Binding dropdownlist in footer

 protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
 if (e.Row.RowType == DataControlRowType.Footer)
        {         
            con.Open();
            string str = "select * from [order]";
            SqlCommand cmd = new SqlCommand(str, con);
            DataTable dt = new DataTable();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dt);       
            DropDownList d1 = (DropDownList)e.Row.FindControl("drop1");
            d1.DataTextField = "product_id";
            d1.DataSource = dt;
            d1.DataBind();
        }         
    }


Make button invisible and dropdownlist visible in gridview button click


 protected void btnadd_Click(object sender, EventArgs e)
    {
        Button b1 = (Button)sender;
        GridViewRow gr = (GridViewRow)b1.NamingContainer;
        DropDownList drp = (DropDownList)GridView1.Rows[gr.RowIndex].FindControl("drp");
        drp.Visible = true;
        b1.Visible = false;      
    }