conn.Open();
        SqlDataAdapter da = new SqlDataAdapter(sql, conn);
        DataTable dt = new DataTable();
        conn.Close();
        da.Fill(dt);

        GridView1.DataSource = dt;
        GridView1.DataBind();
    }