ASP.NET DROPDOWNLIST & GRIDVIEW

protected void d_selected_indexchanged(object sender, EventArgs e)
   {
       string str = "Select *from " + DropDownList1.SelectedValue.ToString().Trim() + "";
       SqlCommand cmd = new SqlCommand(str,Db.GetConnection());

       SqlDataAdapter ad = new SqlDataAdapter(cmd);
       DataTable dt = new DataTable();

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